--- gcc/gcc.info-11 2018/04/24 17:56:44 1.1.1.3 +++ gcc/gcc.info-11 2018/04/24 18:18:44 1.1.1.7 @@ -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 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 @@ -11,1043 +15,814 @@ 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: Standard Names, Next: Pattern Ordering, Prev: Constraints, Up: Machine Desc +File: gcc.info, Node: Bug Reporting, Next: Sending Patches, Prev: Bug Lists, Up: Bugs -Standard Names for Patterns Used in Generation -============================================== +How to Report Bugs +================== - Here is a table of the instruction names that are meaningful in the -RTL generation pass of the compiler. Giving one of these names to an -instruction pattern tells the RTL generation pass that it can use the -pattern in to accomplish a certain task. - -`movM' - Here M stands for a two-letter machine mode name, in lower case. - This instruction pattern moves data with that machine mode from - operand 1 to operand 0. For example, `movsi' moves full-word data. - - If operand 0 is a `subreg' with mode M of a register whose own - mode is wider than M, the effect of this instruction is to store - the specified value in the part of the register that corresponds - to mode M. The effect on the rest of the register is undefined. - - This class of patterns is special in several ways. First of all, - each of these names *must* be defined, because there is no other - way to copy a datum from one place to another. - - Second, these patterns are not used solely in the RTL generation - pass. Even the reload pass can generate move insns to copy values - from stack slots into temporary registers. When it does so, one - of the operands is a hard register and the other is an operand - that can need to be reloaded into a register. - - Therefore, when given such a pair of operands, the pattern must - generate RTL which needs no reloading and needs no temporary - registers--no registers other than the operands. For example, if - you support the pattern with a `define_expand', then in such a - case the `define_expand' mustn't call `force_reg' or any other such - function which might generate new pseudo registers. - - This requirement exists even for subword modes on a RISC machine - where fetching those modes from memory normally requires several - insns and some temporary registers. Look in `spur.md' to see how - the requirement can be satisfied. - - During reload a memory reference with an invalid address may be - passed as an operand. Such an address will be replaced with a - valid address later in the reload pass. In this case, nothing may - be done with the address except to use it as it stands. If it is - copied, it will not be replaced with a valid address. No attempt - should be made to make such an address into a valid address and no - routine (such as `change_address') that will do so may be called. - Note that `general_operand' will fail when applied to such an - address. - - The global variable `reload_in_progress' (which must be explicitly - declared if required) can be used to determine whether such special - handling is required. - - The variety of operands that have reloads depends on the rest of - the machine description, but typically on a RISC machine these can - only be pseudo registers that did not get hard registers, while on - other machines explicit memory references will get optional - reloads. - - If a scratch register is required to move an object to or from - memory, it can be allocated using `gen_reg_rtx' prior to reload. - But this is impossible during and after reload. If there are - cases needing scratch registers after reload, you must define - `SECONDARY_INPUT_RELOAD_CLASS' and/or - `SECONDARY_OUTPUT_RELOAD_CLASS' to detect them, and provide - patterns `reload_inM' or `reload_outM' to handle them. *Note - Register Classes::. - - The constraints on a `moveM' must permit moving any hard register - to any other hard register provided that `HARD_REGNO_MODE_OK' - permits mode M in both registers and `REGISTER_MOVE_COST' applied - to their classes returns a value of 2. - - It is obligatory to support floating point `moveM' instructions - into and out of any registers that can hold fixed point values, - because unions and structures (which have modes `SImode' or - `DImode') can be in those registers and they may have floating - point members. - - There may also be a need to support fixed point `moveM' - instructions in and out of floating point registers. - Unfortunately, I have forgotten why this was so, and I don't know - whether it is still true. If `HARD_REGNO_MODE_OK' rejects fixed - point values in floating point registers, then the constraints of - the fixed point `moveM' instructions must be designed to avoid - ever trying to reload into a floating point register. - -`reload_inM' -`reload_outM' - Like `movM', but used when a scratch register is required to move - between operand 0 and operand 1. Operand 2 describes the scratch - register. See the discussion of the `SECONDARY_RELOAD_CLASS' - macro in *note Register Classes::.. - -`movstrictM' - Like `movM' except that if operand 0 is a `subreg' with mode M of - a register whose natural mode is wider, the `movstrictM' - instruction is guaranteed not to alter any of the register except - the part which belongs to mode M. - -`load_multiple' - Load several consecutive memory locations into consecutive - registers. Operand 0 is the first of the consecutive registers, - operand 1 is the first memory location, and operand 2 is a - constant: the number of consecutive registers. - - Define this only if the target machine really has such an - instruction; do not define this if the most efficient way of - loading consecutive registers from memory is to do them one at a - time. - - On some machines, there are restrictions as to which consecutive - registers can be stored into memory, such as particular starting or - ending register numbers or only a range of valid counts. For those - machines, use a `define_expand' (*note Expander Definitions::.) - and make the pattern fail if the restrictions are not met. - - Write the generated insn as a `parallel' with elements being a - `set' of one register from the appropriate memory location (you may - also need `use' or `clobber' elements). Use a `match_parallel' - (*note RTL Template::.) to recognize the insn. See `a29k.md' and - `rs6000.md' for examples of the use of this insn pattern. - -`store_multiple' - Similar to `load_multiple', but store several consecutive registers - into consecutive memory locations. Operand 0 is the first of the - consecutive memory locations, operand 1 is the first register, and - operand 2 is a constant: the number of consecutive registers. - -`addM3' - Add operand 2 and operand 1, storing the result in operand 0. All - operands must have mode M. This can be used even on two-address - machines, by means of constraints requiring operands 1 and 0 to be - the same location. - -`subM3', `mulM3' -`divM3', `udivM3', `modM3', `umodM3' -`sminM3', `smaxM3', `uminM3', `umaxM3' -`andM3', `iorM3', `xorM3' - Similar, for other arithmetic operations. - -`mulhisi3' - Multiply operands 1 and 2, which have mode `HImode', and store a - `SImode' product in operand 0. - -`mulqihi3', `mulsidi3' - Similar widening-multiplication instructions of other widths. - -`umulqihi3', `umulhisi3', `umulsidi3' - Similar widening-multiplication instructions that do unsigned - multiplication. - -`divmodM4' - Signed division that produces both a quotient and a remainder. - Operand 1 is divided by operand 2 to produce a quotient stored in - operand 0 and a remainder stored in operand 3. - - For machines with an instruction that produces both a quotient and - a remainder, provide a pattern for `divmodM4' but do not provide - patterns for `divM3' and `modM3'. This allows optimization in the - relatively common case when both the quotient and remainder are - computed. - - If an instruction that just produces a quotient or just a remainder - exists and is more efficient than the instruction that produces - both, write the output routine of `divmodM4' to call - `find_reg_note' and look for a `REG_UNUSED' note on the quotient - or remainder and generate the appropriate instruction. - -`udivmodM4' - Similar, but does unsigned division. - -`ashlM3' - Arithmetic-shift operand 1 left by a number of bits specified by - operand 2, and store the result in operand 0. Here M is the mode - of operand 0 and operand 1; operand 2's mode is specified by the - instruction pattern, and the compiler will convert the operand to - that mode before generating the instruction. - -`ashrM3', `lshlM3', `lshrM3', `rotlM3', `rotrM3' - Other shift and rotate instructions, analogous to the `ashlM3' - instructions. - - Logical and arithmetic left shift are the same. Machines that do - not allow negative shift counts often have only one instruction for - shifting left. On such machines, you should define a pattern named - `ashlM3' and leave `lshlM3' undefined. - -`negM2' - Negate operand 1 and store the result in operand 0. - -`absM2' - Store the absolute value of operand 1 into operand 0. - -`sqrtM2' - Store the square root of operand 1 into operand 0. - - The `sqrt' built-in function of C always uses the mode which - corresponds to the C data type `double'. - -`ffsM2' - Store into operand 0 one plus the index of the least significant - 1-bit of operand 1. If operand 1 is zero, store zero. M is the - mode of operand 0; operand 1's mode is specified by the instruction - pattern, and the compiler will convert the operand to that mode - before generating the instruction. - - The `ffs' built-in function of C always uses the mode which - corresponds to the C data type `int'. - -`one_cmplM2' - Store the bitwise-complement of operand 1 into operand 0. - -`cmpM' - Compare operand 0 and operand 1, and set the condition codes. The - RTL pattern should look like this: - - (set (cc0) (compare (match_operand:M 0 ...) - (match_operand:M 1 ...))) - -`tstM' - Compare operand 0 against zero, and set the condition codes. The - RTL pattern should look like this: - - (set (cc0) (match_operand:M 0 ...)) - - `tstM' patterns should not be defined for machines that do not use - `(cc0)'. Doing so would confuse the optimizer since it would no - longer be clear which `set' operations were comparisons. The - `cmpM' patterns should be used instead. - -`movstrM' - Block move instruction. The addresses of the destination and - source strings are the first two operands, and both are in mode - `Pmode'. The number of bytes to move is the third operand, in mode - M. - - The fourth operand is the known shared alignment of the source and - destination, in the form of a `const_int' rtx. Thus, if the - compiler knows that both source and destination are word-aligned, - it may provide the value 4 for this operand. - - These patterns need not give special consideration to the - possibility that the source and destination strings might overlap. - -`cmpstrM' - Block compare instruction, with five operands. Operand 0 is the - output; it has mode M. The remaining four operands are like the - operands of `movstrM'. The two memory blocks specified are - compared byte by byte in lexicographic order. The effect of the - instruction is to store a value in operand 0 whose sign indicates - the result of the comparison. - -`floatMN2' - Convert signed integer operand 1 (valid for fixed point mode M) to - floating point mode N and store in operand 0 (which has mode N). - -`floatunsMN2' - Convert unsigned integer operand 1 (valid for fixed point mode M) - to floating point mode N and store in operand 0 (which has mode N). - -`fixMN2' - Convert operand 1 (valid for floating point mode M) to fixed point - mode N as a signed number and store in operand 0 (which has mode - N). This instruction's result is defined only when the value of - operand 1 is an integer. - -`fixunsMN2' - Convert operand 1 (valid for floating point mode M) to fixed point - mode N as an unsigned number and store in operand 0 (which has - mode N). This instruction's result is defined only when the value - of operand 1 is an integer. - -`ftruncM2' - Convert operand 1 (valid for floating point mode M) to an integer - value, still represented in floating point mode M, and store it in - operand 0 (valid for floating point mode M). - -`fix_truncMN2' - Like `fixMN2' but works for any floating point value of mode M by - converting the value to an integer. - -`fixuns_truncMN2' - Like `fixunsMN2' but works for any floating point value of mode M - by converting the value to an integer. - -`truncMN' - Truncate operand 1 (valid for mode M) to mode N and store in - operand 0 (which has mode N). Both modes must be fixed point or - both floating point. - -`extendMN' - Sign-extend operand 1 (valid for mode M) to mode N and store in - operand 0 (which has mode N). Both modes must be fixed point or - both floating point. - -`zero_extendMN' - Zero-extend operand 1 (valid for mode M) to mode N and store in - operand 0 (which has mode N). Both modes must be fixed point. - -`extv' - Extract a bit field from operand 1 (a register or memory operand), - where operand 2 specifies the width in bits and operand 3 the - starting bit, and store it in operand 0. Operand 0 must have mode - `word_mode'. Operand 1 may have mode `byte_mode' or `word_mode'; - often `word_mode' is allowed only for registers. Operands 2 and 3 - must be valid for `word_mode'. - - The RTL generation pass generates this instruction only with - constants for operands 2 and 3. - - The bit-field value is sign-extended to a full word integer before - it is stored in operand 0. - -`extzv' - Like `extv' except that the bit-field value is zero-extended. - -`insv' - Store operand 3 (which must be valid for `word_mode') into a bit - field in operand 0, where operand 1 specifies the width in bits and - operand 2 the starting bit. Operand 0 may have mode `byte_mode' or - `word_mode'; often `word_mode' is allowed only for registers. - Operands 1 and 2 must be valid for `word_mode'. - - The RTL generation pass generates this instruction only with - constants for operands 1 and 2. - -`sCOND' - Store zero or nonzero in the operand according to the condition - codes. Value stored is nonzero iff the condition COND is true. - COND is the name of a comparison operation expression code, such - as `eq', `lt' or `leu'. - - You specify the mode that the operand must have when you write the - `match_operand' expression. The compiler automatically sees which - mode you have used and supplies an operand of that mode. - - The value stored for a true condition must have 1 as its low bit, - or else must be negative. Otherwise the instruction is not - suitable and you should omit it from the machine description. You - describe to the compiler exactly which value is stored by defining - the macro `STORE_FLAG_VALUE' (*note Misc::.). If a description - cannot be found that can be used for all the `sCOND' patterns, you - should omit those operations from the machine description. - - These operations may fail, but should do so only in relatively - uncommon cases; if they would fail for common cases involving - integer comparisons, it is best to omit these patterns. - - If these operations are omitted, the compiler will usually - generate code that copies the constant one to the target and - branches around an assignment of zero to the target. If this code - is more efficient than the potential instructions used for the - `sCOND' pattern followed by those required to convert the result - into a 1 or a zero in `SImode', you should omit the `sCOND' - operations from the machine description. - -`bCOND' - Conditional branch instruction. Operand 0 is a `label_ref' that - refers to the label to jump to. Jump if the condition codes meet - condition COND. - - Some machines do not follow the model assumed here where a - comparison instruction is followed by a conditional branch - instruction. In that case, the `cmpM' (and `tstM') patterns should - simply store the operands away and generate all the required insns - in a `define_expand' (*note Expander Definitions::.) for the - conditional branch operations. All calls to expand `vCOND' - patterns are immediately preceded by calls to expand either a - `cmpM' pattern or a `tstM' pattern. - - Machines that use a pseudo register for the condition code value, - or where the mode used for the comparison depends on the condition - being tested, should also use the above mechanism. *Note Jump - Patterns:: - - The above discussion also applies to `sCOND' patterns. - -`call' - Subroutine call instruction returning no value. Operand 0 is the - function to call; operand 1 is the number of bytes of arguments - pushed (in mode `SImode', except it is normally a `const_int'); - operand 2 is the number of registers used as operands. - - On most machines, operand 2 is not actually stored into the RTL - pattern. It is supplied for the sake of some RISC machines which - need to put this information into the assembler code; they can put - it in the RTL instead of operand 1. - - Operand 0 should be a `mem' RTX whose address is the address of the - function. Note, however, that this address can be a `symbol_ref' - expression even if it would not be a legitimate memory address on - the target machine. If it is also not a valid argument for a call - instruction, the pattern for this operation should be a - `define_expand' (*note Expander Definitions::.) that places the - address into a register and uses that register in the call - instruction. - -`call_value' - Subroutine call instruction returning a value. Operand 0 is the - hard register in which the value is returned. There are three more - operands, the same as the three operands of the `call' instruction - (but with numbers increased by one). - - Subroutines that return `BLKmode' objects use the `call' insn. - -`call_pop', `call_value_pop' - Similar to `call' and `call_value', except used if defined and if - `RETURN_POPS_ARGS' is non-zero. They should emit a `parallel' - that contains both the function call and a `set' to indicate the - adjustment made to the frame pointer. - - For machines where `RETURN_POPS_ARGS' can be non-zero, the use of - these patterns increases the number of functions for which the - frame pointer can be eliminated, if desired. - -`return' - Subroutine return instruction. This instruction pattern name - should be defined only if a single instruction can do all the work - of returning from a function. - - Like the `movM' patterns, this pattern is also used after the RTL - generation phase. In this case it is to support machines where - multiple instructions are usually needed to return from a - function, but some class of functions only requires one - instruction to implement a return. Normally, the applicable - functions are those which do not need to save any registers or - allocate stack space. - - For such machines, the condition specified in this pattern should - only be true when `reload_completed' is non-zero and the function's - epilogue would only be a single instruction. For machines with - register windows, the routine `leaf_function_p' may be used to - determine if a register window push is required. - - Machines that have conditional return instructions should define - patterns such as - - (define_insn "" - [(set (pc) - (if_then_else (match_operator 0 "comparison_operator" - [(cc0) (const_int 0)]) - (return) - (pc)))] - "CONDITION" - "...") - - where CONDITION would normally be the same condition specified on - the named `return' pattern. - -`nop' - No-op instruction. This instruction pattern name should always be - defined to output a no-op in assembler code. `(const_int 0)' will - do as an RTL pattern. - -`indirect_jump' - An instruction to jump to an address which is operand zero. This - pattern name is mandatory on all machines. - -`casesi' - Instruction