--- gcc/gcc.info-12 2018/04/24 18:11:43 1.1.1.6 +++ gcc/gcc.info-12 2018/04/24 18:24:52 1.1.1.8 @@ -1,12 +1,13 @@ -This is Info file gcc.info, produced by Makeinfo-1.54 from the input +This is Info file gcc.info, produced by Makeinfo-1.55 from the input file gcc.texi. This file documents the use and the internals of the GNU compiler. - Published by the Free Software Foundation 675 Massachusetts Avenue -Cambridge, MA 02139 USA + Published by the Free Software Foundation 59 Temple Place - Suite 330 +Boston, MA 02111-1307 USA - Copyright (C) 1988, 1989, 1992, 1993 Free Software Foundation, Inc. + Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995 Free Software +Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are @@ -14,991 +15,1096 @@ preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also -that the sections entitled "GNU General Public License" and "Protect -Your Freedom--Fight `Look And Feel'" are included exactly as in the -original, and provided that the entire resulting derived work is -distributed under the terms of a permission notice identical to this -one. +that the sections entitled "GNU General Public License," "Funding for +Free Software," and "Protect Your Freedom--Fight `Look And Feel'" are +included exactly as in the original, and provided that the entire +resulting derived work is distributed under the terms of a permission +notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the sections entitled "GNU General Public -License" and "Protect Your Freedom--Fight `Look And Feel'", and this -permission notice, may be included in translations approved by the Free -Software Foundation instead of in the original English. +License," "Funding for Free Software," and "Protect Your Freedom--Fight +`Look And Feel'", and this permission notice, may be included in +translations approved by the Free Software Foundation instead of in the +original English.  -File: gcc.info, Node: Regs and Memory, Next: Arithmetic, Prev: Constants, Up: RTL +File: gcc.info, Node: Temporaries, Prev: Static Definitions, Up: C++ Misunderstandings -Registers and Memory +Temporaries May Vanish Before You Expect +---------------------------------------- + + It is dangerous to use pointers or references to *portions* of a +temporary object. The compiler may very well delete the object before +you expect it to, leaving a pointer to garbage. The most common place +where this problem crops up is in classes like the libg++ `String' +class, that define a conversion function to type `char *' or `const +char *'. However, any class that returns a pointer to some internal +structure is potentially subject to this problem. + + For example, a program may use a function `strfunc' that returns +`String' objects, and another function `charfunc' that operates on +pointers to `char': + + String strfunc (); + void charfunc (const char *); + +In this situation, it may seem natural to write +`charfunc (strfunc ());' based on the knowledge that class `String' has +an explicit conversion to `char' pointers. However, what really +happens is akin to `charfunc (strfunc ().convert ());', where the +`convert' method is a function to do the same data conversion normally +performed by a cast. Since the last use of the temporary `String' +object is the call to the conversion function, the compiler may delete +that object before actually calling `charfunc'. The compiler has no +way of knowing that deleting the `String' object will invalidate the +pointer. The pointer then points to garbage, so that by the time +`charfunc' is called, it gets an invalid argument. + + Code like this may run successfully under some other compilers, +especially those that delete temporaries relatively late. However, the +GNU C++ behavior is also standard-conforming, so if your program depends +on late destruction of temporaries it is not portable. + + If you think this is surprising, you should be aware that the ANSI +C++ committee continues to debate the lifetime-of-temporaries problem. + + For now, at least, the safe way to write such code is to give the +temporary a name, which forces it to remain until the end of the scope +of the name. For example: + + String& tmp = strfunc (); + charfunc (tmp); + + +File: gcc.info, Node: Protoize Caveats, Next: Non-bugs, Prev: C++ Misunderstandings, Up: Trouble + +Caveats of using `protoize' +=========================== + + The conversion programs `protoize' and `unprotoize' can sometimes +change a source file in a way that won't work unless you rearrange it. + + * `protoize' can insert references to a type name or type tag before + the definition, or in a file where they are not defined. + + If this happens, compiler error messages should show you where the + new references are, so fixing the file by hand is straightforward. + + * There are some C constructs which `protoize' cannot figure out. + For example, it can't determine argument types for declaring a + pointer-to-function variable; this you must do by hand. `protoize' + inserts a comment containing `???' each time it finds such a + variable; so you can find all such variables by searching for this + string. ANSI C does not require declaring the argument types of + pointer-to-function types. + + * Using `unprotoize' can easily introduce bugs. If the program + relied on prototypes to bring about conversion of arguments, these + conversions will not take place in the program without prototypes. + One case in which you can be sure `unprotoize' is safe is when you + are removing prototypes that were made with `protoize'; if the + program worked before without any prototypes, it will work again + without them. + + You can find all the places where this problem might occur by + compiling the program with the `-Wconversion' option. It prints a + warning whenever an argument is converted. + + * Both conversion programs can be confused if there are macro calls + in and around the text to be converted. In other words, the + standard syntax for a declaration or definition must not result + from expanding a macro. This problem is inherent in the design of + C and cannot be fixed. If only a few functions have confusing + macro calls, you can easily convert them manually. + + * `protoize' cannot get the argument types for a function whose + definition was not actually compiled due to preprocessing + conditionals. When this happens, `protoize' changes nothing in + regard to such a function. `protoize' tries to detect such + instances and warn about them. + + You can generally work around this problem by using `protoize' step + by step, each time specifying a different set of `-D' options for + compilation, until all of the functions have been converted. + There is no automatic way to verify that you have got them all, + however. + + * Confusion may result if there is an occasion to convert a function + declaration or definition in a region of source code where there + is more than one formal parameter list present. Thus, attempts to + convert code containing multiple (conditionally compiled) versions + of a single function header (in the same vicinity) may not produce + the desired (or expected) results. + + If you plan on converting source files which contain such code, it + is recommended that you first make sure that each conditionally + compiled region of source code which contains an alternative + function header also contains at least one additional follower + token (past the final right parenthesis of the function header). + This should circumvent the problem. + + * `unprotoize' can become confused when trying to convert a function + definition or declaration which contains a declaration for a + pointer-to-function formal argument which has the same name as the + function being defined or declared. We recommand you avoid such + choices of formal parameter names. + + * You might also want to correct some of the indentation by hand and + break long lines. (The conversion programs don't write lines + longer than eighty characters in any case.) + + +File: gcc.info, Node: Non-bugs, Next: Warnings and Errors, Prev: Protoize Caveats, Up: Trouble + +Certain Changes We Don't Want to Make +===================================== + + This section lists changes that people frequently request, but which +we do not make because we think GNU CC is better without them. + + * Checking the number and type of arguments to a function which has + an old-fashioned definition and no prototype. + + Such a feature would work only occasionally--only for calls that + appear in the same file as the called function, following the + definition. The only way to check all calls reliably is to add a + prototype for the function. But adding a prototype eliminates the + motivation for this feature. So the feature is not worthwhile. + + * Warning about using an expression whose type is signed as a shift + count. + + Shift count operands are probably signed more often than unsigned. + Warning about this would cause far more annoyance than good. + + * Warning about assigning a signed value to an unsigned variable. + + Such assignments must be very common; warning about them would + cause more annoyance than good. + + * Warning about unreachable code. + + It's very common to have unreachable code in machine-generated + programs. For example, this happens normally in some files of GNU + C itself. + + * Warning when a non-void function value is ignored. + + Coming as I do from a Lisp background, I balk at the idea that + there is something dangerous about discarding a value. There are + functions that return values which some callers may find useful; + it makes no sense to clutter the program with a cast to `void' + whenever the value isn't useful. + + * Assuming (for optimization) that the address of an external symbol + is never zero. + + This assumption is false on certain systems when `#pragma weak' is + used. + + * Making `-fshort-enums' the default. + + This would cause storage layout to be incompatible with most other + C compilers. And it doesn't seem very important, given that you + can get the same result in other ways. The case where it matters + most is when the enumeration-valued object is inside a structure, + and in that case you can specify a field width explicitly. + + * Making bitfields unsigned by default on particular machines where + "the ABI standard" says to do so. + + The ANSI C standard leaves it up to the implementation whether a + bitfield declared plain `int' is signed or not. This in effect + creates two alternative dialects of C. + + The GNU C compiler supports both dialects; you can specify the + signed dialect with `-fsigned-bitfields' and the unsigned dialect + with `-funsigned-bitfields'. However, this leaves open the + question of which dialect to use by default. + + Currently, the preferred dialect makes plain bitfields signed, + because this is simplest. Since `int' is the same as `signed int' + in every other context, it is cleanest for them to be the same in + bitfields as well. + + Some computer manufacturers have published Application Binary + Interface standards which specify that plain bitfields should be + unsigned. It is a mistake, however, to say anything about this + issue in an ABI. This is because the handling of plain bitfields + distinguishes two dialects of C. Both dialects are