--- gcc/gcc.info-10 2018/04/24 17:55:51 1.1.1.3 +++ gcc/gcc.info-10 2018/04/24 18:24:16 1.1.1.8 @@ -1,9 +1,13 @@ -This is Info file gcc.info, produced by Makeinfo-1.47 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 59 Temple Place - Suite 330 +Boston, MA 02111-1307 USA + + 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 @@ -11,867 +15,855 @@ 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 "Boycott" -are included exactly as in the original, and provided that the entire +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 "Boycott", and this permission notice, may be included in +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: RTL Template, Next: Output Template, Prev: Example, Up: Machine Desc +File: gcc.info, Node: Asm Labels, Next: Explicit Reg Vars, Prev: Extended Asm, Up: C Extensions + +Controlling Names Used in Assembler Code +======================================== + + You can specify the name to be used in the assembler code for a C +function or variable by writing the `asm' (or `__asm__') keyword after +the declarator as follows: + + int foo asm ("myfoo") = 2; + +This specifies that the name to be used for the variable `foo' in the +assembler code should be `myfoo' rather than the usual `_foo'. + + On systems where an underscore is normally prepended to the name of +a C function or variable, this feature allows you to define names for +the linker that do not start with an underscore. + + You cannot use `asm' in this way in a function *definition*; but you +can get the same effect by writing a declaration for the function +before its definition and putting `asm' there, like this: + + extern func () asm ("FUNC"); + + func (x, y) + int x, y; + ... + + It is up to you to make sure that the assembler names you choose do +not conflict with any other assembler symbols. Also, you must not use a +register name; that would produce completely invalid assembler code. +GNU CC does not as yet have the ability to store static variables in +registers. Perhaps that will be added. + + +File: gcc.info, Node: Explicit Reg Vars, Next: Alternate Keywords, Prev: Asm Labels, Up: C Extensions -RTL Template for Generating and Recognizing Insns -================================================= +Variables in Specified Registers +================================ - The RTL template is used to define which insns match the particular -pattern and how to find their operands. For named patterns, the RTL -template also says how to construct an insn from specified operands. - - Construction involves substituting specified operands into a copy of -the template. Matching involves determining the values that serve as -the operands in the insn being matched. Both of these activities are -controlled by special expression types that direct matching and -substitution of the operands. - -`(match_operand:M N PREDICATE CONSTRAINT)' - This expression is a placeholder for operand number N of the insn. - When constructing an insn, operand number N will be substituted - at this point. When matching an insn, whatever appears at this - position in the insn will be taken as operand number N; but it - must satisfy PREDICATE or this instruction pattern will not match - at all. - - Operand numbers must be chosen consecutively counting from zero in - each instruction pattern. There may be only one `match_operand' - expression in the pattern for each operand number. Usually - operands are numbered in the order of appearance in `match_operand' - expressions. - - PREDICATE is a string that is the name of a C function that - accepts two arguments, an expression and a machine mode. During - matching, the function will be called with the putative operand as - the expression and M as the mode argument (if M is not specified, - `VOIDmode' will be used, which normally causes PREDICATE to accept - any mode). If it returns zero, this instruction pattern fails to - match. PREDICATE may be an empty string; then it means no test is - to be done on the operand, so anything which occurs in this - position is valid. - - Most of the time, PREDICATE will reject modes other than M--but - not always. For example, the predicate `address_operand' uses M - as the mode of memory ref that the address should be valid for. - Many predicates accept `const_int' nodes even though their mode is - `VOIDmode'. - - CONSTRAINT controls reloading and the choice of the best register - class to use for a value, as explained later (*note - Constraints::.). - - People are often unclear on the difference between the constraint - and the predicate. The predicate helps decide whether a given - insn matches the pattern. The constraint plays no role in this - decision; instead, it controls various decisions in the case of an - insn which does match. - - On CISC machines, PREDICATE is most often `"general_operand"'. - This function checks that the putative operand is either a - constant, a register or a memory reference, and that it is valid - for mode M. - - For an operand that must be a register, PREDICATE should be - `"register_operand"'. It would be valid to use - `"general_operand"', since the reload pass would copy any - non-register operands through registers, but this would make GNU - CC do extra work, it would prevent invariant operands (such as - constant) from being removed from loops, and it would prevent the - register allocator from doing the best possible job. On RISC - machines, it is usually most efficient to allow PREDICATE to - accept only objects that the constraints allow. - - For an operand that must be a constant, either use - `"immediate_operand"' for PREDICATE, or make the instruction - pattern's extra condition require a constant, or both. You cannot - expect the constraints to do this work! If the constraints allow - only constants, but the predicate allows something else, the - compiler will crash when that case arises. - -`(match_scratch:M N CONSTRAINT)' - This expression is also a placeholder for operand number N and - indicates that operand must be a `scratch' or `reg' expression. - - When matching patterns, this is completely equivalent to - - (match_operand:M N "scratch_operand" PRED) - - but, when generating RTL, it produces a (`scratch':M) expression. - - If the last few expressions in a `parallel' are `clobber' - expressions whose operands are either a hard register or - `match_scratch', the combiner can add them when necessary. *Note - Side Effects::. - -`(match_dup N)' - This expression is also a placeholder for operand number N. It is - used when the operand needs to appear more than once in the insn. - - In construction, `match_dup' behaves exactly like `match_operand': - the operand is substituted into the insn being constructed. But - in matching, `match_dup' behaves differently. It assumes that - operand number N has already been determined by a `match_operand' - appearing earlier in the recognition template, and it matches only - an identical-looking expression. - -`(match_operator:M N PREDICATE [OPERANDS...])' - This pattern is a kind of placeholder for a variable RTL expression - code. - - When constructing an insn, it stands for an RTL expression whose - expression code is taken from that of operand N, and whose - operands are constructed from the patterns OPERANDS. - - When matching an expression, it matches an expression if the - function PREDICATE returns nonzero on that expression *and* the - patterns OPERANDS match the operands of the expression. - - Suppose that the function `commutative_operator' is defined as - follows, to match any expression whose operator is one of the - commutative arithmetic operators of RTL and whose mode is MODE: - - int - commutative_operator (x, mode) - rtx x; - enum machine_mode mode; - { - enum rtx_code code = GET_CODE (x); - if (GET_MODE (x) != mode) - return 0; - return GET_RTX_CLASS (code) == 'c' || code == EQ || code == NE; - } - - Then the following pattern will match any RTL expression consisting - of a commutative operator applied to two general operands: - - (match_operator:SI 3 "commutative_operator" - [(match_operand:SI 1 "general_operand" "g") - (match_operand:SI 2 "general_operand" "g")]) - - Here the vector `[OPERANDS...]' contains two patterns because the - expressions to be matched all contain two operands. - - When this pattern does match, the two operands of the commutative - operator are recorded as operands 1 and 2 of the insn. (This is - done by the two instances of `match_operand'.) Operand 3 of the - insn will be the entire commutative expression: use `GET_CODE - (operands[3])' to see which commutative operator was used. - - The machine mode M of `match_operator' works like that of - `match_operand': it is passed as the second argument to the - predicate function, and that function is solely responsible for - deciding whether the expression to be matched "has" that mode. - - When constructing an insn, argument 3 of the gen-function will - specify the operation (i.e. the expression code) for the - expression to be made. It should be an RTL expression, whose - expression code is copied into a new expression whose operands are - arguments 1 and 2 of the gen-function. The subexpressions of - argument 3 are not used; only its expression code matters. - - When `match_operator' is used in a pattern for matching an insn, - it usually best if the operand number of the `match_operator' is - higher than that of the actual operands of the insn. This improves - register allocation because the register allocator often looks at - operands 1 and 2 of insns to see if it can do register tying. - - There is no way to specify constraints in `match_operator'. The - operand of the insn which corresponds to the `match_operator' - never has any constraints because it is never reloaded as a whole. - However, if parts of its OPERANDS are matched by `match_operand' - patterns, those parts may have constraints of their own. - -`(match_parallel N PREDICATE [SUBPAT...])' - This pattern is a placeholder for an insn that consists of a - `parallel' expression with a variable number of elements. This - expression should only appear at the top level of an insn pattern. - - When constructing an insn, operand number N will be substituted at - this point. When matching an insn, it matches if the body of the - insn is a `parallel' expression with at least as many elements as - the vector of SUBPAT expressions in the `match_parallel', if each - SUBPAT matches the corresponding element of the `parallel', *and* - the function PREDICATE returns nonzero on the `parallel' that is - the body of the insn. It is the responsibility of the predicate - to validate elements of the `parallel' beyond those listed in the - `match_parallel'. - - A typical use of `match_parallel' is to match load and store - multiple expressions, which can contains a variable number of - elements in a `parallel'. For example, - - (define_insn "" - [(match_parallel 0 "load_multiple_operation" - [(set (match_operand:SI 1 "gen_reg_operand" "=r") - (match_operand:SI 2 "memory_operand" "m")) - (use (reg:SI 179)) - (clobber (reg:SI 179))])] - "" - "loadm 0,0,%1,%2") - - This example comes from `a29k.md'. The function - `load_multiple_operations' is defined in `a29k.c' and checks that - subsequent elements in the `parallel' are the same as the `set' in - the pattern, except that they are referencing subsequent registers - and memory locations. - - An insn that matches this pattern might look like: - - (parallel [(set (reg:SI 20) (mem:SI (reg:SI 100))) - (use (reg:SI 179)) - (clobber (reg:SI 179)) - (set (reg:SI 21) (mem:SI (plus:SI (reg:SI 100) (const_int 4)))) - (set (reg:SI 22) (mem:SI (plus:SI (reg:SI 100) (const_int 8))))]) - -`(address (match_operand:M N "address_operand" ""))' - This complex of expressions is a placeholder for an operand number - N in a "load address" instruction: an operand which specifies a - memory location in the usual way, but for which the actual operand - value used is the address of the location, not the contents of the - location. - - `address' expressions never appear in RTL code, only in machine - descriptions. And they are used only in machine descriptions that - do not use the operand constraint feature. When operand - constraints are in use, the letter `p' in the constraint serves - this purpose. - - M is the machine mode of the *memory location being addressed*, - not the machine mode of the address itself. That mode is always - the same on a given target machine (it is `Pmode', which normally - is `SImode'), so there is no point in mentioning it; thus, no - machine mode is written in the `address' expression. If some day - support is added for machines in which addresses of different - kinds of objects appear differently or are used differently (such - as the PDP-10), different formats would perhaps need different - machine modes and these modes might be written in the `address' - expression. - - -File: gcc.info, Node: Output Template, Next: Output Statement, Prev: RTL Template, Up: Machine Desc - -Output Templates and Operand Substitution -========================================= - - The "output template" is a string which specifies how to output the -assembler code for an instruction pattern. Most of the template is a -fixed string which is output literally. The character `%' is used to -specify where to substitute an operand; it can also be used to identify -places where different variants of the assembler require different -syntax. - - In the simplest case, a `%' followed by a digit N says to output -operand N at that point in the string. - - `%' followed by a letter and a digit says to output an operand in an -alternate fashion. Four letters have standard, built-in meanings -described below. The machine description macro `PRINT_OPERAND' can -define additional letters with nonstandard meanings. - - `%cDIGIT' can be used to substitute an operand that is a constant -value without the syntax that normally indicates an immediate operand. - - `%nDIGIT' is like `%cDIGIT' except that the value of the constant is -negated before printing. - - `%aDIGIT' can be used to substitute an operand as if it were a -memory reference, with the actual operand treated as the address. This -may be useful when outputting a "load address" instruction, because -often the assembler syntax for such an instruction requires you to -write the operand as if it were a memory reference. - - `%lDIGIT' is used to substitute a `label_ref' into a jump -instruction. - - `%' followed by a punctuation character specifies a substitution that -does not use an operand. Only one case is standard: `%%' outputs a `%' -into the assembler code. Other nonstandard cases can be defined in the -`PRINT_OPERAND' macro. You must also define which punctuation -characters are valid with the `PRINT_OPERAND_PUNCT_VALID_P' macro. - - The template may generate multiple assembler instructions. Write -the text for the instructions, with `\;' between them. - - When the RTL contains two operands which are required by constraint -to match each other, the output template must refer only to the -lower-numbered operand. Matching operands are not always identical, and -the rest of the compiler arranges to put the proper RTL expression for -printing into the lower-numbered operand. - - One use of nonstandard letters or punctuation following `%' is to -distinguish between different assembler languages for the same machine; -for example, Motorola syntax versus MIT syntax for the 68000. Motorola -syntax requires periods in most opcode names, while MIT syntax does -not. For example, the opcode `movel' in MIT syntax is `move.l' in -Motorola syntax. The same file of patterns is used for both kinds of -output syntax, but the character sequence `%.' is used in each place -where Motorola syntax wants a period. The `PRINT_OPERAND' macro for -Motorola syntax defines the sequence to output a period; the macro for -MIT syntax defines it to do nothing. - - -File: gcc.info, Node: Output Statement, Next: Constraints, Prev: Output Template, Up: Machine Desc - -C Statements for Generating Assembler Output -============================================ - - Often a single fixed template string cannot produce correct and -efficient assembler code for all the cases that are recognized by a -single instruction pattern. For example, the opcodes may depend on the -kinds of operands; or some unfortunate combinations of operands may -require extra machine instructions. - - If the output control string starts with a `@', then it is actually -a series of templates, each on a separate line. (Blank lines and -leading spaces and tabs are ignored.) The templates correspond to the -pattern's constraint alternatives (*note Multi-Alternative::.). For -example, if a target machine has a two-address add instruction `addr' -to add into a register and another `addm' to add a register to memory, -you might write this pattern: - - (define_insn "addsi3" - [(set (match_operand:SI 0 "general_operand" "=r,m") - (plus:SI (match_operand:SI 1 "general_operand" "0,0") - (match_operand:SI 2 "general_operand" "g,r")))] - "" - "@ - addr %1,%0 - addm %1,%0") - - If the output control string starts with a `*', then it is not an -output template but rather a piece of C program that should compute a -template. It should execute a `return' statement to return the -template-string you want. Most such templates use C string literals, -which require doublequote characters to delimit them. To include these -doublequote characters in the string, prefix each one with `\'. - - The operands may be found in the array `operands', whose C data type -is `rtx []'. - - It is very common to select different ways of generating assembler -code based on whether an immediate operand is within a certain range. -Be careful when doing this, because the result of `INTVAL' is an -integer on the host machine. If the host machine has more bits in an -`int' than the target machine has in the mode in which the constant -will be used, then some of the bits you get from `INTVAL' will be -superfluous. For proper results, you must carefully disregard the -values of those bits. - - It is possible to output an assembler instruction and then go on to -output or compute more of them, using the subroutine `output_asm_insn'. - This receives two arguments: a template-string and a vector of -operands. The vector may be `operands', or it may be another array of -`rtx' that you declare locally and initialize yourself. - - When an insn pattern has multiple alternatives in its constraints, -often the appearance of the assembler code is determined mostly by -which alternative was matched. When this is so, the C code can test -the variable `which_alternative', which is the ordinal number of the -alternative that was actually satisfied (0 for the first, 1 for the -second alternative, etc.). - - For example, suppose there are two opcodes for storing zero, `clrreg' -for registers and `clrmem' for memory locations. Here is how a pattern -could use `which_alternative' to choose between them: - - (define_insn "" - [(set (match_operand:SI 0 "general_operand" "=r,m") - (const_int 0))] - "" - "* - return (which_alternative == 0 - ? \"clrreg %0\" : \"clrmem %0\"); - ") - - The example above, where the assembler code to generate was *solely* -determined by the alternative, could also have been specified as -follows, having the output control string start with a `@': - - (define_insn "" - [(set (match_operand:SI 0 "general_operand" "=r,m") - (const_int 0))] - "" - "@ - clrreg %0 - clrmem %0") - - -File: gcc.info, Node: Constraints, Next: Standard Names, Prev: Output Statement, Up: Machine Desc - -Operand Constraints -=================== - - Each `match_operand' in an instruction pattern can specify a -constraint for the type of operands allowed. Constraints can say -whether an operand may be in a register, and which kinds of register; -whether the operand can be a memory reference, and which kinds of -address; whether the operand may be an immediate constant, and which -possible values it may have. Constraints can also require two operands -to match. + GNU C allows you to put a few global variables into specified +hardware registers. You can also specify the register in which an +ordinary register variable should be allocated. + + * Global register variables reserve registers throughout the program. + This may be useful in programs such as programming language + interpreters which have a couple of global variables that are + accessed very often. + + * Local register variables in specific registers do not reserve the + registers. The compiler's data flow analysis is capable of + determining where the specified registers contain live values, and + where they are available for other uses. + + These local variables are sometimes convenient for use with the + extended `asm' feature (*note Extended Asm::.), if you want to + write one output of the assembler instruction directly into a + particular register. (This will work provided the register you + specify fits the constraints specified for that operand in the + `asm'.) * Menu: -* Simple Constraints:: Basic use of constraints. -* Multi-Alternative:: When an insn has two alternative constraint-patterns. -* Class Preferences:: Constraints guide which hard register to put things in. -* Modifiers:: More precise control over effects of constraints. -* No Constraints:: Describing a clean machine without constraints. - - -File: gcc.info, Node: Simple Constraints, Next: Multi-Alternative, Prev: Constraints, Up: Constraints - -Simple Constraints ------------------- - - The simplest kind of constraint is a string full of letters, each of -which describes one kind of operand that is permitted. Here are the -letters that are allowed: - -`m' - A memory operand is allowed, with any kind of address that the - machine supports in general. - -`o' - A memory operand is allowed, but only if the address is - "offsettable". This means that adding a small integer (actually, - the width in bytes of the operand, as determined by its machine - mode) may be added to the address and the result is also a valid - memory address. - - For example, an address which is constant is offsettable; so is an - address that is the sum of a register and a constant (as long as a - slightly larger constant is also within the range of - address-offsets supported by the machine); but an autoincrement or - autodecrement address is not offsettable. More complicated - indirect/indexed addresses may or may not be offsettable depending - on the other addressing modes that the machine supports. - - Note that in an output operand which can be matched by another - operand, the constraint letter `o' is valid only when accompanied - by both `<' (if the target machine has predecrement addressing) - and `>' (if the target machine has preincrement addressing). - -`V' - A memory operand that is not offsettable. In other words, - anything that would fit the `m' constraint but not the `o' - constraint. - -`<' - A memory operand with autodecrement addressing (either - predecrement or postdecrement) is allowed. - -`>' - A memory operand with autoincrement addressing (either - preincrement or postincrement) is allowed. - -`r' - A register operand is allowed provided that it is in a general - register. - -`d', `a', `f', ... - Other letters can be defined in machine-dependent fashion to stand - for particular classes of registers. `d', `a' and `f' are defined - on the 68000/68020 to stand for data, address and floating point - registers. - -`i' - An immediate integer operand (one with constant value) is allowed. - This includes symbolic constants whose values will be known only at - assembly time. - -`n' - An immediate integer operand with a known numeric value is allowed. - Many systems cannot support assembly-time constants for operands - less than a word wide. Constraints for these operands should use - `n' rather than `i'. - -`I', `J', `K', ... `P' - Other letters in the range `I' through `P' may be defined in a - machine-dependent