to jump through a dispatch table, including bounds - checking. This instruction takes five operands: - - 1. The index to dispatch on, which has mode `SImode'. - - 2. The lower bound for indices in the table, an integer constant. - - 3. The total range of indices in the table--the largest index - minus the smallest one (both inclusive). - - 4. A label that precedes the table itself. - - 5. A label to jump to if the index has a value outside the - bounds. (If the machine-description macro - `CASE_DROPS_THROUGH' is defined, then an out-of-bounds index - drops through to the code following the jump table instead of - jumping to this label. In that case, this label is not - actually used by the `casesi' instruction, but it is always - provided as an operand.) - - The table is a `addr_vec' or `addr_diff_vec' inside of a - `jump_insn'. The number of elements in the table is one plus the - difference between the upper bound and the lower bound. - -`tablejump' - Instruction to jump to a variable address. This is a low-level - capability which can be used to implement a dispatch table when - there is no `casesi' pattern. - - This pattern requires two operands: the address or offset, and a - label which should immediately precede the jump table. If the - macro `CASE_VECTOR_PC_RELATIVE' is defined then the first operand - is an offset which counts from the address of the table; - otherwise, it is an absolute address to jump to. In either case, - the first operand has mode `Pmode'. - - The `tablejump' insn is always the last insn before the jump table - it uses. Its assembler code normally has no need to use the - second operand, but you should incorporate it in the RTL pattern so - that the jump optimizer will not delete the table as unreachable - code. - -`save_stack_block' -`save_stack_function' -`save_stack_nonlocal' -`restore_stack_block' -`restore_stack_function' -`restore_stack_nonlocal' - Most machines save and restore the stack pointer by copying it to - or from an object of mode `Pmode'. Do not define these patterns on - such machines. - - Some machines require special handling for stack pointer saves and - restores. On those machines, define the patterns corresponding to - the non-standard cases by using a `define_expand' (*note Expander - Definitions::.) that produces the required insns. The three types - of saves and restores are: - - 1. `save_stack_block' saves the stack pointer at the start of a - block that allocates a variable-sized object and - `restore_stack_block' restores the stack pointer when the - block is exited. - - 2. `save_stack_function' and `restore_stack_function' operate - similarly for the outermost block of a function and are used - when the function allocates variable-sized objects or calls - `alloca'. Only the epilogue uses the restored stack pointer, - allowing a simpler save or restore sequence on some machines. - - 3. `save_stack_nonlocal' is used in functions that contain labels - branched to by nested functions. It saves the stack pointer - in such a way that the inner function can use - `restore_stack_nonlocal' to restore the stack pointer. The - compiler generates code to restore the frame and argument - pointer registers, but some machines require saving and - restoring additional data such as register window information - or stack backchains. Place insns in these patterns to save - and restore any such required data. - - When saving the stack pointer, operand 0 is the save area and - operand 1 is the stack pointer. The mode used to allocate the - save area is the mode of operand 0. You must specify an integral - mode, or `VOIDmode' if no save area is needed for a particular - type of save (either because no save is needed or because a - machine-specific save area can be used). Operand 0 is the stack - pointer and operand 1 is the save area for restore operations. If - `save_stack_block' is defined, operand 0 must not be `VOIDmode' - since these saves can be arbitrarily nested. - - A save area is a `mem' that is at a constant offset from - `virtual_stack_vars_rtx' when the stack pointer is saved for use by - nonlocal gotos and a `reg' in the other two cases. - -`allocate_stack' - Subtract operand 0 from the stack pointer to create space for for - dynamically allocated data. - - Do not define this pattern if all that must be done is the - subtraction. On some machines require other operations such as - stack probes or maintaining the back chain. Define this pattern - to emit those operations in addition to updating the stack pointer. + The fundamental principle of reporting bugs usefully is this: +*report all the facts*. If you are not sure whether to state a fact or +leave it out, state it! + + Often people omit facts because they think they know what causes the +problem and they conclude that some details don't matter. Thus, you +might assume that the name of the variable you use in an example does +not matter. Well, probably it doesn't, but one cannot be sure. +Perhaps the bug is a stray memory reference which happens to fetch from +the location where that name is stored in memory; perhaps, if the name +were different, the contents of that location would fool the compiler +into doing the right thing despite the bug. Play it safe and give a +specific, complete example. That is the easiest thing for you to do, +and the most helpful. + + Keep in mind that the purpose of a bug report is to enable someone to +fix the bug if it is not known. It isn't very important what happens if +the bug is already known. Therefore, always write your bug reports on +the assumption that the bug is not known. + + Sometimes people give a few sketchy facts and ask, "Does this ring a +bell?" This cannot help us fix a bug, so it is basically useless. We +respond by asking for enough details to enable us to investigate. You +might as well expedite matters by sending them to begin with. + + Try to make your bug report self-contained. If we have to ask you +for more information, it is best if you include all the previous +information in your response, as well as the information that was +missing. + + Please report each bug in a separate message. This makes it easier +for us to track which bugs have been fixed and to forward your bugs +reports to the appropriate maintainer. + + To enable someone to investigate the bug, you should include all +these things: + + * The version of GNU CC. You can get this by running it with the + `-v' option. + + Without this, we won't know whether there is any point in looking + for the bug in the current version of GNU CC. + + * A complete input file that will reproduce the bug. If the bug is + in the C preprocessor, send a source file and any header files + that it requires. If the bug is in the compiler proper (`cc1'), + run your source file through the C preprocessor by doing `gcc -E + SOURCEFILE > OUTFILE', then include the contents of OUTFILE in the + bug report. (When you do this, use the same `-I', `-D' or `-U' + options that you used in actual compilation.) + + A single statement is not enough of an example. In order to + compile it, it must be embedded in a complete file of compiler + input; and the bug might depend on the details of how this is done. + + Without a real example one can compile, all anyone can do about + your bug report is wish you luck. It would be futile to try to + guess how to provoke the bug. For example, bugs in register + allocation and reloading frequently depend on every little detail + of the function they happen in. + + Even if the input file that fails comes from a GNU program, you + should still send the complete test case. Don't ask the GNU CC + maintainers to do the extra work of obtaining the program in + question--they are all overworked as it is. Also, the problem may + depend on what is in the header files on your system; it is + unreliable for the GNU CC maintainers to try the problem with the + header files available to them. By sending CPP output, you can + eliminate this source of uncertainty and save us a certain + percentage of wild goose chases. + + * The command arguments you gave GNU CC or GNU C++ to compile that + example and observe the bug. For example, did you use `-O'? To + guarantee you won't omit something important, list all the options. + + If we were to try to guess the arguments, we would probably guess + wrong and then we would not encounter the bug. + + * The type of machine you are using, and the operating system name + and version number. + + * The operands you gave to the `configure' command when you installed + the compiler. + + * A complete list of any modifications you have made to the compiler + source. (We don't promise to investigate the bug unless it + happens in an unmodified compiler. But if you've made + modifications and don't tell us, then you are sending us on a wild + goose chase.) + + Be precise about these changes. A description in English is not + enough--send a context diff for them. + + Adding files of your own (such as a machine description for a + machine we don't support) is a modification of the compiler source. + + * Details of any other deviations from the standard procedure for + installing GNU CC. + + * A description of what behavior you observe that you believe is + incorrect. For example, "The compiler gets a fatal signal," or, + "The assembler instruction at line 208 in the output is incorrect." + + Of course, if the bug is that the compiler gets a fatal signal, + then one can't miss it. But if the bug is incorrect output, the + maintainer might not notice unless it is glaringly wrong. None of + us has time to study all the assembler code from a 50-line C + program just on the chance that one instruction might be wrong. + We need *you* to do this part! + + Even if the problem you experience is a fatal signal, you should + still say so explicitly. Suppose something strange is going on, + such as, your copy of the compiler is out of synch, or you have + encountered a bug in the C library on your system. (This has + happened!) Your copy might crash and the copy here would not. If + you said to expect a crash, then when the compiler here fails to + crash, we would know that the bug was not happening. If you don't + say to expect a crash, then we would not know whether the bug was + happening. We would not be able to draw any conclusion from our + observations. + + If the problem is a diagnostic when compiling GNU CC with some + other compiler, say whether it is a warning or an error. + + Often the observed symptom is incorrect output when your program + is run. Sad to say, this is not enough information unless the + program is short and simple. None of us has time to study a large + program to figure out how it would work if compiled correctly, + much less which line of it was compiled wrong. So you will have + to do that. Tell us which source line it is, and what incorrect + result happens when that line is executed. A person who + understands the program can find this as easily as finding a bug + in the program itself. + + * If you send examples of assembler code output from GNU CC or GNU + C++, please use `-g' when you make them. The debugging information + includes source line numbers which are essential for correlating + the output with the input. + + * If you wish to mention something in the GNU CC source, refer to it + by context, not by line number. + + The line numbers in the development sources don't match those in + your sources. Your line numbers would convey no useful + information to the maintainers. + + * Additional information from a debugger might enable someone to + find a problem on a machine which he does not have available. + However, you need to think when you collect this information if + you want it to have any chance of being useful. + + For example, many people send just a backtrace, but that is never + useful by itself. A simple backtrace with arguments conveys little + about GNU CC because the compiler is largely data-driven; the same + functions are called over and over for different RTL insns, doing + different things depending on the details of the insn. + + Most of the arguments listed in the backtrace are useless because + they are pointers to RTL list structure. The numeric values of the + pointers, which the debugger prints in the backtrace, have no + significance whatever; all that matters is the contents of the + objects they point to (and most of the contents are other such + pointers). + + In addition, most compiler passes consist of one or more loops that + scan the RTL insn sequence. The most vital piece of information + about such a loop--which insn it has reached--is usually in a + local variable, not in an argument. + + What you need to provide in addition to a backtrace are the values + of the local variables for several stack frames up. When a local + variable or an argument is an RTX, first print its value and then + use the GDB command `pr' to print the RTL expression that it points + to. (If GDB doesn't run on your machine, use your debugger to call + the function `debug_rtx' with the RTX as an argument.) In + general, whenever a variable is a pointer, its value is no use + without the data it points to. + + Here are some things that are not necessary: + + * A description of the envelope of the bug. + + Often people who encounter a bug spend a lot of time investigating + which changes to the input file will make the bug go away and which + changes will not affect it. + + This is often time consuming and not very useful, because the way + we will find the bug is by running a single example under the + debugger with breakpoints, not by pure deduction from a series of + examples. You might as well save your time for something else. + + Of course, if you can find a simpler example to report *instead* of + the original one, that is a convenience. Errors in the output + will be easier to spot, running under the debugger will take less + time, etc. Most GNU CC bugs involve just one function, so the + most straightforward way to simplify an example is to delete all + the function definitions except the one where the bug occurs. + Those earlier in the file may be replaced by external declarations + if the crucial function depends on them. (Exception: inline + functions may affect compilation of functions defined later in the + file.) + + However, simplification is not vital; if you don't want to do this, + report the bug anyway and send the entire test case you used. + + * In particular, some people insert conditionals `#ifdef BUG' around + a statement which, if removed, makes the bug not happen. These + are just clutter; we won't pay any attention to them anyway. + Besides, you should send us cpp output, and that can't have + conditionals. + + * A patch for the bug. + + A patch for the bug is useful if it is a good one. But don't omit + the necessary information, such as the test case, on the + assumption that a patch is all we need. We might see problems + with your patch and decide to fix the problem another way, or we + might not understand it at all. + + Sometimes with a program as complicated as GNU CC it is very hard + to construct an example that will make the program follow a + certain path through the code. If you don't send the example, we + won't be able to construct one, so we won't be able to verify that + the bug is fixed. + + And if we can't understand what bug you are trying to fix, or why + your patch should be an improvement, we won't install it. A test + case will help us to understand. + + *Note Sending Patches::, for guidelines on how to make it easy for + us to understand and install your patches. + + * A guess about what the bug is or what it depends on. + + Such guesses are usually wrong. Even I can't guess right about + such things without first using the debugger to find the facts. + + * A core dump file. + + We have no way of examining a core dump for your type of machine + unless we have an identical system--and if we do have one, we + should be able to reproduce the crash ourselves.  -File: gcc.info, Node: Pattern Ordering, Next: Dependent Patterns, Prev: Standard Names, Up: Machine Desc +File: gcc.info, Node: Sending Patches, Prev: Bug Reporting, Up: Bugs + +Sending Patches for GNU CC +========================== -When the Order of Patterns Matters -================================== + If you would like to write bug fixes or improvements for the GNU C +compiler, that is very helpful. When you send your changes, please +follow these guidelines to avoid causing extra work for us in studying +the patches. + + If you don't follow these guidelines, your information might still be +useful, but using it will take extra work. Maintaining GNU C is a lot +of work in the best of circumstances, and we can't keep up unless you do +your best to help. + + * Send an explanation with your changes of what problem they fix or + what improvement they bring about. For a bug fix, just include a + copy of the bug report, and explain why the change fixes the bug. + + (Referring to a bug report is not as good as including it, because + then we will have to look it up, and we have probably already + deleted it if we've already fixed the bug.) + + * Always include a proper bug report for the problem you think you + have fixed. We need to convince ourselves that the change is + right before installing it. Even if it is right, we might have + trouble judging it if we don't have a way to reproduce the problem. + + * Include all the comments that are appropriate to help people + reading the source in the future understand why this change was + needed. + + * Don't mix together changes made for different reasons. Send them + *individually*. + + If you make two changes for separate reasons, then we might not + want to install them both. We might want to install just one. If + you send them all jumbled together in a single set of diffs, we + have to do extra work to disentangle them--to figure out which + parts of the change serve which purpose. If we don't have time + for this, we might have to ignore your changes entirely. + + If you send each change as soon as you have written it, with its + own explanation, then the two changes never get tangled up, and we + can consider each one properly without any extra work to + disentangle them. + + Ideally, each change you send should be impossible to subdivide + into parts that we might want to consider separately, because each + of its parts gets its motivation from the other parts. + + * Send each change as soon as that change is finished. Sometimes + people think they are helping us by accumulating many changes to + send them all together. As explained above, this is absolutely + the worst thing you could do. + + Since you should send each change separately, you might as well + send it right away. That gives us the option of installing it + immediately if it is important. + + * Use `diff -c' to make your diffs. Diffs without context are hard + for us to install reliably. More than that, they make it hard for + us to study the diffs to decide whether we want to install them. + Unidiff format is better than contextless diffs, but not as easy + to read as `-c' format. + + If you have GNU diff, use `diff -cp', which shows the name of the + function that each change occurs in. + + * Write the change log entries for your changes. We get lots of + changes, and we don't have time to do all the change log writing + ourselves. + + Read the `ChangeLog' file to see what sorts of information to put + in, and to learn the style that we use. The purpose of the change + log is to show people where to find what was changed. So you need + to be specific about what functions you changed; in large + functions, it's often helpful to indicate where within the + function the change was. + + On the other hand, once you have shown people where to find the + change, you need not explain its