--- gcc/gcc.info-10 2018/04/24 18:00:53 1.1.1.4 +++ gcc/gcc.info-10 2018/04/24 18:17:56 1.1.1.7 @@ -1,9 +1,13 @@ -This is Info file gcc.info, produced by Makeinfo-1.49 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. - Copyright (C) 1988, 1989, 1992 Free Software Foundation, Inc. + Published by the Free Software Foundation 675 Massachusetts Avenue +Cambridge, MA 02139 USA + + Copyright (C) 1988, 1989, 1992, 1993, 1994 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 @@ -11,1071 +15,945 @@ 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: Conversions, Next: RTL Declarations, Prev: Bit Fields, Up: RTL +File: gcc.info, Node: External Bugs, Next: Incompatibilities, Prev: Interoperation, Up: Trouble -Conversions -=========== +Problems Compiling Certain Programs +=================================== - 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. + Certain programs have problems compiling. - -File: gcc.info, Node: RTL Declarations, Next: Side Effects, Prev: Conversions, Up: RTL + * Parse errors may occur compiling X11 on a Decstation running + Ultrix 4.2 because of problems in DEC's versions of the X11 header + files `X11/Xlib.h' and `X11/Xutil.h'. People recommend adding + `-I/usr/include/mit' to use the MIT versions of the header files, + using the `-traditional' switch to turn off ANSI C, or fixing the + header files by adding this: + + #ifdef __STDC__ + #define NeedFunctionPrototypes 0 + #endif + + * If you have trouble compiling Perl on a SunOS 4 system, it may be + because Perl specifies `-I/usr/ucbinclude'. This accesses the + unfixed header files. Perl specifies the options + + -traditional -Dvolatile=__volatile__ + -I/usr/include/sun -I/usr/ucbinclude + -fpcc-struct-return + + most of which are unnecessary with GCC 2.4.5 and newer versions. + You can make a properly working Perl by setting `ccflags' to + `-fwritable-strings' (implied by the `-traditional' in the + original options) and `cppflags' to empty in `config.sh', then + typing `./doSH; make depend; make'. + + * On various 386 Unix systems derived from System V, including SCO, + ISC, and ESIX, you may get error messages about running out of + virtual memory while compiling certain programs. + + You can prevent this problem by linking GNU CC with the GNU malloc + (which thus replaces the malloc that comes with the system). GNU + malloc is available as a separate package, and also in the file + `src/gmalloc.c' in the GNU Emacs 19 distribution. + + If you have installed GNU malloc as a separate library package, + use this option when you relink GNU CC: + + MALLOC=/usr/local/lib/libgmalloc.a + + Alternatively, if you have compiled `gmalloc.c' from Emacs 19, copy + the object file to `gmalloc.o' and use this option when you relink + GNU CC: -Declarations -============ + MALLOC=gmalloc.o - Declaration expression codes do not represent arithmetic operations -but rather state assertions about their operands. + +File: gcc.info, Node: Incompatibilities, Next: Fixed Headers, Prev: External Bugs, Up: Trouble + +Incompatibilities of GNU CC +=========================== -`(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. + There are several noteworthy incompatibilities between GNU C and most +existing (non-ANSI) versions of C. The `-traditional' option +eliminates many of these incompatibilities, *but not all*, by telling +GNU C to behave like the other C compilers. + + * GNU CC normally makes string constants read-only. If several + identical-looking string constants are used, GNU CC stores only one + copy of the string. + + One consequence is that you cannot call `mktemp' with a string + constant argument. The function `mktemp' always alters the string + its argument points to. + + Another consequence is that `sscanf' does not work on some systems + when passed a string constant as its format control string or + input. This is because `sscanf' incorrectly tries to write into + the string constant. Likewise `fscanf' and `scanf'. + + The best solution to these problems is to change the program to use + `char'-array variables with initialization strings for these + purposes instead of string constants. But if this is not possible, + you can use the `-fwritable-strings' flag, which directs GNU CC to + handle string constants the same way most C compilers do. + `-traditional' also has this effect, among others. + + * `-2147483648' is positive. + + This is because 2147483648 cannot fit in the type `int', so + (following the ANSI C rules) its data type is `unsigned long int'. + Negating this value yields 2147483648 again. + + * GNU CC does not substitute macro arguments when they appear inside + of string constants. For example, the following macro in GNU CC + + #define foo(a) "a" + + will produce output `"a"' regardless of what the argument A is. + + The `-traditional' option directs GNU CC to handle such cases + (among others) in the old-fashioned (non-ANSI) fashion. + + * When you use `setjmp' and `longjmp', the only automatic variables + guaranteed to remain valid are those declared `volatile'. This is + a consequence of automatic register allocation. Consider this + function: + + jmp_buf j; + + foo () + { + int a, b; + + a = fun1 (); + if (setjmp (j)) + return a; + + a = fun2 (); + /* `longjmp (j)' may occur in `fun3'. */ + return a + fun3 (); + } + + Here `a' may or may not be restored to its first value when the + `longjmp' occurs. If `a' is allocated in a register, then its + first value is restored; otherwise, it keeps the last value stored + in it. + + If you use the `-W' option with the `-O' option, you will get a + warning when GNU CC thinks such a problem might be possible. + + The `-traditional' option directs GNU C to put variables in the + stack by default, rather than in registers, in functions that call + `setjmp'. This results in the behavior found in traditional C + compilers. + + * Programs that use preprocessor directives in the middle of macro + arguments do not work with GNU CC. For example, a program like + this will not work: + + foobar ( + #define luser + hack) + + ANSI