fashion to permit immediate integer operands with - explicit integer values in specified ranges. For example, on the - 68000, `I' is defined to stand for the range of values 1 to 8. - This is the range permitted as a shift count in the shift - instructions. - -`E' - An immediate floating operand (expression code `const_double') is - allowed, but only if the target floating point format is the same - as that of the host machine (on which the compiler is running). - -`F' - An immediate floating operand (expression code `const_double') is - allowed. - -`G', `H' - `G' and `H' may be defined in a machine-dependent fashion to - permit immediate floating operands in particular ranges of values. - -`s' - An immediate integer operand whose value is not an explicit - integer is allowed. - - This might appear strange; if an insn allows a constant operand - with a value not known at compile time, it certainly must allow - any known value. So why use `s' instead of `i'? Sometimes it - allows better code to be generated. - - For example, on the 68000 in a fullword instruction it is possible - to use an immediate operand; but if the immediate value is between - -128 and 127, better code results from loading the value into a - register and using the register. This is because the load into - the register can be done with a `moveq' instruction. We arrange - for this to happen by defining the letter `K' to mean "any integer - outside the range -128 to 127", and then specifying `Ks' in the - operand constraints. - -`g' - Any register, memory or immediate integer operand is allowed, - except for registers that are not general registers. - -`X' - Any operand whatsoever is allowed, even if it does not satisfy - `general_operand'. This is normally used in the constraint of a - `match_scratch' when certain alternatives will not actually - require a scratch register. - -`0', `1', `2', ... `9' - An operand that matches the specified operand number is allowed. - If a digit is used together with letters within the same - alternative, the digit should come last. - - This is called a "matching constraint" and what it really means is - that the assembler has only a single operand that fills two roles - considered separate in the RTL insn. For example, an add insn has - two input operands and one output operand in the RTL, but on most - CISC machines an add instruction really has only two operands, one - of them an input-output operand: - - addl #35,r12 - - Matching constraints are used in these circumstances. More - precisely, the two operands that match must include one input-only - operand and one output-only operand. Moreover, the digit must be a - smaller number than the number of the operand that uses it in the - constraint. - - For operands to match in a particular case usually means that they - are identical-looking RTL expressions. But in a few special cases - specific kinds of dissimilarity are allowed. For example, `*x' as - an input operand will match `*x++' as an output operand. For - proper results in such cases, the output template should always - use the output-operand's number when printing the operand. - -`p' - An operand that is a valid memory address is allowed. This is for - "load address" and "push address" instructions. - - `p' in the constraint must be accompanied by `address_operand' as - the predicate in the `match_operand'. This predicate interprets - the mode specified in the `match_operand' as the mode of the memory - reference for which the address would be valid. - -`Q', `R', `S', ... `U' - Letters in the range `Q' through `U' may be defined in a - machine-dependent fashion to stand for arbitrary operand types. - The machine description macro `EXTRA_CONSTRAINT' is passed the - operand as its first argument and the constraint letter as its - second operand. - - A typical use for this would be to distinguish certain types of - memory references that affect other insn operands. - - Do not define these constraint letters to accept register - references (`reg'); the reload pass does not expect this and would - not handle it properly. - - In order to have valid assembler code, each operand must satisfy its -constraint. But a failure to do so does not prevent the pattern from -applying to an insn. Instead, it directs the compiler to modify the -code so that the constraint will be satisfied. Usually this is done by -copying an operand into a register. - - Contrast, therefore, the two instruction patterns that follow: - - (define_insn "" - [(set (match_operand:SI 0 "general_operand" "=r") - (plus:SI (match_dup 0) - (match_operand:SI 1 "general_operand" "r")))] - "" - "...") - -which has two operands, one of which must appear in two places, and - - (define_insn "" - [(set (match_operand:SI 0 "general_operand" "=r") - (plus:SI (match_operand:SI 1 "general_operand" "0") - (match_operand:SI 2 "general_operand" "r")))] - "" - "...") - -which has three operands, two of which are required by a constraint to -be identical. If we are considering an insn of the form - - (insn N PREV NEXT - (set (reg:SI 3) - (plus:SI (reg:SI 6) (reg:SI 109))) - ...) - -the first pattern would not apply at all, because this insn does not -contain two identical subexpressions in the right place. The pattern -would say, "That does not look like an add instruction; try other -patterns." The second pattern would say, "Yes, that's an add -instruction, but there is something wrong with it." It would direct -the reload pass of the compiler to generate additional insns to make -the constraint true. The results might look like this: - - (insn N2 PREV N - (set (reg:SI 3) (reg:SI 6)) - ...) +* Global Reg Vars:: +* Local Reg Vars:: + + +File: gcc.info, Node: Global Reg Vars, Next: Local Reg Vars, Up: Explicit Reg Vars + +Defining Global Register Variables +---------------------------------- + + You can define a global register variable in GNU C like this: + + register int *foo asm ("a5"); + +Here `a5' is the name of the register which should be used. Choose a +register which is normally saved and restored by function calls on your +machine, so that library routines will not clobber it. + + Naturally the register name is cpu-dependent, so you would need to +conditionalize your program according to cpu type. The register `a5' +would be a good choice on a 68000 for a variable of pointer type. On +machines with register windows, be sure to choose a "global" register +that is not affected magically by the function call mechanism. + + In addition, operating systems on one type of cpu may differ in how +they name the registers; then you would need additional conditionals. +For example, some 68000 operating systems call this register `%a5'. + + Eventually there may be a way of asking the compiler to choose a +register automatically, but first we need to figure out how it should +choose and how to enable you to guide the choice. No solution is +evident. + + Defining a global register variable in a certain register reserves +that register entirely for this use, at least within the current +compilation. The register will not be allocated for any other purpose +in the functions in the current compilation. The register will not be +saved and restored by these functions. Stores into this register are +never deleted even if they would appear to be dead, but references may +be deleted or moved or simplified. + + It is not safe to access the global register variables from signal +handlers, or from more than one thread of control, because the system +library routines may temporarily use the register for other things +(unless you recompile them specially for the task at hand). + + It is not safe for one function that uses a global register variable +to call another such function `foo' by way of a third function `lose' +that was compiled without knowledge of this variable (i.e. in a +different source file in which the variable wasn't declared). This is +because `lose' might save the register and put some other value there. +For example, you can't