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GNU Info File | 1996-10-12 | 51KB | 878 lines

This is Info file gcc.info, produced by Makeinfo-1.64 from the input file /ade-src/fsf/gcc/./gcc.texi. This file documents the use and the internals of the GNU compiler. Published by the Free Software Foundation 59 Temple Place - Suite 330 Boston, MA 02111-1307 USA Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are 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: Side Effects, Next: Incdec, Prev: RTL Declarations, Up: RTL Side Effect Expressions ======================= The expression codes described so far represent values, not actions. But machine instructions never produce values; they are meaningful only for their side effects on the state of the machine. Special expression codes are used to represent side effects. The body of an instruction is always one of these side effect codes; the codes described above, which represent values, appear only as the operands of these. `(set LVAL X)' Represents the action of storing the value of X into the place represented by LVAL. LVAL must be an expression representing a place that can be stored in: `reg' (or `subreg' or `strict_low_part'), `mem', `pc' or `cc0'. If LVAL is a `reg', `subreg' or `mem', it has a machine mode; then X must be valid for that mode. If LVAL is a `reg' whose machine mode is less than the full width of the register, then it means that the part of the register specified by the machine mode is given the specified value and the rest of the register receives an undefined value. Likewise, if LVAL is a `subreg' whose machine mode is narrower than the mode of the register, the rest of the register can be changed in an undefined way. If LVAL is a `strict_low_part' of a `subreg', then the part of the register specified by the machine mode of the `subreg' is given the value X and the rest of the register is not changed. If LVAL is `(cc0)', it has no machine mode, and X may be either a `compare' expression or a value that may have any mode. The latter case represents a "test" instruction. The expression `(set (cc0) (reg:M N))' is equivalent to `(set (cc0) (compare (reg:M N) (const_int 0)))'. Use the former expression to save space during the compilation. If LVAL is `(pc)', we have a jump instruction, and the possibilities for X are very limited. It may be a `label_ref' expression (unconditional jump). It may be an `if_then_else' (conditional jump), in which case either the second or the third operand must be `(pc)' (for the case which does not jump) and the other of the two must be a `label_ref' (for the case which does jump). X may also be a `mem' or `(plus:SI (pc) Y)', where Y may be a `reg' or a `mem'; these unusual patterns are used to represent jumps through branch tables. If LVAL is neither `(cc0)' nor `(pc)', the mode of LVAL must not be `VOIDmode' and the mode of X must be valid for the mode of LVAL. LVAL is customarily accessed with the `SET_DEST' macro and X with the `SET_SRC' macro. `(return)' As the sole expression in a pattern, represents a return from the current function, on machines where this can be done with one instruction, such as Vaxes. On machines where a multi-instruction "epilogue" must be executed in order to return from the function, returning is done by jumping to a label which precedes the epilogue, and the `return' expression code is never used. Inside an `if_then_else' expression, represents the value to be placed in `pc' to return to the caller. Note that an insn pattern of `(return)' is logically equivalent to `(set (pc) (return))', but the latter form is never used. `(call FUNCTION NARGS)' Represents a function call. FUNCTION is a `mem' expression whose address is the address of the function to be called. NARGS is an expression which can be used for two purposes: on some machines it represents the number of bytes of stack argument; on others, it represents the number of argument registers. Each machine has a standard machine mode which FUNCTION must have. The machine description defines macro `FUNCTION_MODE' to expand into the requisite mode name. The purpose of this mode is to specify what kind of addressing is allowed, on machines where the allowed kinds of addressing depend on the machine mode being addressed. `(clobber X)' Represents the storing or possible storing of an unpredictable, undescribed value into X, which must be a `reg', `scratch' or `mem' expression. One place this is used is in string instructions that store standard values into particular hard registers. It may not be worth the trouble to describe the values that are stored, but it is essential to inform the compiler that the registers will be altered, lest it attempt to keep data in them across the string instruction. If X is `(mem:BLK (const_int 0))', it means that all memory locations must be presumed clobbered. Note that the machine description classifies certain hard registers as "call-clobbered". All function call instructions are assumed by default to clobber these registers, so there is no need to use `clobber' expressions to indicate this fact. Also, each function call is assumed to have the potential to alter any memory location, unless the function is declared `const'. If the last group of expressions in a `parallel' are each a `clobber' expression whose arguments are `reg' or `match_scratch' (*note RTL Template::.) expressions, the combiner phase can add the appropriate `clobber' expressions to an insn it has constructed when doing so will cause a pattern to be matched. This feature can be used, for example, on a machine that whose multiply and add instructions don't use an MQ register but which has an add-accumulate instruction that does clobber the MQ register. Similarly, a