purpose. Thus, if you add a new + function, all you need to say about it is that it is new. If you + feel that the purpose needs explaining, it probably does--but the + explanation will be much more useful if you put it in comments in + the code. + + If you would like your name to appear in the header line for who + made the change, send us the header line. + + * When you write the fix, keep in mind that we can't install a + change that would break other systems. + + People often suggest fixing a problem by changing + machine-independent files such as `toplev.c' to do something + special that a particular system needs. Sometimes it is totally + obvious that such changes would break GNU CC for almost all users. + We can't possibly make a change like that. At best it might tell + us how to write another patch that would solve the problem + acceptably. + + Sometimes people send fixes that *might* be an improvement in + general--but it is hard to be sure of this. It's hard to install + such changes because we have to study them very carefully. Of + course, a good explanation of the reasoning by which you concluded + the change was correct can help convince us. + + The safest changes are changes to the configuration files for a + particular machine. These are safe because they can't create new + bugs on other machines. - Sometimes an insn can match more than one instruction pattern. Then -the pattern that appears first in the machine description is the one -used. Therefore, more specific patterns (patterns that will match fewer -things) and faster instructions (those that will produce better code -when they do match) should usually go first in the description. - - In some cases the effect of ordering the patterns can be used to hide -a pattern when it is not valid. For example, the 68000 has an -instruction for converting a fullword to floating point and another for -converting a byte to floating point. An instruction converting an -integer to floating point could match either one. We put the pattern -to convert the fullword first to make sure that one will be used rather -than the other. (Otherwise a large integer might be generated as a -single-byte immediate quantity, which would not work.) Instead of using -this pattern ordering it would be possible to make the pattern for -convert-a-byte smart enough to deal properly with any constant value. + Please help us keep up with the workload by designing the patch in + a form that is good to install.  -File: gcc.info, Node: Dependent Patterns, Next: Jump Patterns, Prev: Pattern Ordering, Up: Machine Desc +File: gcc.info, Node: Service, Next: VMS, Prev: Bugs, Up: Top -Interdependence of Patterns -=========================== +How To Get Help with GNU CC +*************************** + + If you need help installing, using or changing GNU CC, there are two +ways to find it: - Every machine description must have a named pattern for each of the -conditional branch names `bCOND'. The recognition template must always -have the form - - (set (pc) - (if_then_else (COND (cc0) (const_int 0)) - (label_ref (match_operand 0 "" "")) - (pc))) - -In addition, every machine description must have an anonymous pattern -for each of the possible reverse-conditional branches. Their templates -look like - - (set (pc) - (if_then_else (COND (cc0) (const_int 0)) - (pc) - (label_ref (match_operand 0 "" "")))) - -They are necessary because jump optimization can turn direct-conditional -branches into reverse-conditional branches. - - It is often convenient to use the `match_operator' construct to -reduce the number of patterns that must be specified for branches. For -example, - - (define_insn "" - [(set (pc) - (if_then_else (match_operator 0 "comparison_operator" - [(cc0) (const_int 0)]) - (pc) - (label_ref (match_operand 1 "" ""))))] - "CONDITION" - "...") - - In some cases machines support instructions identical except for the -machine mode of one or more operands. For example, there may be -"sign-extend halfword" and "sign-extend byte" instructions whose -patterns are - - (set (match_operand:SI 0 ...) - (extend:SI (match_operand:HI 1 ...))) - - (set (match_operand:SI 0 ...) - (extend:SI (match_operand:QI 1 ...))) - -Constant integers do not specify a machine mode, so an instruction to -extend a constant value could match either pattern. The pattern it -actually will match is the one that appears first in the file. For -correct results, this must be the one for the widest possible mode -(`HImode', here). If the pattern matches the `QImode' instruction, the -results will be incorrect if the constant value does not actually fit -that mode. - - Such instructions to extend constants are rarely generated because -they are optimized away, but they do occasionally happen in nonoptimized -compilations. - - If a constraint in a pattern allows a constant, the reload pass may -replace a register with a constant permitted by the constraint in some -cases. Similarly for memory references. You must ensure that the -predicate permits all objects allowed by the constraints to prevent the -compiler from crashing. - - Because of this substitution, you should not provide separate -patterns for increment and decrement instructions. Instead, they -should be generated from the same pattern that supports -register-register add insns by examining the operands and generating -the appropriate machine instruction. + * Send a message to a suitable network mailing list. First try + `bug-gcc@prep.ai.mit.edu', and if that brings no response, try + `help-gcc@prep.ai.mit.edu'. + + * Look in the service directory for someone who might help you for a + fee. The service directory is found in the file named `SERVICE' + in the GNU CC distribution.  -File: gcc.info, Node: Jump Patterns, Next: Insn Canonicalizations, Prev: Dependent Patterns, Up: Machine Desc +File: gcc.info, Node: VMS, Next: Portability, Prev: Service, Up: Top + +Using GNU CC on VMS +******************* -Defining Jump Instruction Patterns -================================== + Here is how to use GNU CC on VMS. - For most machines, GNU CC assumes that the machine has a condition -code. A comparison insn sets the condition code, recording the results -of both signed and unsigned comparison of the given operands. A -separate branch insn tests the condition code and branches or not -according its value. The branch insns come in distinct signed and -unsigned flavors. Many common machines, such as the Vax, the 68000 and -the 32000, work this way. - - Some machines have distinct signed and unsigned compare -instructions, and only one set of conditional branch instructions. The -easiest way to handle these machines is to treat them just like the -others until the final stage where assembly code is written. At this -time, when outputting code for the compare instruction, peek ahead at -the following branch using `next_cc0_user (insn)'. (The variable -`insn' refers to the insn being output, in the output-writing code in -an instruction pattern.) If the RTL says that is an unsigned branch, -output an unsigned compare; otherwise output a signed compare. When -the branch itself is output, you can treat signed and unsigned branches -identically. - - The reason you can do this is that GNU CC always generates a pair of -consecutive RTL insns, possibly separated by `note' insns, one to set -the condition code and one to test it, and keeps the pair inviolate -until the end. - - To go with this technique, you must define the machine-description -macro `NOTICE_UPDATE_CC' to do `CC_STATUS_INIT'; in other words, no -compare instruction is superfluous. - - Some machines have compare-and-branch instructions and no condition -code. A similar technique works for them. When it is time to "output" a -compare instruction, record its operands in two static variables. When -outputting the branch-on-condition-code instruction that follows, -actually output a compare-and-branch instruction that uses the -remembered operands. - - It also works to define patterns for compare-and-branch instructions. -In optimizing compilation, the pair of compare and branch instructions -will be combined according to these patterns. But this does not happen -if optimization is not requested. So you must use one of the solutions -above in addition to any special patterns you define. - - In many RISC machines, most instructions do not affect the condition -code and there may not even be a separate condition code register. On -these machines, the restriction that the definition and use of the -condition code be adjacent insns is not necessary and can prevent -important optimizations. For example, on the IBM RS/6000, there is a -delay for taken branches unless the condition code register is set three -instructions earlier than the conditional branch. The instruction -scheduler cannot perform this optimization if it is not permitted to -separate the definition and use of the condition code register. - - On these machines, do not use `(cc0)', but instead use a register to -represent the condition code. If there is a specific condition code -register in the machine, use a hard register. If the condition code or -comparison result can be placed in any general register, or if there are -multiple condition registers, use a pseudo register. - - On some machines, the type of branch instruction generated may -depend on the way the condition code was produced; for example, on the -68k and Sparc, setting the condition code directly from an add or -subtract instruction does not clear the overflow bit the way that a test -instruction does, so a different branch instruction must be used for -some conditional branches. For machines that use `(cc0)', the set and -use of the condition code must be adjacent (separated only by `note' -insns) allowing flags in `cc_status' to be used. (*Note Condition -Code::.) Also, the comparison and branch insns can be located from -each other by using the functions `prev_cc0_setter' and `next_cc0_user'. - - However, this is not true on machines that do not use `(cc0)'. On -those machines, no assumptions can be made about the adjacency of the -compare and branch insns and the above methods cannot be used. Instead, -we use the machine mode of the condition code register to record -different formats of the condition code register. - - Registers used to store the condition code value should have a mode -that is in class `MODE_CC'. Normally, it will be `CCmode'. If -additional modes are required (as for the add example mentioned above in -the Sparc), define the macro `EXTRA_CC_MODES' to list the additional -modes required (*note Condition Code::.). Also define `EXTRA_CC_NAMES' -to list the names of those modes and `SELECT_CC_MODE' to choose a mode -given an operand of a compare. - - If it is known during RTL generation that a different mode will be -required (for example, if the machine has separate compare instructions -for signed and unsigned quantities, like most IBM processors), they can -be specified at that time. - - If the cases that require different modes would be made by -instruction combination, the macro `SELECT_CC_MODE' determines which -machine mode should be used for the comparison result. The patterns -should be written using that mode. To support the case of the add on -the Sparc discussed above, we have the pattern - - (define_insn "" - [(set (reg:CC_NOOV 0) - (compare:CC_NOOV (plus:SI (match_operand:SI 0 "register_operand" "%r") - (match_operand:SI 1 "arith_operand" "rI")) - (const_int 0)))] - "" - "...") +* Menu: - The `SELECT_CC_MODE' macro on the Sparc returns `CC_NOOVmode' for -comparisons whose argument is a `plus'. +* Include Files and VMS:: Where the preprocessor looks for the include files. +* Global Declarations:: How to do globaldef, globalref and globalvalue with + GNU CC. +* VMS Misc:: Misc information.  -File: gcc.info, Node: Insn Canonicalizations, Next: Peephole Definitions, Prev: Jump Patterns, Up: Machine Desc +File: gcc.info, Node: Include Files and VMS, Next: Global Declarations, Up: VMS -Canonicalization of Instructions -================================ +Include Files and VMS +===================== - There are often cases where multiple RTL expressions could represent -an operation performed by a single machine instruction. This situation -is most commonly encountered with logical, branch, and -multiply-accumulate instructions. In such cases, the compiler attempts -to convert these multiple RTL expressions into a single canonical form -to reduce the number of insn patterns required. - - In addition to algebraic simplifications, following canonicalizations -are performed: - - * For commutative and comparison operators, a constant is always - made the second operand. If a machine only supports a constant as - the second operand, only patterns that match a constant in the - second operand need be supplied. - - For these operators, if only one operand is a `neg', `not', - `mult', `plus', or `minus' expression, it will be the first - operand. - - * For the `compare' operator, a constant is always the second operand - on machines where `cc0' is used (*note Jump Patterns::.). On other - machines, there are rare cases where the compiler might want to - construct a `compare' with a constant as the first operand. - However, these cases are not common enough for it to be worthwhile - to provide a pattern matching a constant as the first operand - unless the machine actually has such an instruction. - - An operand of `neg', `not', `mult', `plus', or `minus' is made the - first operand under the same conditions as above. - - * `(minus X (const_int N))' is converted to `(plus X (const_int - -N))'. - - * Within address computations (i.e., inside `mem'), a left shift is - converted into the appropriate multiplication by a power of two. - - De`Morgan's Law is used to move bitwise negation inside a bitwise - logical-and or logical-or operation. If this results in only one - operand being a `not' expression, it will be the first one. - - A machine that has an instruction that performs a bitwise - logical-and of one operand with the bitwise negation of the other - should specify the pattern for that instruction as - - (define_insn "" - [(set (match_operand:M 0 ...) - (and:M (not:M (match_operand:M 1 ...)) - (match_operand:M 2 ...)))] - "..." - "...") - - Similarly, a pattern for a "NAND" instruction should be written - - (define_insn "" - [(set (match_operand:M 0 ...) - (ior:M (not:M (match_operand:M 1 ...)) - (not:M (match_operand:M 2 ...))))] - "..." - "...") - - In both cases, it is not necessary to include patterns for the many - logically equivalent RTL expressions. - - * The only possible RTL expressions involving both bitwise - exclusive-or and bitwise negation are `(xor:M X) Y)' and `(not:M - (xor:M X Y))'. - - * The sum of three items, one of which is a constant, will only - appear in the form - - (plus:M (plus:M X Y) CONSTANT) - - * On machines that do not use `cc0', `(compare X (const_int 0))' - will be converted to X. - - * Equality comparisons of a group of bits (usually a single bit) - with zero will be written using `zero_extract' rather than the - equivalent `and' or `sign_extract' operations. + Due to the differences between the filesystems of Unix and VMS, GNU +CC attempts to translate file names in `#include' into names that VMS +will understand. The basic strategy is to prepend a prefix to the +specification of the include file, convert the whole filename to a VMS +filename, and then try to open the file. GNU CC tries various prefixes +one by one until one of them succeeds: + + 1. The first prefix is the `GNU_CC_INCLUDE:' logical name: this is + where GNU C header files are traditionally stored. If you wish to + store header files in non-standard locations, then you can assign + the logical `GNU_CC_INCLUDE' to be a search list, where each + element of the list is suitable for use with a rooted logical. + + 2. The next prefix tried is `SYS$SYSROOT:[SYSLIB.]'. This is where + VAX-C header files are traditionally stored. + + 3. If the include file specification by itself is a valid VMS + filename, the preprocessor then uses this name with no prefix in + an attempt to open the include file. + + 4. If the file specification is not a valid VMS filename (i.e. does + not contain a device or a directory specifier, and contains a `/' + character), the preprocessor tries to convert it from Unix syntax + to VMS syntax. + + Conversion works like this: the first directory name becomes a + device, and the rest of the directories are converted into + VMS-format directory names. For example, the name `X11/foobar.h' + is translated to `X11:[000000]foobar.h' or `X11:foobar.h', + whichever one can be opened. This strategy allows you to assign a + logical name to point to the actual location of the header files. + + 5. If none of these strategies succeeds, the `#include' fails. + + Include directives of the form: + + #include foobar + +are a common source of incompatibility between VAX-C and GNU CC. VAX-C +treats this much like a standard `#include ' directive. That +is incompatible with the ANSI C behavior implemented by GNU CC: to +expand the name `foobar' as a macro. Macro expansion should eventually +yield one of the two standard formats for `#include': + + #include "FILE" + #include + + If you have this problem, the best solution is to modify the source +to convert the `#include' directives to one of the two standard forms. +That will work with either compiler. If you want a quick and dirty fix, +define the file names as macros with the proper expansion, like this: + + #define stdio + +This will work, as long as the name doesn't conflict with anything else +in the program. + + Another source of incompatibility is that VAX-C assumes that: + + #include "foobar" + +is actually asking for the file `foobar.h'. GNU CC does not make this +assumption, and instead takes what you ask for literally; it tries to +read the file `foobar'. The best way to avoid this problem is to +always specify the desired file extension in your include directives. + + GNU CC for VMS is distributed with a set of include files that is +sufficient to compile most general purpose programs. Even though the +GNU CC distribution does not contain header files to define constants +and structures for some VMS system-specific functions, there is no +reason why you cannot use GNU CC with any of these functions. You first +may have to generate or create header files, either by using the public +domain utility `UNSDL' (which can be found on a DECUS tape), or by +extracting the relevant modules from one of the system macro libraries, +and using an editor to construct a C header file. + + A `#include' file name cannot contain a DECNET node name. The +preprocessor reports an I/O error if you attempt to use a node name, +whether explicitly, or implicitly via a logical name.  -File: gcc.info, Node: Peephole Definitions, Next: Expander Definitions, Prev: Insn Canonicalizations, Up: Machine Desc +File: gcc.info, Node: Global Declarations, Next: VMS Misc, Prev: Include Files and VMS, Up: VMS -Defining Machine-Specific Peephole Optimizers -============================================= +Global Declarations and VMS +=========================== - In addition to instruction patterns the `md' file may contain -definitions of machine-specific peephole optimizations. + GNU CC does not provide the `globalref', `globaldef' and +`globalvalue' keywords of VAX-C. You can get the same effect with an +obscure feature of GAS, the GNU assembler. (This requires GAS version +1.39 or later.) The following macros allow you to use this feature in +a fairly natural way: + + #ifdef __GNUC__ + #define GLOBALREF(TYPE,NAME) \ + TYPE NAME \ + asm ("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME) + #define GLOBALDEF(TYPE,NAME,VALUE) \ + TYPE NAME \ + asm ("_$$PsectAttributes_GLOBALSYMBOL$$" #NAME) \ + = VALUE + #define GLOBALVALUEREF(TYPE,NAME) \ + const TYPE NAME[1] \ + asm ("_$$PsectAttributes_GLOBALVALUE$$" #NAME) + #define GLOBALVALUEDEF(TYPE,NAME,VALUE) \ + const TYPE NAME[1] \ + asm ("_$$PsectAttributes_GLOBALVALUE$$" #NAME) \ + = {VALUE} + #else + #define GLOBALREF(TYPE,NAME) \ + globalref TYPE NAME + #define GLOBALDEF(TYPE,NAME,VALUE) \ + globaldef TYPE NAME = VALUE + #define GLOBALVALUEDEF(TYPE,NAME,VALUE) \ + globalvalue TYPE NAME = VALUE + #define GLOBALVALUEREF(TYPE,NAME) \ + globalvalue TYPE NAME + #endif - The combiner does not notice certain peephole optimizations when the -data flow in the program does not suggest that it should try them. For -example, sometimes two consecutive insns related in purpose can be -combined even though the second one does not appear to use a register -computed in the first one. A machine-specific peephole optimizer can -detect such opportunities. - - A definition looks like this: - - (define_peephole - [INSN-PATTERN-1 - INSN-PATTERN-2 - ...] - "CONDITION" - "TEMPLATE" - "OPTIONAL INSN-ATTRIBUTES") - -The last string operand may be omitted if you are not using any -machine-specific information in this machine description. If present, -it must obey the same rules as in a `define_insn'. - - In this skeleton, INSN-PATTERN-1 and so on are patterns to match -consecutive insns. The optimization applies to a sequence of insns when -INSN-PATTERN-1 matches the first one, INSN-PATTERN-2 matches the next, -and so on. - - Each of the insns matched by a peephole must also match a -`define_insn'. Peepholes are checked only at the last stage just -before code generation, and only optionally. Therefore, any insn which -would match a peephole but no `define_insn' will cause a crash in code -generation in an unoptimized compilation, or at various optimization -stages. - - The operands of the insns are matched with `match_operands', -`match_operator', and `match_dup', as usual. What is not usual is that -the operand numbers apply to all the insn patterns in the definition. -So, you can check for identical operands in two insns by using -`match_operand' in one insn and `match_dup' in the other. - - The operand constraints used in `match_operand' patterns do not have -any direct effect on the applicability of the peephole, but they will -be validated afterward, so make sure your constraints are general enough -to apply whenever the peephole matches. If the peephole matches but -the constraints are not satisfied, the compiler will crash. - - It is safe to omit constraints in all the operands of the peephole; -or you can write constraints which serve as a double-check on the -criteria previously tested. - - Once a sequence of insns matches the patterns, the CONDITION is -checked. This is a C expression which makes the final decision whether -to perform the optimization (we do so if the expression is nonzero). If -CONDITION is omitted (in other words, the string is empty) then the -optimization is applied to every sequence of insns that matches the -patterns. - - The defined peephole optimizations are applied after register -allocation is complete. Therefore, the peephole definition can check -which operands have ended up in which kinds of registers, just by -looking at the operands. - - The way to refer to the operands in CONDITION is to write -`operands[I]' for operand number I (as matched by `(match_operand I -...)'). Use the variable `insn' to refer to the last of the insns -being matched; use `prev_nonnote_insn' to find the preceding insns. - - When optimizing computations with intermediate results, you can use -CONDITION to match only when the intermediate results are not used -elsewhere. Use the C expression `dead_or_set_p (INSN, OP)', where INSN -is the insn in which you expect the value to be used for the last time -(from the value of `insn', together with use of `prev_nonnote_insn'), -and OP is the intermediate value (from `operands[I]'). - - Applying the optimization means replacing the sequence of insns with -one new insn. The TEMPLATE controls ultimate output of assembler code -for this combined insn. It works exactly like the template of a -`define_insn'. Operand numbers in this template are the same ones used -in matching the original sequence of insns. - - The result of a defined peephole optimizer does not need to match -any of the insn patterns in the machine description; it does not even -have an opportunity to match them. The peephole optimizer definition -itself serves as the insn pattern to control how the insn is output. - - Defined peephole optimizers are run as assembler code is being -output, so the insns they produce are never combined or rearranged in -any way. - - Here is an example, taken from the 68000 machine description: - - (define_peephole - [(set (reg:SI 15) (plus:SI (reg:SI 15) (const_int 4))) - (set (match_operand:DF 0 "register_operand" "=f") - (match_operand:DF 1 "register_operand" "ad"))] - "FP_REG_P (operands[0]) && ! FP_REG_P (operands[1])" - "* - { - rtx xoperands[2]; - xoperands[1] = gen_rtx (REG, SImode, REGNO (operands[1]) + 1); - #ifdef MOTOROLA - output_asm_insn (\"move.l %1,(sp)\", xoperands); - output_asm_insn (\"move.l %1,-(sp)\", operands); - return \"fmove.d (sp)+,%0\"; +(The `_$$PsectAttributes_GLOBALSYMBOL' prefix at the start of the name +is removed by the assembler, after it has modified the attributes of +the symbol). These macros are provided in the VMS binaries +distribution in a header file `GNU_HACKS.H'. An example of the usage +is: + + GLOBALREF (int, ijk); + GLOBALDEF (int, jkl, 0); + + The macros `GLOBALREF' and `GLOBALDEF' cannot be used +straightforwardly for arrays, since there is no way to insert the array +dimension into the declaration at the right place. However, you can +declare an array with these macros if you first define a typedef for the +array type, like this: + + typedef int intvector[10]; + GLOBALREF (intvector, foo); + + Array and structure initializers will also break the macros; you can +define the initializer to be a macro of its own, or you can expand the +`GLOBALDEF' macro by hand. You may find a case where you wish to use +the `GLOBALDEF' macro with a large array, but you are not interested in +explicitly initializing each element of the array. In such cases you +can use an initializer like: `{0,}', which will initialize the entire +array to `0'. + + A shortcoming of this implementation is that a variable declared with +`GLOBALVALUEREF' or `GLOBALVALUEDEF' is always an array. For example, +the declaration: + + GLOBALVALUEREF(int, ijk); + +declares the variable `ijk' as an array of type `int [1]'. This is +done because a globalvalue is actually a constant; its "value" is what +the linker would normally consider an address. That is not how an +integer value works in C, but it is how an array works. So treating +the symbol as an array name gives consistent results--with the +exception that the value seems to have the wrong type. *Don't try to +access an element of the array.* It doesn't have any elements. The +array "address" may not be the address of actual storage. + + The fact that the symbol is an array may lead to warnings where the +variable is used. Insert type casts to avoid the warnings. Here is an +example; it takes advantage of the ANSI C feature allowing macros that +expand to use the same name as the macro itself. + + GLOBALVALUEREF (int, ss$_normal); + GLOBALVALUEDEF (int, xyzzy,123); + #ifdef __GNUC__ + #define ss$_normal ((int) ss$_normal) + #define xyzzy ((int) xyzzy) + #endif + + Don't use `globaldef' or `globalref' with a variable whose type is +an enumeration type; this is not implemented. Instead, make the +variable an integer, and use a `globalvaluedef' for each of the +enumeration values. An example of this would be: + + #ifdef __GNUC__ + GLOBALDEF (int, color, 0); + GLOBALVALUEDEF (int, RED, 0); + GLOBALVALUEDEF (int, BLUE, 1); + GLOBALVALUEDEF (int, GREEN, 3); #else - output_asm_insn (\"movel %1,sp@\", xoperands); - output_asm_insn (\"movel %1,sp@-\", operands); - return \"fmoved sp@+,%0\"; + enum globaldef color {RED, BLUE, GREEN = 3}; #endif - } - ") - The effect of this optimization is to change + +File: gcc.info, Node: VMS Misc, Prev: Global Declarations, Up: VMS + +Other VMS Issues +================ + + GNU CC automatically arranges for `main' to return 1 by default if +you fail to specify an explicit return value. This will be interpreted +by VMS as a status code indicating a normal successful completion. +Version 1 of GNU CC did not provide this default. + + GNU CC on VMS works only with the GNU assembler, GAS. You need +version 1.37 or later of GAS in order to produce value debugging +information for the VMS debugger. Use the ordinary VMS linker with the +object files produced by GAS. + + Under previous versions of GNU CC, the generated code would +occasionally give strange results when linked to the sharable `VAXCRTL' +library. Now this should work. + + A caveat for use of `const' global variables: the `const' modifier +must be specified in every external declaration of the variable in all +of the source files that use that variable. Otherwise the linker will +issue warnings about conflicting attributes for the variable. Your +program will still work despite the warnings, but the variable will be +placed in writable storage. + + Although the VMS linker does distinguish between upper and lower case +letters in global symbols, most VMS compilers convert all such symbols +into upper case and most run-time library routines also have upper case +names. To be able to reliably call such routines, GNU CC (by means of +the assembler GAS) converts global symbols into upper case like other +VMS compilers. However, since the usual practice in C is to distinguish +case, GNU CC (via GAS) tries to preserve usual C behavior by augmenting +each name that is not all lower case. This means truncating the name +to at most 23 characters and then adding more characters at the end +which encode the case pattern of those 23. Names