expect a global register variable to be +available in the comparison-function that you pass to `qsort', since +`qsort' might have put something else in that register. (If you are +prepared to recompile `qsort' with the same global register variable, +you can solve this problem.) + + If you want to recompile `qsort' or other source files which do not +actually use your global register variable, so that they will not use +that register for any other purpose, then it suffices to specify the +compiler option `-ffixed-REG'. You need not actually add a global +register declaration to their source code. + + A function which can alter the value of a global register variable +cannot safely be called from a function compiled without this variable, +because it could clobber the value the caller expects to find there on +return. Therefore, the function which is the entry point into the part +of the program that uses the global register variable must explicitly +save and restore the value which belongs to its caller. + + On most machines, `longjmp' will restore to each global register +variable the value it had at the time of the `setjmp'. On some +machines, however, `longjmp' will not change the value of global +register variables. To be portable, the function that called `setjmp' +should make other arrangements to save the values of the global register +variables, and to restore them in a `longjmp'. This way, the same +thing will happen regardless of what `longjmp' does. + + All global register variable declarations must precede all function +definitions. If such a declaration could appear after function +definitions, the declaration would be too late to prevent the register +from being used for other purposes in the preceding functions. + + Global register variables may not have initial values, because an +executable file has no means to supply initial contents for a register. + + On the Sparc, there are reports that g3 ... g7 are suitable +registers, but certain library functions, such as `getwd', as well as +the subroutines for division and remainder, modify g3 and g4. g1 and +g2 are local temporaries. + + On the 68000, a2 ... a5 should be suitable, as should d2 ... d7. Of +course, it will not do to use more than a few of those. + + +File: gcc.info, Node: Local Reg Vars, Prev: Global Reg Vars, Up: Explicit Reg Vars + +Specifying Registers for Local Variables +---------------------------------------- + + You can define a local register variable with a specified register +like this: + + register int *foo asm ("a5"); + +Here `a5' is the name of the register which should be used. Note that +this is the same syntax used for defining global register variables, +but for a local variable it would appear within a function. + + Naturally the register name is cpu-dependent, but this is not a +problem, since specific registers are most often useful with explicit +assembler instructions (*note Extended Asm::.). Both of these things +generally require that you conditionalize your program according to cpu +type. + + In addition, operating systems on one type of cpu may differ in how +they name the registers; then you would need additional conditionals. +For example, some 68000 operating systems call this register `%a5'. + + Eventually there may be a way of asking the compiler to choose a +register automatically, but first we need to figure out how it should +choose and how to enable you to guide the choice. No solution is +evident. + + Defining such a register variable does not reserve the register; it +remains available for other uses in places where flow control determines +the variable's value is not live. However, these registers are made +unavailable for use in the reload pass. I would not be surprised if +excessive use of this feature leaves the compiler too few available +registers to compile certain functions. + + +File: gcc.info, Node: Alternate Keywords, Next: Incomplete Enums, Prev: Explicit Reg Vars, Up: C Extensions + +Alternate Keywords +================== + + The option `-traditional' disables certain keywords; `-ansi' +disables certain others. This causes trouble when you want to use GNU C +extensions, or ANSI C features, in a general-purpose header file that +should be usable by all programs, including ANSI C programs and +traditional ones. The keywords `asm', `typeof' and `inline' cannot be +used since they won't work in a program compiled with `-ansi', while +the keywords `const', `volatile', `signed', `typeof' and `inline' won't +work in a program compiled with `-traditional'. + + The way to solve these problems is to put `__' at the beginning and +end of each problematical keyword. For example, use `__asm__' instead +of `asm', `__const__' instead of `const', and `__inline__' instead of +`inline'. + + Other C compilers won't accept these alternative keywords; if you +want to compile with another compiler, you can define the alternate +keywords as macros to replace them with the customary keywords. It +looks like this: + + #ifndef __GNUC__ + #define __asm__ asm + #endif + + `-pedantic' causes warnings for many GNU C extensions. You can +prevent such warnings within one expression by writing `__extension__' +before the expression. `__extension__' has no effect aside from this. + + +File: gcc.info, Node: Incomplete Enums, Next: Function Names, Prev: Alternate Keywords, Up: C Extensions + +Incomplete `enum' Types +======================= + + You can define an `enum' tag without specifying its possible values. +This results in an incomplete type, much like what you get if you write +`struct foo' without describing the elements. A later declaration +which does specify the possible values completes the type. + + You can't allocate variables or storage using the type while it is +incomplete. However, you can work with pointers to that type. + + This extension may not be very useful, but it makes the handling of +`enum' more consistent with the way `struct' and `union' are handled. + + This extension is not supported by GNU C++. + + +File: gcc.info, Node: Function Names, Prev: Incomplete Enums, Up: C Extensions + +Function Names as Strings +========================= + + GNU CC predefines two string variables to be the name of the current +function. The variable `__FUNCTION__' is the name of the function as +it appears in the source. The variable `__PRETTY_FUNCTION__' is the +name of the function pretty printed in a language specific fashion. + + These names are always the same in a C function, but in a C++ +function they may be different. For example, this program: + + extern "C" { + extern int printf (char *, ...); + } - (insn N N2 NEXT - (set (reg:SI 3) - (plus:SI (reg:SI 3) (reg:SI 109))) - ...) - - It is up to you to make sure that each operand, in each pattern, has -constraints that can handle any RTL expression that could be present for -that operand. (When multiple alternatives are in use, each pattern -must, for each possible combination of operand expressions, have at -least one alternative which can handle that combination of operands.) -The constraints don't need to *allow* any possible operand--when this is -the case, they do not constrain--but they must at least point the way to -reloading any possible operand so that it will fit. - - * If the constraint accepts whatever operands the predicate permits, - there is no problem: reloading is never necessary for this operand. - - For example, an operand whose constraints permit everything except - registers is safe provided its predicate rejects registers. - - An operand whose predicate accepts only constant values is safe - provided its constraints include the letter `i'. If any possible - constant value is accepted, then nothing less than `i' will do; if - the predicate is more selective, then the constraints may also be - more selective. - - * Any operand expression can be reloaded by copying it into a - register. So if an operand's constraints allow some kind of - register, it is certain to be safe. It need not permit all - classes