meaningful on + every type of machine. Whether a particular object file was + compiled using signed bitfields or unsigned is of no concern to + other object files, even if they access the same bitfields in the + same data structures. + + A given program is written in one or the other of these two + dialects. The program stands a chance to work on most any machine + if it is compiled with the proper dialect. It is unlikely to work + at all if compiled with the wrong dialect. + + Many users appreciate the GNU C compiler because it provides an + environment that is uniform across machines. These users would be + inconvenienced if the compiler treated plain bitfields differently + on certain machines. + + Occasionally users write programs intended only for a particular + machine type. On these occasions, the users would benefit if the + GNU C compiler were to support by default the same dialect as the + other compilers on that machine. But such applications are rare. + And users writing a program to run on more than one type of + machine cannot possibly benefit from this kind of compatibility. + + This is why GNU CC does and will treat plain bitfields in the same + fashion on all types of machines (by default). + + There are some arguments for making bitfields unsigned by default + on all machines. If, for example, this becomes a universal de + facto standard, it would make sense for GNU CC to go along with + it. This is something to be considered in the future. + + (Of course, users strongly concerned about portability should + indicate explicitly in each bitfield whether it is signed or not. + In this way, they write programs which have the same meaning in + both C dialects.) + + * Undefining `__STDC__' when `-ansi' is not used. + + Currently, GNU CC defines `__STDC__' as long as you don't use + `-traditional'. This provides good results in practice. + + Programmers normally use conditionals on `__STDC__' to ask whether + it is safe to use certain features of ANSI C, such as function + prototypes or ANSI token concatenation. Since plain `gcc' supports + all the features of ANSI C, the correct answer to these questions + is "yes". + + Some users try to use `__STDC__' to check for the availability of + certain library facilities. This is actually incorrect usage in + an ANSI C program, because the ANSI C standard says that a + conforming freestanding implementation should define `__STDC__' + even though it does not have the library facilities. `gcc -ansi + -pedantic' is a conforming freestanding implementation, and it is + therefore required to define `__STDC__', even though it does not + come with an ANSI C library. + + Sometimes people say that defining `__STDC__' in a compiler that + does not completely conform to the ANSI C standard somehow + violates the standard. This is illogical. The standard is a + standard for compilers that claim to support ANSI C, such as `gcc + -ansi'--not for other compilers such as plain `gcc'. Whatever the + ANSI C standard says is relevant to the design of plain `gcc' + without `-ansi' only for pragmatic reasons, not as a requirement. + + * Undefining `__STDC__' in C++. + + Programs written to compile with C++-to-C translators get the + value of `__STDC__' that goes with the C compiler that is + subsequently used. These programs must test `__STDC__' to + determine what kind of C preprocessor that compiler uses: whether + they should concatenate tokens in the ANSI C fashion or in the + traditional fashion. + + These programs work properly with GNU C++ if `__STDC__' is defined. + They would not work otherwise. + + In addition, many header files are written to provide prototypes + in ANSI C but not in traditional C. Many of these header files + can work without change in C++ provided `__STDC__' is defined. If + `__STDC__' is not defined, they will all fail, and will all need + to be changed to test explicitly for C++ as well. + + * Deleting "empty" loops. + + GNU CC does not delete "empty" loops because the most likely reason + you would put one in a program is to have a delay. Deleting them + will not make real programs run any faster, so it would be + pointless. + + It would be different if optimization of a nonempty loop could + produce an empty one. But this generally can't happen. + + * Making side effects happen in the same order as in some other + compiler. + + It is never safe to depend on the order of evaluation of side + effects. For example, a function call like this may very well + behave differently from one compiler to another: + + void func (int, int); + + int i = 2; + func (i++, i++); + + There is no guarantee (in either the C or the C++ standard language + definitions) that the increments will be evaluated in any + particular order. Either increment might happen first. `func' + might get the arguments `2, 3', or it might get `3, 2', or even + `2, 2'. + + * Not allowing structures with volatile fields in registers. + + Strictly speaking, there is no prohibition in the ANSI C standard + against allowing structures with volatile fields in registers, but + it does not seem to make any sense and is probably not what you + wanted to do. So the compiler will give an error message in this + case. + + +File: gcc.info, Node: Warnings and Errors, Prev: Non-bugs, Up: Trouble + +Warning Messages and Error Messages +=================================== + + The GNU compiler can produce two kinds of diagnostics: errors and +warnings. Each kind has a different purpose: + + *Errors* report problems that make it impossible to compile your + program. GNU CC reports errors with the source file name and line + number where the problem is apparent. + + *Warnings* report other unusual conditions in your code that *may* + indicate a problem, although compilation can (and does) proceed. + Warning messages also report the source file name and line number, + but include the text `warning:' to distinguish them from error + messages. + + Warnings may indicate danger points where you should check to make +sure that your program really does what you intend; or the use of +obsolete features; or the use of nonstandard features of GNU C or C++. +Many warnings are issued only if you ask for them, with one of the `-W' +options (for instance, `-Wall' requests a variety of useful warnings). + + GNU CC always tries to compile your program if possible; it never +gratuitously rejects a program whose meaning is clear merely because +(for instance) it fails to conform to a standard. In some cases, +however, the C and C++ standards specify that certain extensions are +forbidden, and a diagnostic *must* be issued by a conforming compiler. +The `-pedantic' option tells GNU CC to issue warnings in such cases; +`-pedantic-errors' says to make them errors instead. This does not +mean that *all* non-ANSI constructs get warnings or errors. + + *Note Options to Request or Suppress Warnings: Warning Options, for +more detail on these and related command-line options. + + +File: gcc.info, Node: Bugs, Next: Service, Prev: Trouble, Up: Top + +Reporting Bugs +************** + + Your bug reports play an essential role in making GNU CC reliable. + + When you encounter a problem, the first thing to do is to see if it +is already known. *Note Trouble::. If it isn't known, then you should +report the problem. + + Reporting a bug may help you by bringing a solution to your problem, +or it may not. (If it does not, look in the service directory; see +*Note Service::.) In any case, the principal function of a bug report +is to help the entire community by making the next version of GNU CC +work better. Bug reports are your contribution to the maintenance of +GNU CC. + + Since the maintainers are very overloaded, we cannot respond to every +bug report. However, if the bug has not been fixed, we are likely to +send you a patch and ask you to tell us whether it works. + + In order for a bug report to serve its purpose, you must include the +information that makes for fixing the bug. + +* Menu: + +* Criteria: Bug Criteria. Have you really found a bug? +* Where: Bug Lists. Where to send your bug report. +* Reporting: Bug Reporting. How to report a bug effectively. +* Patches: Sending Patches. How to send a patch for GNU CC. +* Known: Trouble. Known problems. +* Help: Service. Where to ask for help. + + +File: gcc.info, Node: Bug Criteria, Next: Bug Lists, Up: Bugs + +Have You Found a Bug? +===================== + + If you are not sure whether you have found a bug, here are some +guidelines: + + * If the compiler gets a fatal signal, for any input whatever, that + is a compiler bug. Reliable compilers never crash. + + * If the compiler produces invalid assembly code, for any input + whatever (except an `asm' statement), that is a compiler bug, + unless the compiler reports errors (not just warnings) which would + ordinarily prevent the assembler from being run. + + * If the compiler produces valid assembly code that does not + correctly execute the input source code, that is a compiler bug. + + However, you must double-check to make sure, because you may have + run into an incompatibility between GNU C and traditional C (*note + Incompatibilities::.). These incompatibilities might be considered + bugs, but they are inescapable consequences of valuable features. + + Or you may have a program whose behavior is undefined, which + happened by chance to give the desired results with another C or + C++ compiler. + + For example, in many nonoptimizing compilers, you can write `x;' + at the end of a function instead of `return x;', with the same + results. But the value of the function is undefined if `return' + is omitted; it is not a bug when GNU CC produces different results. + + Problems often result from expressions with two increment + operators, as in `f (*p++, *p++)'. Your previous compiler might + have interpreted that expression the way you intended; GNU CC might + interpret it another way. Neither compiler is wrong. The bug is + in your code. + + After you have localized the error to a single source line, it + should be easy to check for these things. If your program is + correct and well defined, you have found a compiler bug. + + * If the compiler produces an error message for valid input, that is + a compiler bug. + + * If the compiler does not produce an error message for invalid + input, that is a compiler bug. However, you should note that your + idea of "invalid input" might be my idea of "an extension" or + "support for traditional practice". + + * If you are an experienced user of C or C++ compilers, your + suggestions for improvement of GNU CC or GNU C++ are welcome in + any case. + + +File: gcc.info, Node: Bug Lists, Next: Bug Reporting, Prev: Bug Criteria, Up: Bugs + +Where to Report Bugs ==================== - Here are the RTL expression types for describing access to machine -registers and to main memory. + Send bug reports for GNU C to `bug-gcc@prep.ai.mit.edu'. + + Send bug reports for GNU C++ to `bug-g++@prep.ai.mit.edu'. If your +bug involves the C++ class library libg++, send mail to +`bug-lib-g++@prep.ai.mit.edu'. If you're not sure, you can send the +bug report to both lists. + + *Do not send bug reports to `help-gcc@prep.ai.mit.edu' or to the +newsgroup `gnu.gcc.help'.