C does not permit such a construct. It would make sense to + support it when `-traditional' is used, but it is too much work to + implement. + + * Declarations of external variables and functions within a block + apply only to the block containing the declaration. In other + words, they have the same scope as any other declaration in the + same place. + + In some other C compilers, a `extern' declaration affects all the + rest of the file even if it happens within a block. + + The `-traditional' option directs GNU C to treat all `extern' + declarations as global, like traditional compilers. + + * In traditional C, you can combine `long', etc., with a typedef + name, as shown here: + + typedef int foo; + typedef long foo bar; + + In ANSI C, this is not allowed: `long' and other type modifiers + require an explicit `int'. Because this criterion is expressed by + Bison grammar rules rather than C code, the `-traditional' flag + cannot alter it. + + * PCC allows typedef names to be used as function parameters. The + difficulty described immediately above applies here too. + + * PCC allows whitespace in the middle of compound assignment + operators such as `+='. GNU CC, following the ANSI standard, does + not allow this. The difficulty described immediately above + applies here too. + + * GNU CC complains about unterminated character constants inside of + preprocessor conditionals that fail. Some programs have English + comments enclosed in conditionals that are guaranteed to fail; if + these comments contain apostrophes, GNU CC will probably report an + error. For example, this code would produce an error: + + #if 0 + You can't expect this to work. + #endif + + The best solution to such a problem is to put the text into an + actual C comment delimited by `/*...*/'. However, `-traditional' + suppresses these error messages. + + * Many user programs contain the declaration `long time ();'. In the + past, the system header files on many systems did not actually + declare `time', so it did not matter what type your program + declared it to return. But in systems with ANSI C headers, `time' + is declared to return `time_t', and if that is not the same as + `long', then `long time ();' is erroneous. + + The solution is to change your program to use `time_t' as the + return type of `time'. + + * When compiling functions that return `float', PCC converts it to a + double. GNU CC actually returns a `float'. If you are concerned + with PCC compatibility, you should declare your functions to return + `double'; you might as well say what you mean. + + * When compiling functions that return structures or unions, GNU CC + output code normally uses a method different from that used on most + versions of Unix. As a result, code compiled with GNU CC cannot + call a structure-returning function compiled with PCC, and vice + versa. + + The method used by GNU CC is as follows: a structure or union + which is 1, 2, 4 or 8 bytes long is returned like a scalar. A + structure or union with any other size is stored into an address + supplied by the caller (usually in a special, fixed register, but + on some machines it is passed on the stack). The + machine-description macros `STRUCT_VALUE' and + `STRUCT_INCOMING_VALUE' tell GNU CC where to pass this address. + + By contrast, PCC on most target machines returns structures and + unions of any size by copying the data into an area of static + storage, and then returning the address of that storage as if it + were a pointer value. The caller must copy the data from that + memory area to the place where the value is wanted. GNU CC does + not use this method because it is slower and nonreentrant. + + On some newer machines, PCC uses a reentrant convention for all + structure and union returning. GNU CC on most of these machines + uses a compatible convention when returning structures and unions + in memory, but still returns small structures and unions in + registers. + + You can tell GNU CC to use a compatible convention for all + structure and union returning with the option + `-fpcc-struct-return'. + + * GNU C complains about program fragments such as `0x74ae-0x4000' + which appear to be two hexadecimal constants separated by the minus + operator. Actually, this string is a single "preprocessing token". + Each such token must correspond to one token in C. Since this + does not, GNU C prints an error message. Although it may appear + obvious that what is meant is an operator and two values, the ANSI + C standard specifically requires that this be treated as erroneous. + + A "preprocessing token" is a "preprocessing number" if it begins + with a digit and is followed by letters, underscores, digits, + periods and `e+', `e-', `E+', or `E-' character sequences. + + To make the above program fragment valid, place whitespace in + front of the minus sign. This whitespace will end the + preprocessing number.  -File: gcc.info, Node: Side Effects, Next: Incdec, Prev: RTL Declarations, Up: RTL +File: gcc.info, Node: Fixed Headers, Next: Disappointments, Prev: Incompatibilities, Up: Trouble -Side Effect Expressions -======================= +Fixed Header Files +================== - 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. + GNU CC needs to install corrected versions of some system header +files. This is because most target systems have some header files that +won't work with GNU CC unless they are changed. Some have bugs, some +are incompatible with ANSI C, and some depend on special features of +other compilers. + + Installing GNU CC automatically creates and installs the fixed header +files, by running a program called `fixincludes' (or for certain +targets an alternative such as `fixinc.svr4'). Normally, you don't +need to pay attention to this. But there are cases where it doesn't do +the right thing automatically. + + * If you update the system's header files, such as by installing a + new system version, the fixed header files of GNU CC are not + automatically updated. The easiest way to update them is to + reinstall GNU CC. (If