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GNU Info File | 1994-11-23 | 47.6 KB | 1,090 lines

This is Info file gcc.info, produced by Makeinfo-1.55 from the input file gcc.texi. This file documents the use and the internals of the GNU compiler. Published by the Free Software Foundation 675 Massachusetts Avenue Cambridge, MA 02139 USA 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 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," "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," "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 Standard Pattern Names For Generation ===================================== 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 perhaps also `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', `lshrM3', `rotlM3', `rotrM3' Other shift and rotate instructions, analogous to the `ashlM3' instructions. `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. Compute the length of a string, with three operands. Operand 0 is the result (of mode M), operand 1 is a `mem' referring to the first character of the string, operand 2 is the character to search for (normally zero), and operand 3 is a constant describing the known alignment of the beginning of the string. `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 `bCOND' 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. `untyped_call' Subroutine call instruction returning a value of any type. Operand 0 is the function to call; operand 1 is a memory location where the result of calling the function is to be stored; operand 2 is a `parallel' expression where each element is a `set' expression that indicates the saving of a function return value into the result block. This instruction pattern should be defined to support `__builtin_apply' on machines where special instructions are needed to call a subroutine with arbitrary arguments or to save the value returned. This instruction pattern is required on machines that have multiple registers that can hold a return value (i.e. `FUNCTION_VALUE_REGNO_P' is true for more than one register). `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. `untyped_return' Untyped subroutine return instruction. This instruction pattern should be defined to support `__builtin_return' on machines where special instructions are needed to return a value of any type. Operand 0 is a memory location where the result of calling a function with `__builtin_apply' is stored; operand 1 is a `parallel' expression where each element is a `set' expression that indicates the restoring of a function return value from the result block. `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' do a similar job 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 (or add if `STACK_GROWS_DOWNWARD' is undefined) operand 0 from the stack pointer to create space for dynamically allocated data. Do not define this pattern if all that must be done is the subtraction. 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. File: gcc.info, Node: Pattern Ordering, Next: Dependent Patterns, Prev: Standard Names, Up: Machine Desc When the Order of Patterns Matters ================================== 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. File: gcc.info, Node: Dependent Patterns, Next: Jump Patterns, Prev: Pattern Ordering, Up: Machine Desc Interdependence of Patterns =========================== 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. File: gcc.info, Node: Jump Patterns, Next: Insn Canonicalizations, Prev: Dependent Patterns, Up: Machine Desc Defining Jump Instruction Patterns ================================== 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)))] "" "...") The `SELECT_CC_MODE' macro on the Sparc returns `CC_NOOVmode' for comparisons whose argument is a `plus'. File: gcc.info, Node: Insn Canonicalizations, Next: Peephole Definitions, Prev: Jump Patterns, Up: Machine Desc Canonicalization of Instructions ================================ 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. File: gcc.info, Node: Peephole Definitions, Next: Expander Definitions, Prev: Insn Canonicalizations, Up: Machine Desc Machine-Specific Peephole Optimizers ==================================== In addition to instruction patterns the `md' file may contain definitions of machine-specific peephole optimizations. 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\"; #else output_asm_insn (\"movel %1,sp@\", xoperands); output_asm_insn (\"movel %1,sp@-\", operands); return \"fmoved sp@+,%0\"; #endif } ") The effect of this optimization is to change 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)))]) ...] ...)