* Most users of GNU CC do not want to receive +bug reports. Those that do, have asked to be on `bug-gcc' and/or +`bug-g++'. + + The mailing lists `bug-gcc' and `bug-g++' both have newsgroups which +serve as repeaters: `gnu.gcc.bug' and `gnu.g++.bug'. Each mailing list +and its newsgroup carry exactly the same messages. + + Often people think of posting bug reports to the newsgroup instead of +mailing them. This appears to work, but it has one problem which can be +crucial: a newsgroup posting does not contain a mail path back to the +sender. Thus, if maintainers need more information, they may be unable +to reach you. For this reason, you should always send bug reports by +mail to the proper mailing list. + + As a last resort, send bug reports on paper to: + + GNU Compiler Bugs + Free Software Foundation + 59 Temple Place - Suite 330 + Boston, MA 02111-1307, USA + + +File: gcc.info, Node: Bug Reporting, Next: Sending Patches, Prev: Bug Lists, Up: Bugs + +How to Report Bugs +================== + + The fundamental principle of reporting bugs usefully is this: +*report all the facts*. If you are not sure whether to state a fact or +leave it out, state it! + + Often people omit facts because they think they know what causes the +problem and they conclude that some details don't matter. Thus, you +might assume that the name of the variable you use in an example does +not matter. Well, probably it doesn't, but one cannot be sure. +Perhaps the bug is a stray memory reference which happens to fetch from +the location where that name is stored in memory; perhaps, if the name +were different, the contents of that location would fool the compiler +into doing the right thing despite the bug. Play it safe and give a +specific, complete example. That is the easiest thing for you to do, +and the most helpful. + + Keep in mind that the purpose of a bug report is to enable someone to +fix the bug if it is not known. It isn't very important what happens if +the bug is already known. Therefore, always write your bug reports on +the assumption that the bug is not known. + + Sometimes people give a few sketchy facts and ask, "Does this ring a +bell?" This cannot help us fix a bug, so it is basically useless. We +respond by asking for enough details to enable us to investigate. You +might as well expedite matters by sending them to begin with. + + Try to make your bug report self-contained. If we have to ask you +for more information, it is best if you include all the previous +information in your response, as well as the information that was +missing. + + Please report each bug in a separate message. This makes it easier +for us to track which bugs have been fixed and to forward your bugs +reports to the appropriate maintainer. + + Do not compress and encode any part of your bug report using programs +such as `uuencode'. If you do so it will slow down the processing of +your bug. If you must submit multiple large files, use `shar', which +allows us to read your message without having to run any decompression +programs. + + To enable someone to investigate the bug, you should include all +these things: + + * The version of GNU CC. You can get this by running it with the + `-v' option. + + Without this, we won't know whether there is any point in looking + for the bug in the current version of GNU CC. + + * A complete input file that will reproduce the bug. If the bug is + in the C preprocessor, send a source file and any header files + that it requires. If the bug is in the compiler proper (`cc1'), + run your source file through the C preprocessor by doing `gcc -E + SOURCEFILE > OUTFILE', then include the contents of OUTFILE in the + bug report. (When you do this, use the same `-I', `-D' or `-U' + options that you used in actual compilation.) + + A single statement is not enough of an example. In order to + compile it, it must be embedded in a complete file of compiler + input; and the bug might depend on the details of how this is done. + + Without a real example one can compile, all anyone can do about + your bug report is wish you luck. It would be futile to try to + guess how to provoke the bug. For example, bugs in register + allocation and reloading frequently depend on every little detail + of the function they happen in. + + Even if the input file that fails comes from a GNU program, you + should still send the complete test case. Don't ask the GNU CC + maintainers to do the extra work of obtaining the program in + question--they are all overworked as it is. Also, the problem may + depend on what is in the header files on your system; it is + unreliable for the GNU CC maintainers to try the problem with the + header files available to them. By sending CPP output, you can + eliminate this source of uncertainty and save us a certain + percentage of wild goose chases. + + * The command arguments you gave GNU CC or GNU C++ to compile that + example and observe the bug. For example, did you use `-O'? To + guarantee you won't omit something important, list all the options. + + If we were to try to guess the arguments, we would probably guess + wrong and then we would not encounter the bug. + + * The type of machine you are using, and the operating system name + and version number. + + * The operands you gave to the `configure' command when you installed + the compiler. + + * A complete list of any modifications you have made to the compiler + source. (We don't promise to investigate the bug unless it + happens in an unmodified compiler. But if you've made + modifications and don't tell us, then you are sending us on a wild + goose chase.) + + Be precise about these changes. A description in English is not + enough--send a context diff for them. + + Adding files of your own (such as a machine description for a + machine we don't support) is a modification of the compiler source. + + * Details of any other deviations from the standard procedure for + installing GNU CC. + + * A description of what behavior you observe that you believe is + incorrect. For example, "The compiler gets a fatal signal," or, + "The assembler instruction at line 208 in the output is incorrect." + + Of course, if the bug is that the compiler gets a fatal signal, + then one can't miss it. But if the bug is incorrect output, the + maintainer might not notice unless it is glaringly wrong. None of + us has time to study all the assembler code from a 50-line C + program just on the chance that one instruction might be wrong. + We need *you* to do this part! + + Even if the problem you experience is a fatal signal, you should + still say so explicitly. Suppose something strange is going on, + such as, your copy of the compiler is out of synch, or you have + encountered a bug in the C library on your system. (This has + happened!) Your copy might crash and the copy here would not. If + you said to expect a crash, then when the compiler here fails to + crash, we would know that the bug was not happening. If you don't + say to expect a crash, then we would not know whether the bug was + happening. We would not be able to draw any conclusion from our + observations. + + If the problem is a diagnostic when compiling GNU CC with some + other compiler, say whether it is a warning or an error. + + Often the observed symptom is incorrect output when your program + is run. Sad to say, this is not enough information unless the + program is short and simple. None of us has time to study a large + program to figure out how it would work if compiled correctly, + much less which line of it was compiled wrong. So you will have + to do that. Tell us which source line it is, and what incorrect + result happens when that line is executed. A person who + understands the program can find this as easily as finding a bug + in the program itself. + + * If you send examples of assembler code output from GNU CC or GNU + C++, please use `-g' when you make them. The debugging information + includes source line numbers which are essential for correlating + the output with the input. + + * If you wish to mention something in the GNU CC source, refer to it + by context, not by line number. + + The line numbers in the development sources don't match those in + your sources. Your line numbers would convey no useful + information to the maintainers. + + * Additional information from a debugger might enable someone to + find a problem on a machine which he does not have available. + However, you need to think when you collect this information if + you want it to have any chance of being useful. + + For example, many people send just a backtrace, but that is never + useful by itself. A simple backtrace with arguments conveys little + about GNU CC because the compiler is largely data-driven; the same + functions are called over and over for different RTL insns, doing + different things depending on the details of the insn. + + Most of the arguments listed in the backtrace are useless because + they are pointers to RTL list structure. The numeric values of the + pointers, which the debugger prints in the backtrace, have no + significance whatever; all that matters is the contents of the + objects they point to (and most of the contents are other such + pointers). + + In addition, most compiler passes consist of one or more loops that + scan the RTL insn sequence. The most vital piece of information + about such a loop--which insn it has reached--is usually in a + local variable, not in an argument. + + What you need to provide in addition to a backtrace are the values + of the local variables for several stack frames up. When a local + variable or an argument is an RTX, first print its value and then + use the GDB command `pr' to print the RTL expression that it points + to. (If GDB doesn't run on your machine, use your debugger to call + the function `debug_rtx' with the RTX as an argument.) In + general, whenever a variable is a pointer, its value is no use + without the data it points to. + + Here are some things that are not necessary: + + * A description of the envelope of the bug. + + Often people who encounter a bug spend a lot of time investigating + which changes to the input file will make the bug go away and which + changes will not affect it. + + This is often time consuming and not very useful, because the way + we will find the bug is by running a single example under the + debugger with breakpoints, not by pure deduction from a series of + examples. You might as well save your time for something else. + + Of course, if you can find a simpler example to report *instead* of + the original one, that is a convenience. Errors in the output + will be easier to spot, running under the debugger will take less + time, etc. Most GNU CC bugs involve just one function, so the + most straightforward way to simplify an example is to delete all + the function definitions except the one where the bug occurs. + Those earlier in the file may be replaced by external declarations + if the crucial function depends on them. (Exception: inline + functions may affect compilation of functions defined later in the + file.) + + However, simplification is not vital; if you don't want to do this, + report the bug anyway and send the entire test case you used. + + * In particular, some people insert conditionals `#ifdef BUG' around + a statement which, if removed, makes the