combined instruction might require a temporary register while the constituent instructions might not. When a `clobber' expression for a register appears inside a `parallel' with other side effects, the register allocator guarantees that the register is unoccupied both before and after that insn. However, the reload phase may allocate a register used for one of the inputs unless the `&' constraint is specified for the selected alternative (*note Modifiers::.). You can clobber either a specific hard register, a pseudo register, or a `scratch' expression; in the latter two cases, GNU CC will allocate a hard register that is available there for use as a temporary. For instructions that require a temporary register, you should use `scratch' instead of a pseudo-register because this will allow the combiner phase to add the `clobber' when required. You do this by coding (`clobber' (`match_scratch' ...)). If you do clobber a pseudo register, use one which appears nowhere else--generate a new one each time. Otherwise, you may confuse CSE. There is one other known use for clobbering a pseudo register in a `parallel': when one of the input operands of the insn is also clobbered by the insn. In this case, using the same pseudo register in the clobber and elsewhere in the insn produces the expected results. `(use X)' Represents the use of the value of X. It indicates that the value in X at this point in the program is needed, even though it may not be apparent why this is so. Therefore, the compiler will not attempt to delete previous instructions whose only effect is to store a value in X. X must be a `reg' expression. During the delayed branch scheduling phase, X may be an insn. This indicates that X previously was located at this place in the code and its data dependencies need to be taken into account. These `use' insns will be deleted before the delayed branch scheduling phase exits. `(parallel [X0 X1 ...])' Represents several side effects performed in parallel. The square brackets stand for a vector; the operand of `parallel' is a vector of expressions. X0, X1 and so on are individual side effect expressions--expressions of code `set', `call', `return', `clobber' or `use'. "In parallel" means that first all the values used in the individual side-effects are computed, and second all the actual side-effects are performed. For example, (parallel [(set (reg:SI 1) (mem:SI (reg:SI 1))) (set (mem:SI (reg:SI 1)) (reg:SI 1))]) says unambiguously that the values of hard register 1 and the memory location addressed by it are interchanged. In both places where `(reg:SI 1)' appears as a memory address it refers to the value in register 1 *before* the execution of the insn. It follows that it is *incorrect* to use `parallel' and expect the result of one `set' to be available for the next one. For example, people sometimes attempt to represent a jump-if-zero instruction this way: (parallel [(set (cc0) (reg:SI 34)) (set (pc) (if_then_else (eq (cc0) (const_int 0)) (label_ref ...) (pc)))]) But this is incorrect, because it says that the jump condition depends on the condition code value *before* this instruction, not on the new value that is set by this instruction. Peephole optimization, which takes place together with final assembly code output, can produce insns whose patterns consist of a `parallel' whose elements are the operands needed to output the resulting assembler code--often `reg', `mem' or constant expressions. This would not be well-formed RTL at any other stage in compilation, but it is ok then because no further optimization remains to be done. However, the definition of the macro `NOTICE_UPDATE_CC', if any, must deal with such insns if you define any peephole optimizations. `(sequence [INSNS ...])' Represents a sequence of insns. Each of the INSNS that appears in the vector is suitable for appearing in the chain of insns, so it must be an `insn', `jump_insn', `call_insn', `code_label', `barrier' or `note'. A `sequence' RTX is never placed in an actual insn during RTL generation. It represents the sequence of insns that result from a `define_expand' *before* those insns are passed to `emit_insn' to insert them in the chain of insns. When actually inserted, the individual sub-insns are separated out and the `sequence' is forgotten. After delay-slot scheduling is completed, an insn and all the insns that reside in its delay slots are grouped together into a `sequence'. The insn requiring the delay slot is the first insn in the vector; subsequent insns are to be placed in the delay slot. `INSN_ANNULLED_BRANCH_P' is set on an insn in a delay slot to indicate that a branch insn should be used that will conditionally annul the effect of the insns in the delay slots. In such a case, `INSN_FROM_TARGET_P' indicates that the insn is from the target of the branch and should be executed only if the branch is taken; otherwise the insn should be executed only if the branch is not taken. *Note Delay Slots::. These expression codes appear in place of a side effect, as the body of an insn, though strictly speaking they do not always describe side effects as such: `(asm_input S)' Represents literal assembler code as described by the string S. `(unspec [OPERANDS ...] INDEX)' `(unspec_volatile [OPERANDS ...] INDEX)' Represents a machine-specific operation on OPERANDS. INDEX selects between multiple machine-specific operations. `unspec_volatile' is used for volatile operations and operations that may trap; `unspec' is used for other operations. These codes may appear inside a `pattern' of an insn, inside a `parallel', or inside an expression. `(addr_vec:M [LR0 LR1 ...])' Represents a table of jump addresses. The vector elements LR0, etc., are `label_ref' expressions. The mode M