you want to be clever, look in the makefile + and you can find a shortcut.) + + * On some systems, in particular SunOS 4, header file directories + contain machine-specific symbolic links in certain places. This + makes it possible to share most of the header files among hosts + running the same version of SunOS 4 on different machine models. + + The programs that fix the header files do not understand this + special way of using symbolic links; therefore, the directory of + fixed header files is good only for the machine model used to + build it. + + In SunOS 4, only programs that look inside the kernel will notice + the difference between machine models. Therefore, for most + purposes, you need not be concerned about this. + + It is possible to make separate sets of fixed header files for the + different machine models, and arrange a structure of symbolic + links so as to use the proper set, but you'll have to do this by + hand. + + * On Lynxos, GNU CC by default does not fix the header files. This + is because bugs in the shell cause the `fixincludes' script to + fail. + + This means you will encounter problems due to bugs in the system + header files. It may be no comfort that they aren't GNU CC's + fault, but it does mean that there's nothing for us to do about + them.  -File: gcc.info, Node: Incdec, Next: Assembler, Prev: Side Effects, Up: RTL +File: gcc.info, Node: Disappointments, Next: C++ Misunderstandings, Prev: Fixed Headers, Up: Trouble + +Disappointments and Misunderstandings +===================================== -Embedded Side-Effects on Addresses -================================== + These problems are perhaps regrettable, but we don't know any +practical way around them. - Four special side-effect expression codes appear as memory addresses. + * Certain local variables aren't recognized by debuggers when you + compile with optimization. -`(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. + This occurs because sometimes GNU CC optimizes the variable out of + existence. There is no way to tell the debugger how to compute the + value such a variable "would have had", and it is not clear that + would be desirable anyway. So GNU CC simply does not mention the + eliminated variable when it writes debugging information. + + You have to expect a certain amount of disagreement between the + executable and your source code, when you use optimization. + + * Users often think it is a bug when GNU CC reports an error for code + like this: + + int foo (struct mumble *); + + struct mumble { ... }; + + int foo (struct mumble *x) + { ... } + + This code really is erroneous, because the scope of `struct + mumble' in the prototype is limited to the argument list + containing it. It does not refer to the `struct mumble' defined + with file scope immediately below--they are two unrelated types + with similar names in different scopes. + + But in the definition of `foo', the file-scope type is used + because that is available to be inherited. Thus, the definition + and the prototype do not match, and you get an error. + + This behavior may seem silly, but it's what the ANSI standard + specifies. It is easy enough for you to make your code work by + moving the definition of `struct mumble' above the prototype. + It's not worth being incompatible with ANSI C just to avoid an + error for the example shown above. + + * Accesses to bitfields even in volatile objects works by accessing + larger objects, such as a byte or a word. You cannot rely on what + size of object is accessed in order to read or write the bitfield; + it may even vary for a given bitfield according to the precise + usage. + + If you care about controlling the amount of memory that is + accessed, use volatile but do not use bitfields. + + * GNU CC comes with shell scripts to fix certain known problems in + system header files. They install corrected copies of various + header files in a special directory where only GNU CC will + normally look for them. The scripts adapt to various systems by + searching all the system header files for the problem cases that + we know about. + + If new system header files are installed, nothing automatically + arranges to update the corrected header files. You will have to + reinstall GNU CC to fix the new header files. More specifically, + go to the build directory and delete the files `stmp-fixinc' and + `stmp-headers', and the subdirectory `include'; then do `make + install' again. + + * On 68000 systems, you can get paradoxical results if you test the + precise values of floating point numbers. For example, you can + find that a floating point value which is not a NaN is not equal + to itself. This results from the fact that the the floating point + registers hold a few more bits of precision than fit in a `double' + in memory. Compiled code moves values between memory and floating + point registers at its convenience, and moving them into memory + truncates them. + + You can partially avoid this problem by using the `-ffloat-store' + option (*note Optimize Options::.). + + * On the MIPS, variable argument functions using `varargs.h' cannot + have a floating point value for the first argument. The reason + for this is that in the absence of a prototype in scope, if the + first argument is a floating point, it is passed in a floating + point register, rather than an integer register. + + If the code is rewritten to use the ANSI standard `stdarg.h' + method of variable arguments, and the prototype is in scope at the + time of the call, everything will work fine.  -File: gcc.info, Node: Assembler, Next: Insns, Prev: IncDec, Up: RTL +File: gcc.info, Node: C++ Misunderstandings, Next: Protoize Caveats, Prev: Disappointments, Up: Trouble -Assembler Instructions as Expressions +Common Misunderstandings with GNU C++ ===================================== - 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. + C++ is a complex language and an evolving one, and its standard +definition (the ANSI C++ draft standard) is also evolving. As a result, +your C++ compiler may occasionally surprise you, even when its behavior +is correct. This section discusses some areas that frequently give +rise to questions of this sort. + +* Menu: + +* Static Definitions:: Static member declarations are not definitions +* Temporaries:: Temporaries may vanish before you expect + + +File: gcc.info, Node: Static Definitions, Next: Temporaries, Up: C++ Misunderstandings + +Declare *and* Define Static Members +----------------------------------- + + When a class has static data members, it is not enough to *declare* +the static member; you must also *define* it. For example: + + class Foo + { + ... + void method(); + static int bar; + }; + + This declaration only establishes that the class `Foo' has an `int' +named `Foo::bar', and a member function named `Foo::method'. But you +still need to define *both* `method' and `bar' elsewhere. According to +the draft ANSI standard, you must supply an initializer in one (and +only one) source file, such as: + + int Foo::bar = 0; + + Other C++ compilers may not correctly implement the standard +behavior. As a result, when you switch to `g++' from one of these +compilers, you may discover that a program that appeared to work +correctly in fact does not conform to the standard: `g++' reports as +undefined symbols any static data members that lack definitions.  -File: gcc.info, Node: Insns, Next: Calls, Prev: Assembler, Up: RTL +File: gcc.info, Node: Temporaries, Prev: Static Definitions, Up: C++ Misunderstandings + +Temporaries May Vanish Before You Expect +---------------------------------------- -Insns -===== + 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-conformant, 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: - The RTL representation of the code for a function is a doubly-linked -chain of objects called "insns". Insns are expressions with special -codes that are used for no other purpose. Some insns are actual -instructions; others represent dispatch tables for `switch' statements; -others represent labels to jump to or various sorts of declarative -information. - - In addition to its own specific data, each insn must have a unique -id-number that distinguishes it from all other insns in the current -function (after delayed branch scheduling, copies of an insn with the -same id-number may be present in multiple places in a function, but -these copies will always be identical and will only appear inside a -`sequence'), and chain pointers to the preceding and following insns. -These three fields occupy the same position in every insn, independent -of the expression code of the insn. They could be accessed with `XEXP' -and `XINT', but instead three special macros are always used: - -`INSN_UID (I)' - Accesses the unique id of insn I. - -`PREV_INSN (I)' - Accesses the chain pointer to the insn preceding I. If I is the - first insn, this is a null pointer. - -`NEXT_INSN (I)' - Accesses the chain pointer to the insn following I. If I is the - last insn, this is a null pointer. - - The first insn in the chain is obtained by calling `get_insns'; the -last insn is the result of calling `get_last_insn'. Within the chain -delimited by these insns, the `NEXT_INSN' and `PREV_INSN' pointers must -always correspond: if INSN is not the first insn, - - NEXT_INSN (PREV_INSN (INSN)) == INSN - -is always true and if INSN is not the last insn, - - PREV_INSN (NEXT_INSN (INSN)) == INSN - -is always true. - - After delay slot scheduling, some of the insns in the chain might be -`sequence' expressions, which contain a vector of insns. The value of -`NEXT_INSN' in all but the last of these insns is the next insn in the -vector; the value of `NEXT_INSN' of the last insn in the vector is the -same as the value of `NEXT_INSN' for the `sequence' in which it is -contained. Similar rules apply for `PREV_INSN'. - - This means that the above invariants are not necessarily true for -insns inside `sequence' expressions. Specifically, if INSN is the -first insn in a `sequence', `NEXT_INSN (PREV_INSN (INSN))' is the insn -containing the `sequence' expression, as is the value of `PREV_INSN -(NEXT_INSN (INSN))' is INSN is the last insn in the `sequence' -expression. You can use these expressions to find the containing -`sequence' expression. - - Every insn has one of the following six expression codes: - -`insn' - The expression code `insn' is used for instructions that do not - jump and do not do function calls. `sequence' expressions are - always contained in insns with code `insn' even if one of those - insns should jump or do function calls. - - Insns with code `insn' have four additional fields beyond the three - mandatory ones listed above. These four are described in a table - below. - -`jump_insn' - The expression code `jump_insn' is used for instructions that may - jump (or, more generally, may contain `label_ref' expressions). If - there is an instruction to return from the current function, it is - recorded as a `jump_insn'. - - `jump_insn' insns have the same extra fields as `insn' insns, - accessed in the same way and in addition contains a field - `JUMP_LABEL' which is defined once jump optimization has completed. - - For simple conditional and unconditional jumps, this field - contains the `code_label' to which this insn will (possibly - conditionally) branch. In a more complex jump, `JUMP_LABEL' - records one of the labels that the insn refers to; the only way to - find the others is to scan the entire body of the insn. - - Return insns count as jumps, but since they do not refer to any - labels, they have zero in the `JUMP_LABEL' field. - -`call_insn' - The expression code `call_insn' is used for instructions that may - do function calls. It is important to distinguish these - instructions because they imply that certain registers and memory - locations may be altered unpredictably. - - A `call_insn' insn may be preceded by insns that contain a single - `use' expression and be followed by insns the contain a single - `clobber' expression. If so, these `use' and `clobber' - expressions are treated as being