bug not happen. These + are just clutter; we won't pay any attention to them anyway. + Besides, you should send us cpp output, and that can't have + conditionals. + + * A patch for the bug. + + A patch for the bug is useful if it is a good one. But don't omit + the necessary information, such as the test case, on the + assumption that a patch is all we need. We might see problems + with your patch and decide to fix the problem another way, or we + might not understand it at all. + + Sometimes with a program as complicated as GNU CC it is very hard + to construct an example that will make the program follow a + certain path through the code. If you don't send the example, we + won't be able to construct one, so we won't be able to verify that + the bug is fixed. + + And if we can't understand what bug you are trying to fix, or why + your patch should be an improvement, we won't install it. A test + case will help us to understand. + + *Note Sending Patches::, for guidelines on how to make it easy for + us to understand and install your patches. + + * A guess about what the bug is or what it depends on. + + Such guesses are usually wrong. Even I can't guess right about + such things without first using the debugger to find the facts. + + * A core dump file. + + We have no way of examining a core dump for your type of machine + unless we have an identical system--and if we do have one, we + should be able to reproduce the crash ourselves. + + +File: gcc.info, Node: Sending Patches, Prev: Bug Reporting, Up: Bugs + +Sending Patches for GNU CC +========================== -`(reg:M N)' - For small values of the integer N (those that are less than - `FIRST_PSEUDO_REGISTER'), this stands for a reference to machine - register number N: a "hard register". For larger values of N, it - stands for a temporary value or "pseudo register". The compiler's - strategy is to generate code assuming an unlimited number of such - pseudo registers, and later convert them into hard registers or - into memory references. - - M is the machine mode of the reference. It is necessary because - machines can generally refer to each register in more than one - mode. For example, a register may contain a full word but there - may be instructions to refer to it as a half word or as a single - byte, as well as instructions to refer to it as a floating point - number of various precisions. - - Even for a register that the machine can access in only one mode, - the mode must always be specified. - - The symbol `FIRST_PSEUDO_REGISTER' is defined by the machine - description, since the number of hard registers on the machine is - an invariant characteristic of the machine. Note, however, that - not all of the machine registers must be general registers. All - the machine registers that can be used for storage of data are - given hard register numbers, even those that can be used only in - certain instructions or can hold only certain types of data. - - A hard register may be accessed in various modes throughout one - function, but each pseudo register is given a natural mode and is - accessed only in that mode. When it is necessary to describe an - access to a pseudo register using a nonnatural mode, a `subreg' - expression is used. - - A `reg' expression with a machine mode that specifies more than - one word of data may actually stand for several consecutive - registers. If in addition the register number specifies a - hardware register, then it actually represents several consecutive - hardware registers starting with the specified one. - - Each pseudo register number used in a function's RTL code is - represented by a unique `reg' expression. - - Some pseudo register numbers, those within the range of - `FIRST_VIRTUAL_REGISTER' to `LAST_VIRTUAL_REGISTER' only appear - during the RTL generation phase and are eliminated before the - optimization phases. These represent locations in the stack frame - that cannot be determined until RTL generation for the function - has been completed. The following virtual register numbers are - defined: - - `VIRTUAL_INCOMING_ARGS_REGNUM' - This points to the first word of the incoming arguments - passed on the stack. Normally these arguments are placed - there by the caller, but the callee may have pushed some - arguments that were previously passed in registers. - - When RTL generation is complete, this virtual register is - replaced by the sum of the register given by - `ARG_POINTER_REGNUM' and the value of `FIRST_PARM_OFFSET'. - - `VIRTUAL_STACK_VARS_REGNUM' - If `FRAME_GROWS_DOWNWARD' is defined, this points to - immediately above the first variable on the stack. - Otherwise, it points to the first variable on the stack. - - `VIRTUAL_STACK_VARS_REGNUM' is replaced with the sum of the - register given by `FRAME_POINTER_REGNUM' and the value - `STARTING_FRAME_OFFSET'. - - `VIRTUAL_STACK_DYNAMIC_REGNUM' - This points to the location of dynamically allocated memory - on the stack immediately after the stack pointer has been - adjusted by the amount of memory desired. - - This virtual register is replaced by the sum of the register - given by `STACK_POINTER_REGNUM' and the value - `STACK_DYNAMIC_OFFSET'. - - `VIRTUAL_OUTGOING_ARGS_REGNUM' - This points to the location in the stack at which outgoing - arguments should be written when the stack is pre-pushed - (arguments pushed using push insns should always use - `STACK_POINTER_REGNUM'). - - This virtual register is replaced by the sum of the register - given by `STACK_POINTER_REGNUM' and the value - `STACK_POINTER_OFFSET'. - -`(subreg:M REG WORDNUM)' - `subreg' expressions are used to refer to a register in a machine - mode other than its natural one, or to refer to one register of a - multi-word `reg' that actually refers to several registers. - - Each pseudo-register has a natural mode. If it is necessary to - operate on it in a different mode--for example, to perform a - fullword move instruction on a pseudo-register that contains a - single byte--the pseudo-register must be enclosed in a `subreg'. - In such a case, WORDNUM is zero. - - Usually M is at least as narrow as the mode of REG, in which case - it is restricting consideration to only the bits of REG that are - in M. - - Sometimes M is wider than the mode of REG. These `subreg' - expressions are often called "paradoxical". They are used in - cases where we want to refer to an object in a wider mode but do - not care what value the additional bits have. The reload pass - ensures that paradoxical references are only made to hard - registers. - - The other use of `subreg' is to extract the individual registers of - a multi-register value. Machine modes such as `DImode' and - `TImode' can indicate values longer than a word, values which - usually require two or more consecutive registers. To access one - of the registers, use a `subreg' with mode `SImode' and a WORDNUM - that says which register. - - Storing in a non-paradoxical `subreg' has undefined results for - bits belonging to the same word as the `subreg'. This laxity makes - it easier to generate efficient code for such instructions. To - represent an instruction that preserves all the bits outside of - those in the `subreg', use `strict_low_part' around the `subreg'. - - The compilation parameter `WORDS_BIG_ENDIAN', if set to 1, says - that word number zero is the most significant part; otherwise, it - is the least significant part. - - Between the combiner pass and the reload pass, it is possible to - have a paradoxical `subreg' which contains a `mem' instead of a - `reg' as its first operand. After the reload pass, it is also - possible to have a non-paradoxical `subreg' which contains a - `mem'; this usually occurs when the `mem' is a stack slot which - replaced a pseudo register. - - Note that it is not valid to access a `DFmode' value in `SFmode' - using a `subreg'. On some machines the most significant part of a - `DFmode' value does not have the same format as a single-precision - floating value. - - It is also not valid to access a single word of a multi-word value - in a hard register when less registers can hold the value than - would be expected from its size. For example, some 32-bit - machines have floating-point registers that can hold an entire - `DFmode' value. If register 10 were such a register `(subreg:SI - (reg:DF 10) 1)' would be invalid because there is no way to - convert that reference to a single machine register. The reload - pass prevents `subreg' expressions such as these from being formed. - - The first operand of a `subreg' expression is customarily accessed - with the `SUBREG_REG' macro and the second operand is customarily - accessed with the `SUBREG_WORD' macro. - -`(scratch:M)' - This represents a scratch register that will be required for the - execution of a single instruction and not used subsequently. It is - converted into a `reg' by either the local register allocator or - the reload pass. - - `scratch' is usually present inside a `clobber' operation (*note - Side Effects::.). - -`(cc0)' - This refers to the machine's condition code register. It has no - operands and may not have a machine mode. There are two ways to - use it: - - * To stand for a complete set of condition code flags. This is - best on most machines, where each comparison sets the entire - series of flags. - - With this technique, `(cc0)' may be validly used in only two - contexts: as the destination of an assignment (in test and - compare instructions) and in comparison operators comparing - against zero (`const_int' with value zero; that is to say, - `const0_rtx'). - - * To stand for a single flag that is the result of a single - condition. This is useful on machines that have only a - single flag bit, and in which comparison instructions must - specify the condition to test. - - With this technique, `(cc0)' may be validly used in only two - contexts: as the destination of an assignment (in test and - compare instructions) where the source is a comparison - operator, and as the first operand of `if_then_else' (in a - conditional branch). - - There is only one expression object of code `cc0'; it is the value - of the variable `cc0_rtx'. Any attempt to create an expression of - code `cc0' will return `cc0_rtx'. - - Instructions can set the condition code implicitly. On many - machines, nearly all instructions set the condition code based on - the value that they compute or store. It is not necessary to - record these actions explicitly in the RTL because the machine - description includes a prescription for recognizing the - instructions that do so (by means of the macro - `NOTICE_UPDATE_CC'). *Note Condition Code::. Only instructions - whose sole purpose is to set the condition code, and instructions - that use the condition code, need mention `(cc0)'. - - On some machines, the condition code register is given a register - number and a `reg' is used instead of `(cc0)'. This is usually the - preferable approach if only a small subset of instructions modify - the condition code. Other machines store condition codes in - general registers; in such cases a pseudo register should be used. - - Some machines, such as the Sparc and RS/6000, have two sets of - arithmetic instructions, one that sets and one that does not set - the condition code. This is best