of registers; the compiler knows how to copy a register - into another register of the proper class in order to make an - instruction valid. - - * A nonoffsettable memory reference can be reloaded by copying the - address into a register. So if the constraint uses the letter - `o', all memory references are taken care of. - - * A constant operand can be reloaded by allocating space in memory to - hold it as preinitialized data. Then the memory reference can be - used in place of the constant. So if the constraint uses the - letters `o' or `m', constant operands are not a problem. - - * If the constraint permits a constant and a pseudo register used in - an insn was not allocated to a hard register and is equivalent to - a constant, the register will be replaced with the constant. If - the predicate does not permit a constant and the insn is - re-recognized for some reason, the compiler will crash. Thus the - predicate must always recognize any objects allowed by the - constraint. - - If the operand's predicate can recognize registers, but the -constraint does not permit them, it can make the compiler crash. When -this operand happens to be a register, the reload pass will be stymied, -because it does not know how to copy a register temporarily into memory. - - -File: gcc.info, Node: Multi-Alternative, Next: Class Preferences, Prev: Simple Constraints, Up: Constraints - -Multiple Alternative Constraints --------------------------------- - - Sometimes a single instruction has multiple alternative sets of -possible operands. For example, on the 68000, a logical-or instruction -can combine register or an immediate value into memory, or it can -combine any kind of operand into a register; but it cannot combine one -memory location into another. - - These constraints are represented as multiple alternatives. An -alternative can be described by a series of letters for each operand. -The overall constraint for an operand is made from the letters for this -operand from the first alternative, a comma, the letters for this -operand from the second alternative, a comma, and so on until the last -alternative. Here is how it is done for fullword logical-or on the -68000: - - (define_insn "iorsi3" - [(set (match_operand:SI 0 "general_operand" "=m,d") - (ior:SI (match_operand:SI 1 "general_operand" "%0,0") - (match_operand:SI 2 "general_operand" "dKs,dmKs")))] - ...) - - The first alternative has `m' (memory) for operand 0, `0' for -operand 1 (meaning it must match operand 0), and `dKs' for operand 2. -The second alternative has `d' (data register) for operand 0, `0' for -operand 1, and `dmKs' for operand 2. The `=' and `%' in the -constraints apply to all the alternatives; their meaning is explained -in the next section (*note Class Preferences::.). - - If all the operands fit any one alternative, the instruction is -valid. Otherwise, for each alternative, the compiler counts how many -instructions must be added to copy the operands so that that -alternative applies. The alternative requiring the least copying is -chosen. If two alternatives need the same amount of copying, the one -that comes first is chosen. These choices can be altered with the `?' -and `!' characters: - -`?' - Disparage slightly the alternative that the `?' appears in, as a - choice when no alternative applies exactly. The compiler regards - this alternative as one unit more costly for each `?' that appears - in it. - -`!' - Disparage severely the alternative that the `!' appears in. This - alternative can still be used if it fits without reloading, but if - reloading is needed, some other alternative will be used. - - When an insn pattern has multiple alternatives in its constraints, -often the appearance of the assembler code is determined mostly by which -alternative was matched. When this is so, the C code for writing the -assembler code can use the variable `which_alternative', which is the -ordinal number of the alternative that was actually satisfied (0 for -the first, 1 for the second alternative, etc.). *Note Output -Statement::. - - -File: gcc.info, Node: Class Preferences, Next: Modifiers, Prev: Multi-Alternative, Up: Constraints - -Register Class Preferences --------------------------- - - The operand constraints have another function: they enable the -compiler to decide which kind of hardware register a pseudo register is -best allocated to. The compiler examines the constraints that apply to -the insns that use the pseudo register, looking for the -machine-dependent letters such as `d' and `a' that specify classes of -registers. The pseudo register is put in whichever class gets the most -"votes". The constraint letters `g' and `r' also vote: they vote in -favor of a general register. The machine description says which -registers are considered general. - - Of course, on some machines all registers are equivalent, and no -register classes are defined. Then none of this complexity is relevant. - - -File: gcc.info, Node: Modifiers, Next: No Constraints, Prev: Class Preferences, Up: Constraints - -Constraint Modifier Characters ------------------------------- - -`=' - Means that this operand is write-only for this instruction: the - previous value is discarded and replaced by output data. - -`+' - Means that this operand is both read and written by the - instruction. - - When the compiler fixes up the operands to satisfy the constraints, - it needs to know which operands are inputs to the instruction and - which are outputs from it. `=' identifies an output; `+' - identifies an operand that is both input and output; all other - operands are assumed to be input only. - -`&' - Means (in a particular alternative) that this operand is written - before the instruction is finished using the input operands. - Therefore, this operand may not lie in a register that is used as - an input operand or as part of any memory address. - - `&' applies only to the alternative in which it is written. In - constraints with multiple alternatives, sometimes one alternative - requires `&' while others do not. See, for example, the `movdf' - insn of the 68000. - - `&' does not obviate the need to write `='. - -`%' - Declares the instruction to be commutative for this operand and the - following operand. This means that the compiler may interchange - the two operands if that is the cheapest way to make all operands - fit the constraints. This is often used in patterns for addition - instructions that really have only two operands: the result must - go in one of the arguments. Here for example, is how the 68000 - halfword-add instruction is defined: - - (define_insn "addhi3" - [(set (match_operand:HI 0 "general_operand" "=m,r") - (plus:HI (match_operand:HI 1 "general_operand" "%0,0") - (match_operand:HI 2 "general_operand" "di,g")))] - ...) - -`#' - Says that all following characters, up to the next comma, are to be - ignored as a constraint. They are significant only for choosing - register preferences. - -`*' - Says that the following character should be ignored when choosing - register preferences. `*' has no effect on the meaning of the - constraint as a constraint, and no effect on reloading. - - Here is an example: the 68000 has an instruction to sign-extend a - halfword in a data register, and can also sign-extend a value by - copying it into an address register. While either kind of - register is acceptable, the constraints on an address-register - destination are less strict, so it is best if register allocation - makes an address register its goal. Therefore, `*' is used so - that the `d' constraint letter (for data register) is ignored when - computing register preferences. - - (define_insn "extendhisi2" - [(set (match_operand:SI 0 "general_operand" "=*d,a") - (sign_extend:SI - (match_operand:HI 1 "general_operand" "0,g")))] - ...) - - -File: gcc.info, Node: No Constraints, Prev: Modifiers, Up: Constraints - -Not Using Constraints ---------------------- - - Some machines are so clean that operand constraints are not -required. For