part of the function call. There - must not even be a `note' between the `call_insn' and the `use' or - `clobber' insns for this special treatment to take place. This is - somewhat of a kludge and will be removed in a later version of GNU - CC. - - `call_insn' insns have the same extra fields as `insn' insns, - accessed in the same way. - -`code_label' - A `code_label' insn represents a label that a jump insn can jump - to. It contains two special fields of data in addition to the - three standard ones. `CODE_LABEL_NUMBER' is used to hold the - "label number", a number that identifies this label uniquely among - all the labels in the compilation (not just in the current - function). Ultimately, the label is represented in the assembler - output as an assembler label, usually of the form `LN' where N is - the label number. - - When a `code_label' appears in an RTL expression, it normally - appears within a `label_ref' which represents the address of the - label, as a number. - - The field `LABEL_NUSES' is only defined once the jump optimization - phase is completed and contains the number of times this label is - referenced in the current function. - -`barrier' - Barriers are placed in the instruction stream when control cannot - flow past them. They are placed after unconditional jump - instructions to indicate that the jumps are unconditional and - after calls to `volatile' functions, which do not return (e.g., - `exit'). They contain no information beyond the three standard - fields. - -`note' - `note' insns are used to represent additional debugging and - declarative information. They contain two nonstandard fields, an - integer which is accessed with the macro `NOTE_LINE_NUMBER' and a - string accessed with `NOTE_SOURCE_FILE'. - - If `NOTE_LINE_NUMBER' is positive, the note represents the - position of a source line and `NOTE_SOURCE_FILE' is the source - file name that the line came from. These notes control generation - of line number data in the assembler output. - - Otherwise, `NOTE_LINE_NUMBER' is not really a line number but a - code with one of the following values (and `NOTE_SOURCE_FILE' must - contain a null pointer): - - `NOTE_INSN_DELETED' - Such a note is completely ignorable. Some passes of the - compiler delete insns by altering them into notes of this - kind. - - `NOTE_INSN_BLOCK_BEG' - `NOTE_INSN_BLOCK_END' - These types of notes indicate the position of the beginning - and end of a level of scoping of variable names. They - control the output of debugging information. - - `NOTE_INSN_LOOP_BEG' - `NOTE_INSN_LOOP_END' - These types of notes indicate the position of the beginning - and end of a `while' or `for' loop. They enable the loop - optimizer to find loops quickly. - - `NOTE_INSN_LOOP_CONT' - Appears at the place in a loop that `continue' statements - jump to. - - `NOTE_INSN_LOOP_VTOP' - This note indicates the place in a loop where the exit test - begins for those loops in which the exit test has been - duplicated. This position becomes another virtual start of - the loop when considering loop invariants. - - `NOTE_INSN_FUNCTION_END' - Appears near the end of the function body, just before the - label that `return' statements jump to (on machine where a - single instruction does not suffice for returning). This - note may be deleted by jump optimization. - - `NOTE_INSN_SETJMP' - Appears following each call to `setjmp' or a related function. - - These codes are printed symbolically when they appear in debugging - dumps. - - The machine mode of an insn is normally `VOIDmode', but some phases -use the mode for various purposes; for example, the reload pass sets it -to `HImode' if the insn needs reloading but not register elimination -and `QImode' if both are required. The common subexpression -elimination pass sets the mode of an insn to `QImode' when it is the -first insn in a block that has already been processed. - - Here is a table of the extra fields of `insn', `jump_insn' and -`call_insn' insns: - -`PATTERN (I)' - An expression for the side effect performed by this insn. This - must be one of the following codes: `set', `call', `use', - `clobber', `return', `asm_input', `asm_output', `addr_vec', - `addr_diff_vec', `trap_if', `unspec', `unspec_volatile', - `parallel', or `sequence'. If it is a `parallel', each element of - the `parallel' must be one these codes, except that `parallel' - expressions cannot be nested and `addr_vec' and `addr_diff_vec' - are not permitted inside a `parallel' expression. - -`INSN_CODE (I)' - An integer that says which pattern in the machine description - matches this insn, or -1 if the matching has not yet been - attempted. - - Such matching is never attempted and this field remains -1 on an - insn whose pattern consists of a single `use', `clobber', - `asm_input', `addr_vec' or `addr_diff_vec' expression. - - Matching is also never attempted on insns that result from an `asm' - statement. These contain at least one `asm_operands' expression. - The function `asm_noperands' returns a non-negative value for such - insns. - - In the debugging output, this field is printed as a number - followed by a symbolic representation that locates the pattern in - the `md' file as some small positive or negative offset from a - named pattern. - -`LOG_LINKS (I)' - A list (chain of `insn_list' expressions) giving information about - dependencies between instructions within a basic block. Neither a - jump nor a label may come between the related insns. - -`REG_NOTES (I)' - A list (chain of `expr_list' and `insn_list' expressions) giving - miscellaneous information about the insn. It is often information - pertaining to the registers used in this insn. - - The `LOG_LINKS' field of an insn is a chain of `insn_list' -expressions. Each of these has two operands: the first is an insn, and -the second is another `insn_list' expression (the next one in the -chain). The last `insn_list' in the chain has a null pointer as second -operand. The significant thing about the chain is which insns appear -in it (as first operands of `insn_list' expressions). Their order is -not significant. - - This list is originally set up by the flow analysis pass; it is a -null pointer until then. Flow only adds links for those data -dependencies which can be used for instruction combination. For each -insn, the flow analysis pass adds a link to insns which store into -registers values that are used for the first time in this insn. The -instruction scheduling pass adds extra links so that every dependence -will be represented. Links represent data dependencies, -antidependencies and output dependencies; the machine mode of the link -distinguishes these three types: antidependencies have mode -`REG_DEP_ANTI', output dependencies have mode `REG_DEP_OUTPUT', and -data dependencies have mode `VOIDmode'. - - The `REG_NOTES' field of an insn is a chain similar to the -`LOG_LINKS' field but it includes `expr_list' expressions in addition -to `insn_list' expressions. There are several kinds of register notes, -which are distinguished by the machine mode, which in a register note -is really understood as being an `enum reg_note'. The first operand OP -of the note is data whose meaning depends on the kind of note. - - The macro `REG_NOTE_KIND (X)' returns the kind of register note. -Its counterpart, the macro `PUT_REG_NOTE_KIND (X, NEWKIND)' sets the -register note type of X to be NEWKIND. - - Register notes are of three classes: They may say something about an -input to an insn, they may say something about an output of an insn, or -they may create a linkage between two insns. There are also a set of -values that are only used in `LOG_LINKS'. - - These register notes annotate inputs to an insn: - -`REG_DEAD' - The value in OP dies in this insn; that is to say, altering the - value immediately after this insn would not affect the future - behavior of the program. - - This does not necessarily mean that the register OP has no useful - value after this insn since it may also be an output of the insn. - In such a case, however, a `REG_DEAD' note would be redundant and - is usually not present until after the reload pass, but no code - relies on this fact. - -`REG_INC' - The register OP is incremented (or decremented; at this level - there is no distinction) by an embedded side effect inside this - insn. This means it appears in a `post_inc', `pre_inc', `post_dec' - or `pre_dec' expression. - -`REG_NONNEG' - The register OP is known to have a nonnegative value when this - insn is reached. This is used so that decrement and branch until - zero instructions, such as the m68k dbra, can be matched. - - The `REG_NONNEG' note is added to insns only if the machine - description contains a pattern named - `decrement_and_branch_until_zero'. - -`REG_NO_CONFLICT' - This insn does not cause a conflict between OP and the item being - set by this insn even though it might appear that it does. In - other words, if the destination register and OP could otherwise be - assigned the same register, this insn does not prevent that - assignment. - - Insns with this note are usually part of a block that begins with a - `clobber' insn specifying a multi-word pseudo register (which will - be the output of the block), a group of insns that each set one - word of the value and have the `REG_NO_CONFLICT' note attached, - and a final insn that copies the output to itself with an attached - `REG_EQUAL' note giving the expression being computed. This block - is encapsulated with `REG_LIBCALL' and `REG_RETVAL' notes on the - first and last insns, respectively. - -`REG_LABEL' - This insn uses OP, a `code_label', but is not a `jump_insn'. The - presence of this note allows jump optimization to be aware that OP - is, in fact, being used. - - The following notes describe attributes of outputs of an insn: - -`REG_EQUIV' -`REG_EQUAL' - This note is only valid on an insn that sets only one register and - indicates that that register will be equal to OP at run time; the - scope of this equivalence differs between the two types of notes. - The value which the insn explicitly copies into the register may - look different from OP, but they will be equal at run time. If the - output of the single `set' is a `strict_low_part' expression, the - note refers to the register that is contained in `SUBREG_REG' of - the `subreg' expression. - - For `REG_EQUIV', the register is equivalent to OP throughout the - entire function, and could validly be replaced in all its - occurrences by OP. ("Validly" here refers to the data flow of the - program; simple replacement may make some insns invalid.) For - example, when a constant is loaded into a register that is never - assigned any other value, this kind of note is used. - - When a parameter is copied into a pseudo-register at entry to a - function, a note of this kind records that the register is - equivalent to the stack slot where the parameter was passed. - Although in this case the register may be set by other insns, it - is still valid to replace the register by the stack slot - throughout the function. - - In the case of `REG_EQUAL', the register that is set by this insn - will be equal to OP at run time at the end of this insn but not - necessarily elsewhere in the function. In this case, OP is - typically an arithmetic expression. For example, when a sequence - of insns such as a library call is used to perform an arithmetic - operation, this kind of note is attached to the insn that produces - or copies the final value. - - These two notes are used in different ways by the compiler passes. - `REG_EQUAL' is used by passes prior to register allocation (such as - common subexpression elimination and loop optimization) to tell - them how to think of that value. `REG_EQUIV' notes are used by - register allocation to indicate that there is an available - substitute expression (either a constant or a `mem' expression for - the location of a parameter on the stack) that may be used in - place of a register if insufficient registers are available. - - Except for stack homes for parameters, which are indicated by a - `REG_EQUIV' note