handled by normally generating - the instruction that does not set the condition code, and making a - pattern that both performs the arithmetic and sets the condition - code register (which would not be `(cc0)' in this case). For - examples, search for `addcc' and `andcc' in `sparc.md'. - -`(pc)' - This represents the machine's program counter. It has no operands - and may not have a machine mode. `(pc)' may be validly used only - in certain specific contexts in jump instructions. - - There is only one expression object of code `pc'; it is the value - of the variable `pc_rtx'. Any attempt to create an expression of - code `pc' will return `pc_rtx'. - - All instructions that do not jump alter the program counter - implicitly by incrementing it, but there is no need to mention - this in the RTL. - -`(mem:M ADDR)' - This RTX represents a reference to main memory at an address - represented by the expression ADDR. M specifies how large a unit - of memory is accessed. - - -File: gcc.info, Node: Arithmetic, Next: Comparisons, Prev: Regs and Memory, Up: RTL - -RTL Expressions for Arithmetic -============================== - - Unless otherwise specified, all the operands of arithmetic -expressions must be valid for mode M. An operand is valid for mode M -if it has mode M, or if it is a `const_int' or `const_double' and M is -a mode of class `MODE_INT'. - - For commutative binary operations, constants should be placed in the -second operand. - -`(plus:M X Y)' - Represents the sum of the values represented by X and Y carried - out in machine mode M. - -`(lo_sum:M X Y)' - Like `plus', except that it represents that sum of X and the - low-order bits of Y. The number of low order bits is - machine-dependent but is normally the number of bits in a `Pmode' - item minus the number of bits set by the `high' code (*note - Constants::.). - - M should be `Pmode'. - -`(minus:M X Y)' - Like `plus' but represents subtraction. - -`(compare:M X Y)' - Represents the result of subtracting Y from X for purposes of - comparison. The result is computed without overflow, as if with - infinite precision. - - Of course, machines can't really subtract with infinite precision. - However, they can pretend to do so when only the sign of the - result will be used, which is the case when the result is stored - in the condition code. And that is the only way this kind of - expression may validly be used: as a value to be stored in the - condition codes. - - The mode M is not related to the modes of X and Y, but instead is - the mode of the condition code value. If `(cc0)' is used, it is - `VOIDmode'. Otherwise it is some mode in class `MODE_CC', often - `CCmode'. *Note Condition Code::. - - Normally, X and Y must have the same mode. Otherwise, `compare' - is valid only if the mode of X is in class `MODE_INT' and Y is a - `const_int' or `const_double' with mode `VOIDmode'. The mode of X - determines what mode the comparison is to be done in; thus it must - not be `VOIDmode'. - - If one of the operands is a constant, it should be placed in the - second operand and the comparison code adjusted as appropriate. - - A `compare' specifying two `VOIDmode' constants is not valid since - there is no way to know in what mode the comparison is to be - performed; the comparison must either be folded during the - compilation or the first operand must be loaded into a register - while its mode is still known. - -`(neg:M X)' - Represents the negation (subtraction from zero) of the value - represented by X, carried out in mode M. - -`(mult:M X Y)' - Represents the signed product of the values represented by X and Y - carried out in machine mode M. - - Some machines support a multiplication that generates a product - wider than the operands. Write the pattern for this as - - (mult:M (sign_extend:M X) (sign_extend:M Y)) - - where M is wider than the modes of X and Y, which need not be the - same. - - Write patterns for unsigned widening multiplication similarly using - `zero_extend'. - -`(div:M X Y)' - Represents the quotient in signed division of X by Y, carried out - in machine mode M. If M is a floating point mode, it represents - the exact quotient; otherwise, the integerized quotient. - - Some machines have division instructions in which the operands and - quotient widths are not all the same; you should represent such - instructions using `truncate' and `sign_extend' as in, - - (truncate:M1 (div:M2 X (sign_extend:M2 Y))) - -`(udiv:M X Y)' - Like `div' but represents unsigned division. - -`(mod:M X Y)' -`(umod:M X Y)' - Like `div' and `udiv' but represent the remainder instead of the - quotient. - -`(smin:M X Y)' -`(smax:M X Y)' - Represents the smaller (for `smin') or larger (for `smax') of X - and Y, interpreted as signed integers in mode M. - -`(umin:M X Y)' -`(umax:M X Y)' - Like `smin' and `smax', but the values are interpreted as unsigned - integers. - -`(not:M X)' - Represents the bitwise complement of the value represented by X, - carried out in mode M, which must be a fixed-point machine mode. - -`(and:M X Y)' - Represents the bitwise logical-and of the values represented by X - and Y, carried out in machine mode M, which must be a fixed-point - machine mode. - -`(ior:M X Y)' - Represents the bitwise inclusive-or of the values represented by X - and Y, carried out in machine mode M, which must be a fixed-point - mode. - -`(xor:M X Y)' - Represents the bitwise exclusive-or of the values represented by X - and Y, carried out in machine mode M, which must be a fixed-point - mode. - -`(ashift:M X C)' - Represents the result of arithmetically shifting X left by C - places. X have mode M, a fixed-point machine mode. C be a - fixed-point mode or be a constant with mode `VOIDmode'; which mode - is determined by the mode called for in the machine description - entry for the left-shift instruction. For example, on the Vax, - the mode of C is `QImode' regardless of M. - -`(lshift:M X C)' - Like `ashift' but for logical left shift. `ashift' and `lshift' - are identical operations; we customarily use `ashift' for both. - -`(lshiftrt:M X C)' -`(ashiftrt:M X C)' - Like `lshift' and `ashift' but for right shift. Unlike the case - for left shift, these two operations are distinct. - -`(rotate:M X C)' -`(rotatert:M X C)' - Similar but represent left and right rotate. If C is a constant, - use `rotate'. - -`(abs:M X)' - Represents the absolute value of X, computed in mode M. - -`(sqrt:M X)' - Represents the square root of X, computed in mode M. Most often M - will be a floating point mode. - -`(ffs:M X)' - Represents one plus the index of the least significant 1-bit in X, - represented as an integer of mode M. (The value is zero if X is - zero.) The mode of X need not be M; depending on the target - machine, various mode combinations may be valid. + If you would like to write bug fixes or improvements for the GNU C +compiler, that is very helpful. Send suggested fixes to the bug report +mailing list, `bug-gcc@prep.ai.mit.edu'. + + Please follow these guidelines so we can study your patches +efficiently. If you don't follow these guidelines, your information +might still be useful, but using it will take extra work. Maintaining +GNU C is a lot of work in the best of circumstances, and we can't keep +up unless you do your best to help. + + * Send an explanation with your changes of what problem they fix or + what improvement they bring about. For a bug fix, just include a + copy of the bug report, and explain why the change fixes the bug. + + (Referring to a bug report is not as good as including it, because + then we will have to look it up, and we have probably already + deleted it if we've already fixed the bug.) + + * Always include a proper bug report for the problem you think you + have fixed. We need to convince ourselves that the change is + right before installing it. Even if it is right, we might have + trouble judging it if we don't have a way to reproduce the problem. + + * Include all the comments that are appropriate to help people + reading the source in the future understand why this change was + needed. + + * Don't mix together changes made for different reasons. Send them + *individually*. + + If you make two changes for separate reasons, then we might not + want to install them both. We might want to install just one. If + you send them all jumbled together in a single set of diffs, we + have to do extra work to disentangle them--to figure out which + parts of the change serve which purpose. If we don't have time + for this, we might have to ignore your changes entirely. + + If you send each change as soon as you have written it, with its + own explanation, then the two changes never get tangled up, and we + can consider each one properly without any extra work to + disentangle them. + + Ideally, each change you send should be impossible to subdivide + into parts that we might want to consider separately, because each + of its parts gets its motivation from the other parts. + + * Send each change as soon as that change is finished. Sometimes + people think they are helping us by accumulating many changes to + send them all together. As explained above, this is absolutely + the worst thing you could do. + + Since you should send each change separately, you might as well + send it right away. That gives us the option of installing it + immediately if it is important. + + * Use `diff -c' to make your diffs. Diffs without context are hard + for us to install reliably. More than that, they make it hard for + us to study the diffs to decide whether we want to install them. + Unidiff format is better than contextless diffs, but not as easy + to read as `-c' format. + + If you have GNU diff, use `diff -cp', which shows the name of the + function that each change occurs in. + + * Write the change log entries for your changes. We get lots of + changes, and we don't have time to do all the change log writing + ourselves. + + Read the `ChangeLog' file to see what sorts of information to put + in, and to learn the style that we use. The purpose of the change + log is to show people where to find what was changed. So you need + to be specific about what functions you changed; in large + functions, it's often helpful to indicate where within the + function the change was. + + On the other hand, once you have shown people where to find the + change, you need not explain its purpose. Thus, if you add a new + function, all you need to say about it is that it is new. If you + feel that the purpose needs explaining, it probably does--but the + explanation will be much more useful if you put it in comments in + the code. + + If you would like your name to appear in the header line for who + made the change, send us the header line. + + * When you write the fix, keep in mind that we can't install a + change that would break other systems. + + People often suggest fixing a problem by changing + machine-independent files such as `toplev.c' to do something + special that a particular system needs. Sometimes it is totally + obvious that such changes would break GNU CC for almost all users. + We can't possibly make a change like that. At best it might tell + us how to write another patch that would solve the problem + acceptably. + + Sometimes people send fixes that *might* be an improvement in + general--but it is hard to be sure of this. It's hard to install + such changes because we have to study them very carefully. Of + course, a good explanation of the reasoning by which you concluded + the change was correct can help convince us. + + The safest changes are changes to the configuration files for a + particular machine. These are safe because they can't create new + bugs on other machines. + + Please help us keep up with the workload by designing the patch in + a form that is good to install. + + +File: gcc.info, Node: Service, Next: VMS, Prev: Bugs, Up: Top + +How To Get Help with GNU CC +*************************** + + If you need help installing, using or changing GNU CC, there are two +ways to find it: + + * Send a message to a suitable network mailing list. First try + `bug-gcc@prep.ai.mit.edu', and if that brings no response, try + `help-gcc@prep.ai.mit.edu'. + + * Look in the service directory for someone who might help you for a + fee. The service directory is found in the file named `SERVICE' + in the GNU CC distribution.  -File: gcc.info, Node: Comparisons, Next: Bit Fields, Prev: Arithmetic, Up: RTL +File: gcc.info, Node: VMS, Next: Portability, Prev: Service, Up: Top -Comparison Operations +Using GNU CC on VMS +******************* + + Here is how to use GNU CC on VMS. + +* Menu: + +* Include Files and VMS:: Where the preprocessor looks for the include files. +* Global Declarations:: How to do globaldef, globalref and globalvalue with + GNU CC. +* VMS Misc:: Misc information. + + +File: gcc.info, Node: Include Files and VMS, Next: Global Declarations, Up: VMS + +Include Files and VMS ===================== - Comparison operators test a relation on two operands and are -considered to represent a machine-dependent nonzero value described by, -but not necessarily equal to, `STORE_FLAG_VALUE' (*note Misc::.) if the -relation holds, or zero if it does not. The mode of the comparison -operation is independent of the mode of the data being compared. If -the comparison operation is being tested (e.g., the first operand of an -`if_then_else'), the mode must be `VOIDmode'. If the comparison -operation is producing data to be stored in some variable, the mode -must be in class `MODE_INT'. All comparison operations producing data -must use the same mode, which is machine-specific. - - There are two ways that comparison operations may be used. The -comparison operators may be used to compare the condition codes `(cc0)' -against zero, as in `(eq (cc0) (const_int 0))'. Such a construct -actually refers to the result of the preceding instruction in which the -condition codes were set. The instructing setting the condition code -must be adjacent to the instruction using the condition code; only -`note' insns may separate them. - - Alternatively, a comparison operation may directly compare two data -objects. The mode of the comparison is determined by the operands; they -must both be valid for a common machine mode. A comparison with both -operands constant would be invalid as the machine mode could not be -deduced from it, but such a comparison should never exist in RTL due to -constant folding. - - In the example above, if `(cc0)' were last set to `(compare X Y)', -the comparison operation is identical to `(eq X Y)'. Usually only one -style of comparisons is supported on a particular machine, but the -combine pass will try to merge the operations to produce the `eq' shown -in case it exists in the context of the particular insn involved. - - Inequality comparisons come in two flavors, signed and unsigned. -Thus, there are distinct expression codes `gt' and `gtu' for signed and -unsigned greater-than. These can produce different results for the same -pair of integer values: for example, 1 is signed greater-than -1 but not -unsigned greater-than, because -1 when regarded as unsigned is actually -`0xffffffff' which is greater than 1. - - The signed comparisons are also used for floating point values. -Floating point comparisons are distinguished by the machine modes of -the operands. - -`(eq:M X Y)' - 1 if the values represented by X and Y are equal, otherwise 0. - -`(ne:M X Y)' - 1 if the values represented by X and Y are not equal, otherwise 0. - -`(gt:M X Y)' - 1 if the X is greater than Y. If they are fixed-point, the - comparison is done in a signed sense. - -`(gtu:M X Y)' - Like `gt' but does unsigned comparison, on fixed-point numbers - only. - -`(lt:M X Y)' -`(ltu:M X Y)' - Like `gt' and `gtu' but test for "less than". - -`(ge:M X Y)' -`(geu:M X Y)' - Like `gt' and `gtu' but test for "greater than or equal". - -`(le:M X Y)' -`(leu:M X Y)' - Like `gt' and `gtu' but test for "less than or equal". - -`(if_then_else COND THEN ELSE)' - This is not a comparison operation but is listed here because it is - always used in conjunction with a comparison operation. To be - precise, COND is a comparison expression. This expression - represents a choice, according to COND, between the value - represented by THEN and the one represented by ELSE. - - On most machines, `if_then_else' expressions are valid only to - express conditional jumps. - -`(cond [TEST1 VALUE1 TEST2 VALUE2 ...] DEFAULT)' - Similar to `if_then_else', but more general. Each of TEST1, - TEST2, ... is performed in turn. The result of this expression is - the VALUE corresponding to the first non-zero test, or DEFAULT if - none of the tests are non-zero expressions. - - This is currently not valid for instruction patterns and is - supported only for insn attributes. *Note Insn Attributes::. - - -File: gcc.info, Node: Bit Fields, Next: Conversions, Prev: Comparisons, Up: RTL - -Bit Fields -========== - - Special expression codes exist to represent bitfield instructions. -These types of expressions are lvalues in RTL; they may appear on the -left side of an assignment, indicating insertion of a value into the -specified bit field. - -`(sign_extract:M LOC SIZE POS)' - This represents a reference to a sign-extended bit field contained - or starting in LOC (a memory or register reference). The bit field - is SIZE bits wide and starts at bit POS. The compilation option - `BITS_BIG_ENDIAN' says which end of the memory unit POS counts - from. - - If LOC is in memory, its mode must be a single-byte integer mode. - If LOC is in a register, the mode to use is specified by the - operand of the `insv' or `extv' pattern (*note Standard Names::.) - and is usually a full-word integer mode. - - The mode of POS is machine-specific and is also specified in the - `insv' or `extv' pattern. - - The mode M is the same as the mode that would be used for LOC if - it were a register. - -`(zero_extract:M LOC SIZE POS)' - Like `sign_extract' but refers to an unsigned or zero-extended bit - field. The same sequence of bits are extracted, but they are - filled to an entire word with zeros instead of by sign-extension. - - -File: gcc.info, Node: Conversions, Next: RTL Declarations, Prev: Bit Fields, Up: RTL - -Conversions -=========== - - All conversions between machine modes must be represented by -explicit conversion operations. For example, an expression which is -the sum of a byte and a full word cannot be written as `(plus:SI -(reg:QI 34) (reg:SI 80))' because the `plus' operation requires two -operands of the same machine mode. Therefore, the byte-sized operand -is enclosed in a conversion operation, as in - - (plus:SI (sign_extend:SI (reg:QI 34)) (reg:SI 80)) - - The conversion operation is not a mere placeholder, because there -may be more than one way of converting from a given starting mode to -the desired final mode. The conversion operation code says how to do -it. - - For all conversion operations, X must not be `VOIDmode' because the -mode in which to do the conversion would not be known. The conversion -must either be done at compile-time or X must be placed into a register. - -`(sign_extend:M X)' - Represents the result of sign-extending the value X to machine - mode M. M must be a fixed-point mode and X a fixed-point value of - a mode narrower than M. - -`(zero_extend:M X)' - Represents the result of zero-extending the value X to machine - mode M. M must be a fixed-point mode and X a fixed-point value of - a mode narrower than M. - -`(float_extend:M X)' - Represents the result of extending the value X to machine mode M. - m must be a floating point mode and X a floating point value of a - mode narrower than M. - -`(truncate:M X)' - Represents the result of truncating the value X to machine mode M. - M must be a fixed-point mode and X a fixed-point value of a mode - wider than M. - -`(float_truncate:M X)' - Represents the result of truncating the value X to machine mode M. - M must be a floating point mode and X a floating point value of a - mode wider than M. - -`(float:M X)' - Represents the result of converting fixed point value X, regarded - as signed, to floating point mode M. - -`(unsigned_float:M X)' - Represents the result of converting fixed point value X, regarded - as unsigned, to floating point mode M. - -`(fix:M X)' - When M is a fixed point mode, represents the result of converting - floating point value X to mode M, regarded as signed. How - rounding is done is not specified, so this operation may be used - validly in compiling C code only for integer-valued operands. - -`(unsigned_fix:M X)' - Represents the result of converting floating point value X to - fixed point mode M, regarded as unsigned. How rounding is done is - not specified. - -`(fix:M X)' - When M is a floating point mode, represents the result of - converting floating point value X (valid for mode M) to an - integer, still represented in floating point mode M, by rounding - towards zero. - - -File: gcc.info, Node: RTL Declarations, Next: Side Effects, Prev: Conversions, Up: RTL - -Declarations -============ - - Declaration expression codes do not represent arithmetic operations -but rather state assertions about their operands. - -`(strict_low_part (subreg:M (reg:N R) 0))' - This expression code is used in only one context: as the - destination operand of a `set' expression. In addition, the - operand of this expression must be a non-paradoxical `subreg' - expression. - - The presence of `strict_low_part' says that the part of the - register which is meaningful in mode N, but is not part of mode M, - is not to be altered. Normally, an assignment to such a subreg is - allowed to have undefined effects on the rest of the register when - M is less than a word. - - -File: gcc.info, Node: Side Effects, Next: Incdec, Prev: RTL Declarations, Up: RTL - -Side Effect Expressions -======================= - - The expression codes described so far represent values, not actions. -But machine instructions never produce values; they are meaningful only -for their side effects on the state of the machine. Special expression -codes are used to represent side effects. - - The body of an instruction is always one of these side effect codes; -the codes described above, which represent values, appear only as the -operands of these. - -`(set LVAL X)' - Represents the action of storing the value of X into the place - represented by LVAL. LVAL must be an expression representing a - place that can be stored in: `reg' (or `subreg' or - `strict_low_part'), `mem', `pc' or `cc0'. - - If LVAL is a `reg', `subreg' or `mem', it has a machine mode; then - X must be valid for that mode. - - If LVAL is a `reg' whose machine mode is less than the full width - of the register, then it means that the part of the register - specified by the machine mode is given the specified value and the - rest of the register receives an undefined value. Likewise, if - LVAL is a `subreg' whose machine mode is narrower than the mode of - the register, the rest of the register can be changed in an - undefined way. - - If LVAL is a `strict_low_part' of a `subreg', then the part of the - register specified by the machine mode of the `subreg' is given - the value X and the rest of the register is not changed. - - If LVAL is `(cc0)', it has no machine mode, and X may be either a - `compare' expression or a value that may have any mode. The - latter case represents a "test" instruction. The expression `(set - (cc0) (reg:M N))' is equivalent to `(set (cc0) (compare (reg:M N) - (const_int 0)))'. Use the former expression to save space during - the compilation. - - If LVAL is `(pc)', we have a jump instruction, and the - possibilities for X are very limited. It may be a `label_ref' - expression (unconditional jump). It may be an `if_then_else' - (conditional jump), in which case either the second or the third - operand must be `(pc)' (for the case which does not jump) and the - other of the two must be a `label_ref' (for the case which does - jump). X may also be a `mem' or `(plus:SI (pc) Y)', where Y may - be a `reg' or a `mem'; these unusual patterns are used to - represent jumps through branch tables. - - If LVAL is neither `(cc0)' nor `(pc)', the mode of LVAL must not - be `VOIDmode' and the mode of X must be valid for the mode of LVAL. - - LVAL is customarily accessed with the `SET_DEST' macro and X with - the `SET_SRC' macro. - -`(return)' - As the sole expression in a pattern, represents a return from the - current function, on machines where this can be done with one - instruction, such as Vaxes. On machines where a multi-instruction - "epilogue" must be executed in order to return from the function, - returning is done by jumping to a label which precedes the - epilogue, and the `return' expression code is never used. - - Inside an `if_then_else' expression, represents the value to be - placed in `pc' to return to the caller. - - Note that an insn pattern of `(return)' is logically equivalent to - `(set (pc) (return))', but the latter form is never used. - -`(call FUNCTION NARGS)' - Represents a function call. FUNCTION is a `mem' expression whose - address is the address of the function to be called. NARGS is an - expression which can be used for two purposes: on some machines it - represents the number of bytes of stack argument; on others, it - represents the number of argument registers. - - Each machine has a standard machine mode which FUNCTION must have. - The machine description defines macro `FUNCTION_MODE' to expand - into the requisite mode name. The purpose of this mode is to - specify what kind of addressing is allowed, on machines where the - allowed kinds of addressing depend on the machine mode being - addressed. - -`(clobber X)' - Represents the storing or possible storing of an unpredictable, - undescribed value into X, which must be a `reg', `scratch' or - `mem' expression. - - One place this is used is in string instructions that store - standard values into particular hard registers. It may not be - worth the trouble to describe the values that are stored, but it - is essential to inform the compiler that the registers will be - altered, lest it attempt to keep data in them across the string - instruction. - - If X is `(mem:BLK (const_int 0))', it means that all memory - locations must be presumed clobbered. - - Note that the machine description classifies certain hard - registers as "call-clobbered". All function call instructions are - assumed by default to clobber these registers, so there is no need - to use `clobber' expressions to indicate this fact. Also, each - function call is assumed to have the potential to alter any memory - location, unless the function is declared `const'. - - If the last group of expressions in a `parallel' are each a - `clobber' expression whose arguments are `reg' or `match_scratch' - (*note RTL Template::.) expressions, the combiner phase can add - the appropriate `clobber' expressions to an insn it has - constructed when doing so will cause a pattern to be matched. - - This feature can be used, for example, on a machine that whose - multiply and add instructions don't use an MQ register but which - has an add-accumulate instruction that does clobber the MQ - register. Similarly, a combined instruction might require a - temporary register while the constituent instructions might not. - - When a `clobber' expression for a register appears inside a - `parallel' with other side effects, the register allocator - guarantees that the register is unoccupied both before and after - that insn. However, the reload phase may allocate a register used - for one of the inputs unless the `&' constraint is specified for - the selected alternative (*note Modifiers::.). You can clobber - either a specific hard register, a pseudo register, or a `scratch' - expression; in the latter two cases, GNU CC will allocate a hard - register that is available there for use as a temporary. - - For instructions that require a temporary register, you should use - `scratch' instead of a pseudo-register because this will allow the - combiner phase to add the `clobber' when required. You do this by - coding (`clobber' (`match_scratch' ...)). If you do clobber a - pseudo register, use one which appears nowhere else--generate a - new one each time. Otherwise, you may confuse CSE. - - There is one other known use for clobbering a pseudo register in a - `parallel': when one of the input operands of the insn is also - clobbered by the insn. In this case, using the same pseudo - register in the clobber and elsewhere in the insn produces the - expected results. - -`(use X)' - Represents the use of the value of X. It indicates that the value - in X at this point in the program is needed, even though it may - not be apparent why this is so. Therefore, the compiler will not - attempt to delete previous instructions whose only effect is to - store a value in X. X must be a `reg' expression. - - During the delayed branch scheduling phase, X may be an insn. - This indicates that X previously was located at this place in the - code and its data dependencies need to be taken into account. - These `use' insns will be deleted before the delayed branch - scheduling phase exits. - -`(parallel [X0 X1 ...])' - Represents several side effects performed in parallel. The square - brackets stand for a vector; the operand of `parallel' is a vector - of expressions. X0, X1 and so on are individual side effect - expressions--expressions of code `set', `call', `return', - `clobber' or `use'. - - "In parallel" means that first all the values used in the - individual side-effects are computed, and second all the actual - side-effects are performed. For example, - - (parallel [(set (reg:SI 1) (mem:SI (reg:SI 1))) - (set (mem:SI (reg:SI 1)) (reg:SI 1))]) - - says unambiguously that the values of hard register 1 and the - memory location addressed by it are interchanged. In both places - where `(reg:SI 1)' appears as a memory address it refers to the - value in register 1 *before* the execution of the insn. - - It follows that it is *incorrect* to use `parallel' and expect the - result of one `set' to be available for the next one. For - example, people sometimes attempt to represent a jump-if-zero - instruction this way: - - (parallel [(set (cc0) (reg:SI 34)) - (set (pc) (if_then_else - (eq (cc0) (const_int 0)) - (label_ref ...) - (pc)))]) - - But this is incorrect, because it says that the jump condition - depends on the condition code value *before* this instruction, not - on the new value that is set by this instruction. - - Peephole optimization, which takes place together with final - assembly code output, can produce insns whose patterns consist of - a `parallel' whose elements are the operands needed to output the - resulting assembler code--often `reg', `mem' or constant - expressions. This would not be well-formed RTL at any other stage - in compilation, but it is ok then because no further optimization - remains to be done. However, the definition of the macro - `NOTICE_UPDATE_CC', if any, must deal with such insns if you - define any peephole optimizations. - -`(sequence [INSNS ...])' - Represents a sequence of insns. Each of the INSNS that appears in - the vector is suitable for appearing in the chain of insns, so it - must be an `insn', `jump_insn', `call_insn', `code_label', - `barrier' or `note'. - - A `sequence' RTX is never placed in an actual insn during RTL - generation. It represents the sequence of insns that result from a - `define_expand' *before* those insns are passed to `emit_insn' to - insert them in the chain of insns. When actually inserted, the - individual sub-insns are separated out and the `sequence' is - forgotten. - - After delay-slot scheduling is completed, an insn and all the - insns that reside in its delay slots are grouped together into a - `sequence'. The insn requiring the delay slot is the first insn - in the vector; subsequent insns are to be placed in the delay slot. - - `INSN_ANNULLED_BRANCH_P' is set on an insn in a delay slot to - indicate that a branch insn should be used that will conditionally - annul the effect of the insns in the delay slots. In such a case, - `INSN_FROM_TARGET_P' indicates that the insn is from the target of - the branch and should be executed only if the branch is taken; - otherwise the insn should be executed only if the branch is not - taken. *Note Delay Slots::. - - These expression codes appear in place of a side effect, as the body -of an insn, though strictly speaking they do not always describe side -effects as such: - -`(asm_input S)' - Represents literal assembler code as described by the string S. - -`(unspec [OPERANDS ...] INDEX)' -`(unspec_volatile [OPERANDS ...] INDEX)' - Represents a machine-specific operation on OPERANDS. INDEX - selects between multiple machine-specific operations. - `unspec_volatile' is used for volatile operations and operations - that may trap; `unspec' is used for other operations. - - These codes may appear inside a `pattern' of an insn, inside a - `parallel', or inside an expression. - -`(addr_vec:M [LR0 LR1 ...])' - Represents a table of jump addresses. The vector elements LR0, - etc., are `label_ref' expressions. The mode M specifies how much - space is given to each address; normally M would be `Pmode'. - -`(addr_diff_vec:M BASE [LR0 LR1 ...])' - Represents a table of jump addresses expressed as offsets from - BASE. The vector elements LR0, etc., are `label_ref' expressions - and so is BASE. The mode M specifies how much space is given to - each address-difference. - - -File: gcc.info, Node: Incdec, Next: Assembler, Prev: Side Effects, Up: RTL - -Embedded Side-Effects on Addresses -================================== - - Four special side-effect expression codes appear as memory addresses. - -`(pre_dec:M X)' - Represents the side effect of decrementing