example, on the Vax, an operand valid in one context is -valid in any other context. On such a machine, every operand -constraint would be `g', excepting only operands of "load address" -instructions which are written as if they referred to a memory -location's contents but actual refer to its address. They would have -constraint `p'. - - For such machines, instead of writing `g' and `p' for all the -constraints, you can choose to write a description with empty -constraints. Then you write `""' for the constraint in every -`match_operand'. Address operands are identified by writing an -`address' expression around the `match_operand', not by their -constraints. - - When the machine description has just empty constraints, certain -parts of compilation are skipped, making the compiler faster. However, -few machines actually do not need constraints; all machine descriptions -now in existence use constraints. + class a { + public: + sub (int i) + { + printf ("__FUNCTION__ = %s\n", __FUNCTION__); + printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__); + } + }; + + int + main (void) + { + a ax; + ax.sub (0); + return 0; + } + +gives this output: + + __FUNCTION__ = sub + __PRETTY_FUNCTION__ = int a::sub (int) + + These names are not macros: they are predefined string variables. +For example, `#ifdef __FUNCTION__' does not have any special meaning +inside a function, since the preprocessor does not do anything special +with the identifier `__FUNCTION__'. + + +File: gcc.info, Node: C++ Extensions, Next: Trouble, Prev: C Extensions, Up: Top + +Extensions to the C++ Language +****************************** + + The GNU compiler provides these extensions to the C++ language (and +you can also use most of the C language extensions in your C++ +programs). If you want to write code that checks whether these +features are available, you can test for the GNU compiler the same way +as for C programs: check for a predefined macro `__GNUC__'. You can +also use `__GNUG__' to test specifically for GNU C++ (*note Standard +Predefined Macros: (cpp.info)Standard Predefined.). + +* Menu: + +* Naming Results:: Giving a name to C++ function return values. +* Min and Max:: C++ Minimum and maximum operators. +* Destructors and Goto:: Goto is safe to use in C++ even when destructors + are needed. +* C++ Interface:: You can use a single C++ header file for both + declarations and definitions. +* Template Instantiation:: Methods for ensuring that exactly one copy of + each needed template instantiation is emitted. +* C++ Signatures:: You can specify abstract types to get subtype + polymorphism independent from inheritance. + + +File: gcc.info, Node: Naming Results, Next: Min and Max, Up: C++ Extensions + +Named Return Values in C++ +========================== + + GNU C++ extends the function-definition syntax to allow you to +specify a name for the result of a function outside the body of the +definition, in C++ programs: + + TYPE + FUNCTIONNAME (ARGS) return RESULTNAME; + { + ... + BODY + ... + } + + You can use this feature to avoid an extra constructor call when a +function result has a class type. For example, consider a function +`m', declared as `X v = m ();', whose result is of class `X': + + X + m () + { + X b; + b.a = 23; + return b; + } + + Although `m' appears to have no arguments, in fact it has one +implicit argument: the address of the return value. At invocation, the +address of enough space to hold `v' is sent in as the implicit argument. +Then `b' is constructed and its `a' field is set to the value 23. +Finally, a copy constructor (a constructor of the form `X(X&)') is +applied to `b', with the (implicit) return value location as the +target, so that `v' is now bound to the return value. + + But this is wasteful. The local `b' is declared just to hold +something that will be copied right out. While a compiler that +combined an "elision" algorithm with interprocedural data flow analysis +could conceivably eliminate all of this, it is much more practical to +allow you to assist the compiler in generating efficient code by +manipulating the return value explicitly, thus avoiding the local +variable and copy constructor altogether. + + Using the extended GNU C++ function-definition syntax, you can avoid +the temporary allocation and copying by naming `r' as your return value +at the outset, and assigning to its `a' field directly: + + X + m () return r; + { + r.a = 23; + } + +The declaration of `r' is a standard, proper declaration, whose effects +are executed *before* any of the body of `m'. + + Functions of this type impose no additional restrictions; in +particular, you can execute `return' statements, or return implicitly by +reaching the end of the function body ("falling off the edge"). Cases +like + + X + m () return r (23); + { + return; + } + +(or even `X m () return r (23); { }') are unambiguous, since the return +value `r' has been initialized in either case. The following code may +be hard to read, but also works predictably: + + X + m () return r; + { + X b; + return b; + } + + The return value slot denoted by `r' is initialized at the outset, +but the statement `return b;' overrides this value. The compiler deals +with this by destroying `r' (calling the destructor if there is one, or +doing nothing if there is not), and then reinitializing `r' with `b'. + + This extension is provided primarily to help people who use +overloaded operators, where there is a great need to control not just +the arguments, but the return values of functions. For classes where +the copy constructor incurs a heavy performance penalty (especially in +the common case where there is a quick default constructor), this is a +major savings. The disadvantage of this extension is that you do not +control when the default constructor for the return value is called: it +is always called at the beginning. + + +File: gcc.info, Node: Min and Max, Next: Destructors and Goto, Prev: Naming Results, Up: C++ Extensions + +Minimum and Maximum Operators in C++ +==================================== + + It is very convenient to have operators which return the "minimum" +or the "maximum" of two arguments. In GNU C++ (but not in GNU C), + +`A ? B' + is the "maximum", returning the larger of the numeric values A and + B. + + These operations are not primitive in ordinary C++, since you can +use a macro to return the minimum of two things in C++, as in the +following example. + + #define MIN(X,Y) ((X) < (Y) ? : (X) : (Y)) + +You might then use `int min = MIN (i, j);' to set MIN to the minimum +value of variables I and J. + + However, side effects in `X' or `Y' may cause unintended behavior. +For example, `MIN (i++, j++)' will fail, incrementing the smaller +counter twice. A GNU C extension allows you to write safe macros that +avoid this kind of problem (*note Naming an Expression's Type: Naming +Types.). However, writing `MIN' and `MAX' as macros also forces you to +use function-call notation notation for a fundamental arithmetic +operation. Using GNU C++ extensions, you can write `int min = i ?' are built into the compiler, they properly +handle expressions with side-effects; `int min = i++ ; + template ostream& operator << (ostream&, const A&); + + This strategy will work with code written for either model. If + you are using code written for the Cfront model, the file + containing a class template and the file containing its member + templates should be implemented in the same translation unit. + + A slight variation on this approach is to use the flag + -falt-external-templates instead; this flag causes template + instances to be emitted in the translation unit that implements + the header where they are first instantiated, rather than the one + which implements the file where the templates are defined. This + header must be the same in all translation units, or things are + likely to break. + + *Note Declarations and Definitions in One Header: C++ Interface, + for more discussion of these pragmas. + + 3. Explicitly