and are not useful to the early optimization - passes and pseudo registers that are equivalent to a memory - location throughout there entire life, which is not detected until - later in the compilation, all equivalences are initially indicated - by an attached `REG_EQUAL' note. In the early stages of register - allocation, a `REG_EQUAL' note is changed into a `REG_EQUIV' note - if OP is a constant and the insn represents the only set of its - destination register. - - Thus, compiler passes prior to register allocation need only check - for `REG_EQUAL' notes and passes subsequent to register allocation - need only check for `REG_EQUIV' notes. - -`REG_UNUSED' - The register OP being set by this insn will not be used in a - subsequent insn. This differs from a `REG_DEAD' note, which - indicates that the value in an input will not be used subsequently. - These two notes are independent; both may be present for the same - register. - -`REG_WAS_0' - The single output of this insn contained zero before this insn. OP - is the insn that set it to zero. You can rely on this note if it - is present and OP has not been deleted or turned into a `note'; - its absence implies nothing. - - These notes describe linkages between insns. They occur in pairs: -one insn has one of a pair of notes that points to a second insn, which -has the inverse note pointing back to the first insn. - -`REG_RETVAL' - This insn copies the value of a multi-insn sequence (for example, a - library call), and OP is the first insn of the sequence (for a - library call, the first insn that was generated to set up the - arguments for the library call). - - Loop optimization uses this note to treat such a sequence as a - single operation for code motion purposes and flow analysis uses - this note to delete such sequences whose results are dead. - - A `REG_EQUAL' note will also usually be attached to this insn to - provide the expression being computed by the sequence. - -`REG_LIBCALL' - This is the inverse of `REG_RETVAL': it is placed on the first - insn of a multi-insn sequence, and it points to the last one. - -`REG_CC_SETTER' -`REG_CC_USER' - On machines that use `cc0', the insns which set and use `cc0' set - and use `cc0' are adjacent. However, when branch delay slot - filling is done, this may no longer be true. In this case a - `REG_CC_USER' note will be placed on the insn setting `cc0' to - point to the insn using `cc0' and a `REG_CC_SETTER' note will be - placed on the insn using `cc0' to point to the insn setting `cc0'. - - These values are only used in the `LOG_LINKS' field, and indicate -the type of dependency that each link represents. Links which indicate -a data dependence (a read after write dependence) do not use any code, -they simply have mode `VOIDmode', and are printed without any -descriptive text. - -`REG_DEP_ANTI' - This indicates an anti dependence (a write after read dependence). - -`REG_DEP_OUTPUT' - This indicates an output dependence (a write after write - dependence). - - For convenience, the machine mode in an `insn_list' or `expr_list' -is printed using these symbolic codes in debugging dumps. - - The only difference between the expression codes `insn_list' and -`expr_list' is that the first operand of an `insn_list' is assumed to -be an insn and is printed in debugging dumps as the insn's unique id; -the first operand of an `expr_list' is printed in the ordinary way as -an expression. + String& tmp = strfunc (); + charfunc (tmp);  -File: gcc.info, Node: Calls, Next: Sharing, Prev: Insns, Up: RTL +File: gcc.info, Node: Protoize Caveats, Next: Non-bugs, Prev: C++ Misunderstandings, Up: Trouble + +Caveats of using `protoize' +=========================== -RTL Representation of Function-Call Insns -========================================= + The conversion programs `protoize' and `unprotoize' can sometimes +change a source file in a way that won't work unless you rearrange it. - Insns that call subroutines have the RTL expression code `call_insn'. -These insns must satisfy special rules, and their bodies must use a -special RTL expression code, `call'. - - A `call' expression has two operands, as follows: - - (call (mem:FM ADDR) NBYTES) - -Here NBYTES is an operand that represents the number of bytes of -argument data being passed to the subroutine, FM is a machine mode -(which must equal as the definition of the `FUNCTION_MODE' macro in the -machine description) and ADDR represents the address of the subroutine. - - For a subroutine that returns no value, the `call' expression as -shown above is the entire body of the insn, except that the insn might -also contain `use' or `clobber' expressions. - - For a subroutine that returns a value whose mode is not `BLKmode', -the value is returned in a hard register. If this register's number is -R, then the body of the call insn looks like this: - - (set (reg:M R) - (call (mem:FM ADDR) NBYTES)) - -This RTL expression makes it clear (to the optimizer passes) that the -appropriate register receives a useful value in this insn. - - When a subroutine returns a `BLKmode' value, it is handled by -passing to the subroutine the address of a place to store the value. So -the call insn itself does not "return" any value, and it has the same -RTL form as a call that returns nothing. - - On some machines, the call instruction itself clobbers some register, -for example to contain the return address. `call_insn' insns on these -machines should have a body which is a `parallel' that contains both -the `call' expression and `clobber' expressions that indicate which -registers are destroyed. Similarly, if the call instruction requires -some register other than the stack pointer that is not explicitly -mentioned it its RTL, a `use' subexpression should mention that -register. - - Functions that are called are assumed to modify all registers listed -in the configuration macro `CALL_USED_REGISTERS' (*note Register -Basics::.) and, with the exception of `const' functions and library -calls, to modify all of memory. - - Insns containing just `use' expressions directly precede the -`call_insn' insn to indicate which registers contain inputs to the -function. Similarly, if registers other than those in -`CALL_USED_REGISTERS' are clobbered by the called function, insns -containing a single `clobber' follow immediately after the call to -indicate which registers. + * `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 preprocessor + 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: Sharing, Prev: Calls, Up: RTL +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 `3, 4', or it might get `4, 3', or even + `3, 3'. + + * 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 +=================================== -Structure Sharing Assumptions -============================= + The GNU compiler can produce two kinds of diagnostics: errors and +warnings. Each kind has a different purpose: - The compiler assumes that certain kinds of RTL expressions are -unique; there do not exist two distinct objects representing the same -value. In other cases, it makes an opposite assumption: that no RTL -expression object of a certain kind appears in more than one place in -the containing structure. - - These assumptions refer to a single function; except for the RTL -objects that describe global variables and external functions, and a -few standard objects such as small integer constants, no RTL objects -are common to two functions. - - * Each pseudo-register has only a single `reg' object to represent - it, and therefore only a single machine mode. - - * For any symbolic label, there is only one `symbol_ref' object - referring to it. - - * There is only one `const_int' expression with value 0, only one - with value 1, and only one with value -1. Some other integer - values are also stored uniquely. - - * There is only one `pc' expression. - - * There is only one `cc0' expression. - - * There is only one `const_double' expression with value 0 for each - floating point mode. Likewise for values 1 and 2. - - * No `label_ref' or `scratch' appears in more than one place in the - RTL structure; in other words, it is safe to do a tree-walk of all - the insns in the function and assume that each time a `label_ref' - or `scratch' is seen it is distinct from all others that are seen. - - * Only one `mem' object is normally created for each static variable - or stack slot, so these objects are frequently shared in all the - places they appear. However, separate but equal objects for these - variables are occasionally made. - - * When a single `asm' statement has multiple output operands, a - distinct `asm_operands' expression is made for each output operand. - However, these all share the vector which contains the sequence of - input operands. This sharing is used later on to test whether two - `asm_operands' expressions come from the same statement, so all - optimizations must carefully preserve the sharing if they copy the - vector at all. - - * No RTL object appears in more than one place in the RTL structure - except as described above. Many passes of the compiler rely on - this by assuming that they can modify RTL objects in place without - unwanted side-effects on other insns. - - * During initial RTL generation, shared structure is freely - introduced. After all the RTL for a function has been generated, - all shared structure is copied by `unshare_all_rtl' in - `emit-rtl.c', after which the above rules are guaranteed to be - followed. - - * During the combiner pass, shared structure within an insn can exist - temporarily. However, the shared structure is copied before the - combiner is finished with the insn. This is done by calling - `copy_rtx_if_shared', which is a subroutine of `unshare_all_rtl'. + *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 +gratuituously 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: Machine Desc, Next: Target Macros, Prev: RTL, Up: Top +File: gcc.info, Node: Bugs, Next: Service, Prev: Trouble, Up: Top -Machine Descriptions -******************** +Reporting Bugs +************** - A machine description has two parts: a file of instruction patterns -(`.md' file) and a C header file of macro definitions. + Your bug reports play an essential role in making GNU CC reliable. - The `.md' file for a target machine contains a pattern for each -instruction that the target machine supports (or at least each -instruction that is worth telling the compiler about). It may also -contain comments. A semicolon causes the rest of the line to be a -comment, unless the semicolon is inside a quoted string. + 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. - See the next chapter for information on the C header file. + In order for a bug report to serve its purpose, you must include the +information that makes for fixing the bug. * Menu: -* Patterns:: How to write instruction patterns. -* Example:: An explained example of a `define_insn' pattern. -* RTL Template:: The RTL template defines what insns match a pattern. -* Output Template:: The output template says how to make assembler code - from such an insn. -* Output Statement:: For more generality, write C code to output - the assembler code. -* Constraints:: When not all operands are general operands. -* Standard Names:: Names mark patterns to use for code generation. -* Pattern Ordering:: When the order of patterns makes a difference. -* Dependent Patterns:: Having one pattern may make you need another. -* Jump Patterns:: Special considerations for patterns for jump insns. -* Insn Canonicalizations::Canonicalization of Instructions -* Peephole Definitions::Defining machine-specific peephole optimizations. -* Expander Definitions::Generating a sequence of several RTL insns - for a standard operation. -* Insn Splitting:: Splitting Instructions into Multiple Instructions -* Insn Attributes:: Specifying the value of attributes for generated insns. +* 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 +==================== + + 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 + 675 Mass Ave + Cambridge, MA 02139 - \ No newline at end of file