which contain at +least one dollar sign are an exception; they are converted directly into +upper case without augmentation. + + Name augmentation yields bad results for programs that use +precompiled libraries (such as Xlib) which were generated by another +compiler. You can use the compiler option `/NOCASE_HACK' to inhibit +augmentation; it makes external C functions and variables +case-independent as is usual on VMS. Alternatively, you could write +all references to the functions and variables in such libraries using +lower case; this will work on VMS, but is not portable to other +systems. The compiler option `/NAMES' also provides control over +global name handling. + + Function and variable names are handled somewhat differently with GNU +C++. The GNU C++ compiler performs "name mangling" on function names, +which means that it adds information to the function name to describe +the data types of the arguments that the function takes. One result of +this is that the name of a function can become very long. Since the +VMS linker only recognizes the first 31 characters in a name, special +action is taken to ensure that each function and variable has a unique +name that can be represented in 31 characters. + + If the name (plus a name augmentation, if required) is less than 32 +characters in length, then no special action is performed. If the name +is longer than 31 characters, the assembler (GAS) will generate a hash +string based upon the function name, truncate the function name to 23 +characters, and append the hash string to the truncated name. If the +`/VERBOSE' compiler option is used, the assembler will print both the +full and truncated names of each symbol that is truncated. + + The `/NOCASE_HACK' compiler option should not be used when you are +compiling programs that use libg++. libg++ has several instances of +objects (i.e. `Filebuf' and `filebuf') which become indistinguishable +in a case-insensitive environment. This leads to cases where you need +to inhibit augmentation selectively (if you were using libg++ and Xlib +in the same program, for example). There is no special feature for +doing this, but you can get the result by defining a macro for each +mixed case symbol for which you wish to inhibit augmentation. The +macro should expand into the lower case equivalent of itself. For +example: + + #define StuDlyCapS studlycaps + + These macro definitions can be placed in a header file to minimize +the number of changes to your source code. + + +File: gcc.info, Node: Portability, Next: Interface, Prev: VMS, Up: Top + +GNU CC and Portability +********************** + + The main goal of GNU CC was to make a good, fast compiler for +machines in the class that the GNU system aims to run on: 32-bit +machines that address 8-bit bytes and have several general registers. +Elegance, theoretical power and simplicity are only secondary. + + GNU CC gets most of the information about the target machine from a +machine description which gives an algebraic formula for each of the +machine's instructions. This is a very clean way to describe the +target. But when the compiler needs information that is difficult to +express in this fashion, I have not hesitated to define an ad-hoc +parameter to the machine description. The purpose of portability is to +reduce the total work needed on the compiler; it was not of interest +for its own sake. + + GNU CC does not contain machine dependent code, but it does contain +code that depends on machine parameters such as endianness (whether the +most significant byte has the highest or lowest address of the bytes in +a word) and the availability of autoincrement addressing. In the +RTL-generation pass, it is often necessary to have multiple strategies +for generating code for a particular kind of syntax tree, strategies +that are usable for different combinations of parameters. Often I have +not tried to address all possible cases, but only the common ones or +only the ones that I have encountered. As a result, a new target may +require additional strategies. You will know if this happens because +the compiler will call `abort'. Fortunately, the new strategies can be +added in a machine-independent fashion, and will affect only the target +machines that need them. + + +File: gcc.info, Node: Interface, Next: Passes, Prev: Portability, Up: Top + +Interfacing to GNU CC Output +**************************** + + GNU CC is normally configured to use the same function calling +convention normally in use on the target system. This is done with the +machine-description macros described (*note Target Macros::.). + + However, returning of structure and union values is done differently +on some target machines. As a result, functions compiled with PCC +returning such types cannot be called from code compiled with GNU CC, +and vice versa. This does not cause trouble often because few Unix +library routines return structures or unions. + + GNU CC code returns structures and unions that are 1, 2, 4 or 8 bytes +long in the same registers used for `int' or `double' return values. +(GNU CC typically allocates variables of such types in registers also.) +Structures and unions of other sizes are returned by storing them into +an address passed by the caller (usually in a register). 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. This is slower than the method used +by GNU CC, and fails to be reentrant. + + On some target machines, such as RISC machines and the 80386, the +standard system convention is to pass to the subroutine the address of +where to return the value. On these machines, GNU CC has been +configured to be compatible with the standard compiler, when this method +is used. It may not be compatible for structures of 1, 2, 4 or 8 bytes. + + GNU CC uses the system's standard convention for passing arguments. +On some machines, the first few arguments are passed in registers; in +others, all are passed on the stack. It would be possible to use +registers for argument passing on any machine, and this would probably +result in a significant speedup. But the result would be complete +incompatibility with code that follows the standard convention. So this +change is practical only if you are switching to GNU CC as the sole C +compiler for the system. We may implement register argument passing on +certain machines once we have a complete GNU system so that we can +compile the libraries with GNU CC. + + On some machines (particularly the Sparc), certain types of arguments +are passed "by invisible reference". This means that the value is +stored in memory, and the address of the memory location is passed to +the subroutine. + + If you use `longjmp', beware of automatic variables. ANSI C says +that automatic variables that are not declared `volatile' have undefined +values after a `longjmp'. And this is all GNU CC promises to do, +because it is very difficult to restore register variables correctly, +and one of GNU CC's features is that it can put variables in registers +without your asking it to. + + If you want a variable to be unaltered by `longjmp', and you don't +want to write `volatile' because old C compilers don't accept it, just +take the address of the variable. If a variable's address is ever +taken, even if just to compute it and ignore it, then the variable +cannot go in a register: + + { + int careful; + &careful; + ... + } - jbsr _foobar - addql #4,sp - movel d1,sp@- - movel d0,sp@- - fmoved sp@+,fp0 - -into - - jbsr _foobar - movel d1,sp@ - movel d0,sp@- - fmoved sp@+,fp0 - - INSN-PATTERN-1 and so on look *almost* like the second operand of -`define_insn'. There is one important difference: the second operand -of `define_insn' consists of one or more RTX's enclosed in square -brackets. Usually, there is only one: then the same action can be -written as an element of a `define_peephole'. But when there are -multiple actions in a `define_insn', they are implicitly enclosed in a -`parallel'. Then you must explicitly write the `parallel', and the -square brackets within it, in the `define_peephole'. Thus, if an insn -pattern looks like this, - - (define_insn "divmodsi4" - [(set (match_operand:SI 0 "general_operand" "=d") - (div:SI (match_operand:SI 1 "general_operand" "0") - (match_operand:SI 2 "general_operand" "dmsK"))) - (set (match_operand:SI 3 "general_operand" "=d") - (mod:SI (match_dup 1) (match_dup 2)))] - "TARGET_68020" - "divsl%.l %2,%3:%0") - -then the way to mention this insn in a peephole is as follows: - - (define_peephole - [... - (parallel - [(set (match_operand:SI 0 "general_operand" "=d") - (div:SI (match_operand:SI 1 "general_operand" "0") - (match_operand:SI 2 "general_operand" "dmsK"))) - (set (match_operand:SI 3 "general_operand" "=d") - (mod:SI (match_dup 1) (match_dup 2)))]) - ...] - ...) + Code compiled with GNU CC may call certain library routines. Most of +them handle arithmetic for which there are no instructions. This +includes multiply and divide on some machines, and floating point +operations on any machine for which floating point support is disabled +with `-msoft-float'. Some standard parts of the C library, such as +`bcopy' or `memcpy', are also called automatically. The usual function +call interface is used for calling the library routines. + + These library routines should be defined in the library `libgcc.a', +which GNU CC automatically searches whenever it links a program. On +machines that have multiply and divide instructions, if hardware +floating point is in use, normally `libgcc.a' is not needed, but it is +searched just in case. + + Each arithmetic function is defined in `libgcc1.c' to use the +corresponding C arithmetic operator. As long as the file is compiled +with another C compiler, which supports all the C arithmetic operators, +this file will work portably. However, `libgcc1.c' does not work if +compiled with GNU CC, because each arithmetic function would compile +into a call to itself! - \ No newline at end of file