instantiate all the template instances you use, and + compile with -fno-implicit-templates. This is probably your best + bet; it may require more knowledge of exactly which templates you + are using, but it's less mysterious than the previous approach, + and it doesn't require any `#pragma's or other g++-specific code. + You can scatter the instantiations throughout your program, you + can create one big file to do all the instantiations, or you can + create tiny files like + + #include "Foo.h" + #include "Foo.cc" + + template class Foo; + + for each instance you need, and create a template instantiation + library from those. I'm partial to the last, but your mileage may + vary. If you are using Cfront-model code, you can probably get + away with not using -fno-implicit-templates when compiling files + that don't `#include' the member template definitions. + + +File: gcc.info, Node: C++ Signatures, Prev: Template Instantiation, Up: C++ Extensions + +Type Abstraction using Signatures +================================= + + In GNU C++, you can use the keyword `signature' to define a +completely abstract class interface as a datatype. You can connect this +abstraction with actual classes using signature pointers. If you want +to use signatures, run the GNU compiler with the `-fhandle-signatures' +command-line option. (With this option, the compiler reserves a second +keyword `sigof' as well, for a future extension.) + + Roughly, signatures are type abstractions or interfaces of classes. +Some other languages have similar facilities. C++ signatures are +related to ML's signatures, Haskell's type classes, definition modules +in Modula-2, interface modules in Modula-3, abstract types in Emerald, +type modules in Trellis/Owl, categories in Scratchpad II, and types in +POOL-I. For a more detailed discussion of signatures, see `Signatures: +A Language Extension for Improving Type Abstraction and Subtype +Polymorphism in C++' by Gerald Baumgartner and Vincent F. Russo (Tech +report CSD-TR-95-051, Dept. of Computer Sciences, Purdue University, +August 1995, a slightly improved version appeared in +*Software--Practice & Experience*, 25(8), pp. 863-889, August 1995). +You can get the tech report by anonymous FTP from `ftp.cs.purdue.edu' +in `pub/gb/Signature-design.ps.gz'. + + Syntactically, a signature declaration is a collection of member +function declarations and nested type declarations. For example, this +signature declaration defines a new abstract type `S' with member +functions `int foo ()' and `int bar (int)': + + signature S + { + int foo (); + int bar (int); + }; + + Since signature types do not include implementation definitions, you +cannot write an instance of a signature directly. Instead, you can +define a pointer to any class that contains the required interfaces as a +"signature pointer". Such a class "implements" the signature type. + + To use a class as an implementation of `S', you must ensure that the +class has public member functions `int foo ()' and `int bar (int)'. +The class can have other member functions as well, public or not; as +long as it offers what's declared in the signature, it is suitable as +an implementation of that signature type. + + For example, suppose that `C' is a class that meets the requirements +of signature `S' (`C' "conforms to" `S'). Then + + C obj; + S * p = &obj; + +defines a signature pointer `p' and initializes it to point to an +object of type `C'. The member function call `int i = p->foo ();' +executes `obj.foo ()'. + + Abstract virtual classes provide somewhat similar facilities in +standard C++. There are two main advantages to using signatures +instead: + + 1. Subtyping becomes independent from inheritance. A class or + signature type `T' is a subtype of a signature type `S' + independent of any inheritance hierarchy as long as all the member + functions declared in `S' are also found in `T'. So you can + define a subtype hierarchy that is completely independent from any + inheritance (implementation) hierarchy, instead of being forced to + use types that mirror the class inheritance hierarchy. + + 2. Signatures allow you to work with existing class hierarchies as + implementations of a signature type. If those class hierarchies + are only available in compiled form, you're out of luck with + abstract virtual classes, since an abstract virtual class cannot + be retrofitted on top of existing class hierarchies. So you would + be required to write interface classes as subtypes of the abstract + virtual class. + + There is one more detail about signatures. A signature declaration +can contain member function *definitions* as well as member function +declarations. A signature member function with a full definition is +called a *default implementation*; classes need not contain that +particular interface in order to conform. For example, a class `C' can +conform to the signature + + signature T + { + int f (int); + int f0 () { return f (0); }; + }; + +whether or not `C' implements the member function `int f0 ()'. If you +define `C::f0', that definition takes precedence; otherwise, the +default implementation `S::f0' applies. + + +File: gcc.info, Node: Trouble, Next: Bugs, Prev: C++ Extensions, Up: Top + +Known Causes of Trouble with GNU CC +*********************************** + + This section describes known problems that affect users of GNU CC. +Most of these are not GNU CC bugs per se--if they were, we would fix +them. But the result for a user may be like the result of a bug. + + Some of these problems are due to bugs in other software, some are +missing features that are too much work to add, and some are places +where people's opinions differ as to what is best. + +* Menu: + +* Actual Bugs:: Bugs we will fix later. +* Installation Problems:: Problems that manifest when you install GNU CC. +* Cross-Compiler Problems:: Common problems of cross compiling with GNU CC. +* Interoperation:: Problems using GNU CC with other compilers, + and with certain linkers, assemblers and debuggers. +* External Bugs:: Problems compiling certain programs. +* Incompatibilities:: GNU CC is incompatible with traditional C. +* Fixed Headers:: GNU C uses corrected versions of system header files. + This is necessary, but doesn't always work smoothly. +* Standard Libraries:: GNU C uses the system C library, which might not be + compliant with the ISO/ANSI C standard. +* Disappointments:: Regrettable things we can't change, but not quite bugs. +* C++ Misunderstandings:: Common misunderstandings with GNU C++. +* Protoize Caveats:: Things to watch out for when using `protoize'. +* Non-bugs:: Things we think are right, but some others disagree. +* Warnings and Errors:: Which problems in your code get warnings, + and which get errors. + + +File: gcc.info, Node: Actual Bugs, Next: Installation Problems, Up: Trouble + +Actual Bugs We Haven't Fixed Yet +================================ + + * The `fixincludes' script interacts badly with automounters; if the + directory of system header files is automounted, it tends to be + unmounted while `fixincludes' is running. This would seem to be a + bug in the automounter. We don't know any good way to work around + it. + + * The `fixproto' script will sometimes add prototypes for the + `sigsetjmp' and `siglongjmp' functions that reference the + `jmp_buf' type before that type is defined. To work around this, + edit the offending file and place the typedef in front of the + prototypes. + + * There are several obscure case of mis-using struct, union, and + enum tags that are not detected as errors by the compiler. + + * When `-pedantic-errors' is specified, GNU C will incorrectly give + an error message when a function name is specified in an expression + involving the comma operator. + + * Loop unrolling doesn't work properly for certain C++ programs. + This is a bug in the C++ front end. It sometimes emits incorrect + debug info, and the loop unrolling code is unable to recover from + this error. - \ No newline at end of file