X by a standard amount - and represents also the value that X has after being decremented. - x must be a `reg' or `mem', but most machines allow only a `reg'. - m must be the machine mode for pointers on the machine in use. - The amount X is decremented by is the length in bytes of the - machine mode of the containing memory reference of which this - expression serves as the address. Here is an example of its use: - - (mem:DF (pre_dec:SI (reg:SI 39))) - - This says to decrement pseudo register 39 by the length of a - `DFmode' value and use the result to address a `DFmode' value. - -`(pre_inc:M X)' - Similar, but specifies incrementing X instead of decrementing it. - -`(post_dec:M X)' - Represents the same side effect as `pre_dec' but a different - value. The value represented here is the value X has before being - decremented. - -`(post_inc:M X)' - Similar, but specifies incrementing X instead of decrementing it. - - These embedded side effect expressions must be used with care. -Instruction patterns may not use them. Until the `flow' pass of the -compiler, they may occur only to represent pushes onto the stack. The -`flow' pass finds cases where registers are incremented or decremented -in one instruction and used as an address shortly before or after; -these cases are then transformed to use pre- or post-increment or --decrement. - - If a register used as the operand of these expressions is used in -another address in an insn, the original value of the register is used. -Uses of the register outside of an address are not permitted within the -same insn as a use in an embedded side effect expression because such -insns behave differently on different machines and hence must be treated -as ambiguous and disallowed. - - An instruction that can be represented with an embedded side effect -could also be represented using `parallel' containing an additional -`set' to describe how the address register is altered. This is not -done because machines that allow these operations at all typically -allow them wherever a memory address is called for. Describing them as -additional parallel stores would require doubling the number of entries -in the machine description. + Due to the differences between the filesystems of Unix and VMS, GNU +CC attempts to translate file names in `#include' into names that VMS +will understand. The basic strategy is to prepend a prefix to the +specification of the include file, convert the whole filename to a VMS +filename, and then try to open the file. GNU CC tries various prefixes +one by one until one of them succeeds: + + 1. The first prefix is the `GNU_CC_INCLUDE:' logical name: this is + where GNU C header files are traditionally stored. If you wish to + store header files in non-standard locations, then you can assign + the logical `GNU_CC_INCLUDE' to be a search list, where each + element of the list is suitable for use with a rooted logical. + + 2. The next prefix tried is `SYS$SYSROOT:[SYSLIB.]'. This is where + VAX-C header files are traditionally stored. + + 3. If the include file specification by itself is a valid VMS + filename, the preprocessor then uses this name with no prefix in + an attempt to open the include file. + + 4. If the file specification is not a valid VMS filename (i.e. does + not contain a device or a directory specifier, and contains a `/' + character), the preprocessor tries to convert it from Unix syntax + to VMS syntax. + + Conversion works like this: the first directory name becomes a + device, and the rest of the directories are converted into + VMS-format directory names. For example, the name `X11/foobar.h' + is translated to `X11:[000000]foobar.h' or `X11:foobar.h', + whichever one can be opened. This strategy allows you to assign a + logical name to point to the actual location of the header files. + + 5. If none of these strategies succeeds, the `#include' fails. + + Include directives of the form: + + #include foobar + +are a common source of incompatibility between VAX-C and GNU CC. VAX-C +treats this much like a standard `#include ' directive. That +is incompatible with the ANSI C behavior implemented by GNU CC: to +expand the name `foobar' as a macro. Macro expansion should eventually +yield one of the two standard formats for `#include': + + #include "FILE" + #include + + If you have this problem, the best solution is to modify the source +to convert the `#include' directives to one of the two standard forms. +That will work with either compiler. If you want a quick and dirty fix, +define the file names as macros with the proper expansion, like this: + + #define stdio + +This will work, as long as the name doesn't conflict with anything else +in the program. + + Another source of incompatibility is that VAX-C assumes that: + + #include "foobar" + +is actually asking for the file `foobar.h'. GNU CC does not make this +assumption, and instead takes what you ask for literally; it tries to +read the file `foobar'. The best way to avoid this problem is to +always specify the desired file extension in your include directives. + + GNU CC for VMS is distributed with a set of include files that is +sufficient to compile most general purpose programs. Even though the +GNU CC distribution does not contain header files to define constants +and structures for some VMS system-specific functions, there is no +reason why you cannot use GNU CC with any of these functions. You first +may have to generate or create header files, either by using the public +domain utility `UNSDL' (which can be found on a DECUS tape), or by +extracting the relevant modules from one of the system macro libraries, +and using an editor to construct a C header file. + + A `#include' file name cannot contain a DECNET node name. The +preprocessor reports an I/O error if you attempt to use a node name, +whether explicitly, or implicitly via a logical name.  -File: gcc.info, Node: Assembler, Next: Insns, Prev: Incdec, Up: RTL +File: gcc.info, Node: Global Declarations, Next: VMS Misc, Prev: Include Files and VMS, Up: VMS -Assembler Instructions as Expressions -===================================== +Global Declarations and VMS +=========================== - The RTX code `asm_operands' represents a value produced by a -user-specified assembler instruction. It is used to represent an `asm' -statement with arguments. An `asm' statement with a single output -operand, like this: - - asm ("foo %1,%2,%0" : "=a" (outputvar) : "g" (x + y), "di" (*z)); - -is represented using a single `asm_operands' RTX which represents the -value that is stored in `outputvar': - - (set RTX-FOR-OUTPUTVAR - (asm_operands "foo %1,%2,%0" "a" 0 - [RTX-FOR-ADDITION-RESULT RTX-FOR-*Z] - [(asm_input:M1 "g") - (asm_input:M2 "di")])) - -Here the operands of the `asm_operands' RTX are the assembler template -string, the output-operand's constraint, the index-number of the output -operand among the output operands specified, a vector of input operand -RTX's, and a vector of input-operand modes and constraints. The mode -M1 is the mode of the sum `x+y'; M2 is that of `*z'. - - When an `asm' statement has multiple output values, its insn has -several such `set' RTX's inside of a `parallel'. Each `set' contains a -`asm_operands'; all of these share the same assembler template and -vectors, but each contains the constraint for the respective output -operand. They are also distinguished by the output-operand index -number, which is 0, 1, ... for successive output operands. + GNU CC does not provide the `globalref', `globaldef' and +`globalvalue' keywords of VAX-C. You can get the same effect with an +obscure feature of GAS, the GNU assembler. (This requires GAS version +1.39 or later.) The following macros allow you to use this feature in +a fairly natural way: + + #ifdef __GNUC__ + #define GLOBALREF(TYPE,NAME) \ + TYPE NAME \ + asm ("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME) + #define GLOBALDEF(TYPE,NAME,VALUE) \ + TYPE NAME \ + asm ("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME) \ + = VALUE + #define GLOBALVALUEREF(TYPE,NAME) \ + const TYPE NAME[1] \ + asm ("_$$PsectAttributes_GLOBALVALUE$$" #NAME) + #define GLOBALVALUEDEF(TYPE,NAME,VALUE) \ + const TYPE NAME[1] \ + asm ("_$$PsectAttributes_GLOBALVALUE$$" #NAME) \ + = {VALUE} + #else + #define GLOBALREF(TYPE,NAME) \ + globalref TYPE NAME + #define GLOBALDEF(TYPE,NAME,VALUE) \ + globaldef TYPE NAME = VALUE + #define GLOBALVALUEDEF(TYPE,NAME,VALUE) \ + globalvalue TYPE NAME = VALUE + #define GLOBALVALUEREF(TYPE,NAME) \ + globalvalue TYPE NAME + #endif + +(The `_$$PsectAttributes_GLOBALSYMBOL' prefix at the start of the name +is removed by the assembler, after it has modified the attributes of +the symbol). These macros are provided in the VMS binaries +distribution in a header file `GNU_HACKS.H'. An example of the usage +is: + + GLOBALREF (int, ijk); + GLOBALDEF (int, jkl, 0); + + The macros `GLOBALREF' and `GLOBALDEF' cannot be used +straightforwardly for arrays, since there is no way to insert the array +dimension into the declaration at the right place. However, you can +declare an array with these macros if you first define a typedef for the +array type, like this: + + typedef int intvector[10]; + GLOBALREF (intvector, foo); + + Array and structure initializers will also break the macros; you can +define the initializer to be a macro of its own, or you can expand the +`GLOBALDEF' macro by hand. You may find a case where you wish to use +the `GLOBALDEF' macro with a large array, but you are not interested in +explicitly initializing each element of the array. In such cases you +can use an initializer like: `{0,}', which will initialize the entire +array to `0'. + + A shortcoming of this implementation is that a variable declared with +`GLOBALVALUEREF' or `GLOBALVALUEDEF' is always an array. For example, +the declaration: + + GLOBALVALUEREF(int, ijk); + +declares the variable `ijk' as an array of type `int [1]'. This is +done because a globalvalue is actually a constant; its "value" is what +the linker would normally consider an address. That is not how an +integer value works in C, but it is how an array works. So treating +the symbol as an array name gives consistent results--with the +exception that the value seems to have the wrong type. *Don't try to +access an element of the array.* It doesn't have any elements. The +array "address" may not be the address of actual storage. + + The fact that the symbol is an array may lead to warnings where the +variable is used. Insert type casts to avoid the warnings. Here is an +example; it takes advantage of the ANSI C feature allowing macros that +expand to use the same name as the macro itself. + + GLOBALVALUEREF (int, ss$_normal); + GLOBALVALUEDEF (int, xyzzy,123); + #ifdef __GNUC__ + #define ss$_normal ((int) ss$_normal) + #define xyzzy ((int) xyzzy) + #endif + + Don't use `globaldef' or `globalref' with a variable whose type is +an enumeration type; this is not implemented. Instead, make the +variable an integer, and use a `globalvaluedef' for each of the +enumeration values. An example of this would be: + + #ifdef __GNUC__ + GLOBALDEF (int, color, 0); + GLOBALVALUEDEF (int, RED, 0); + GLOBALVALUEDEF (int, BLUE, 1); + GLOBALVALUEDEF (int, GREEN, 3); + #else + enum globaldef color {RED, BLUE, GREEN = 3}; + #endif