specifies how much space is given to each address; normally M would be `Pmode'. `(addr_diff_vec:M BASE [LR0 LR1 ...])' Represents a table of jump addresses expressed as offsets from BASE. The vector elements LR0, etc., are `label_ref' expressions and so is BASE. The mode M specifies how much space is given to each address-difference. File: gcc.info, Node: Incdec, Next: Assembler, Prev: Side Effects, Up: RTL Embedded Side-Effects on Addresses ================================== Four special side-effect expression codes appear as memory addresses. `(pre_dec:M X)' Represents the side effect of decrementing X by a standard amount and represents also the value that X has after being decremented. X must be a `reg' or `mem', but most machines allow only a `reg'. M must be the machine mode for pointers on the machine in use. The amount X is decremented by is the length in bytes of the machine mode of the containing memory reference of which this expression serves as the address. Here is an example of its use: (mem:DF (pre_dec:SI (reg:SI 39))) This says to decrement pseudo register 39 by the length of a `DFmode' value and use the result to address a `DFmode' value. `(pre_inc:M X)' Similar, but specifies incrementing X instead of decrementing it. `(post_dec:M X)' Represents the same side effect as `pre_dec' but a different value. The value represented here is the value X has before being decremented. `(post_inc:M X)' Similar, but specifies incrementing X instead of decrementing it. These embedded side effect expressions must be used with care. Instruction patterns may not use them. Until the `flow' pass of the compiler, they may occur only to represent pushes onto the stack. The `flow' pass finds cases where registers are incremented or decremented in one instruction and used as an address shortly before or after; these cases are then transformed to use pre- or post-increment or -decrement. If a register used as the operand of these expressions is used in another address in an insn, the original value of the register is used. Uses of the register outside of an address are not permitted within the same insn as a use in an embedded side effect expression because such insns behave differently on different machines and hence must be treated as ambiguous and disallowed. An instruction that can be represented with an embedded side effect could also be represented using `parallel' containing an additional `set' to describe how the address register is altered. This is not done because machines that allow these operations at all typically allow them wherever a memory address is called for. Describing them as additional parallel stores would require doubling the number of entries in the machine description. File: gcc.info, Node: Assembler, Next: Insns, Prev: Incdec, Up: RTL Assembler Instructions as Expressions ===================================== The RTX code `asm_operands' represents a value produced by a user-specified assembler instruction. It is used to represent an `asm' statement with arguments. An `asm' statement with a single output operand, like this: asm ("foo %1,%2,%0" : "=a" (outputvar) : "g" (x + y), "di" (*z)); is represented using a single `asm_operands' RTX which represents the value that is stored in `outputvar': (set RTX-FOR-OUTPUTVAR (asm_operands "foo %1,%2,%0" "a" 0 [RTX-FOR-ADDITION-RESULT RTX-FOR-*Z] [(asm_input:M1 "g") (asm_input:M2 "di")])) Here the operands of the `asm_operands' RTX are the assembler template string, the output-operand's constraint, the index-number of the output operand among the output operands specified, a vector of input operand RTX's, and a vector of input-operand modes and constraints. The mode M1 is the mode of the sum `x+y'; M2 is that of `*z'. When an `asm' statement has multiple output values, its insn has several such `set' RTX's inside of a `parallel'. Each `set' contains a `asm_operands'; all of these share the same assembler template and vectors, but each contains the constraint for the respective output operand. They are also distinguished by the output-operand index number, which is 0, 1, ... for successive output operands. File: gcc.info, Node: Insns, Next: Calls, Prev: Assembler, Up: RTL Insns ===== The RTL representation of the code for a function is a doubly-linked chain of objects called "insns". Insns are expressions with special codes that are used for no other purpose. Some insns are actual instructions; others represent dispatch tables for `switch' statements; others represent labels to jump to or various sorts of declarative information. In addition to its own specific data, each insn must have a unique id-number that distinguishes it from all other insns in the current function (after delayed branch scheduling, copies of an insn with the same id-number may be present in multiple places in a function, but these copies will always be identical and will only appear inside a `sequence'), and chain pointers to the preceding and following insns. These three fields occupy the same position in every insn, independent of the expression code of the insn. They could be accessed with `XEXP' and `XINT', but instead three special macros are always used: `INSN_UID (I)' Accesses the unique id of insn I. `PREV_INSN (I)' Accesses the chain pointer to the insn preceding I. If I is the first insn, this is a null pointer. `NEXT_INSN (I)' Accesses the chain pointer to the insn following I. If I is the last insn, this is a null pointer. The first insn in the chain is obtained by calling `get_insns'; the last insn is the result of calling `get_last_insn'. Within the chain delimited by these insns, the `NEXT_INSN' and `PREV_INSN' pointers must always correspond: if INSN is not the first insn, NEXT_INSN (PREV_INSN (INSN)) == INSN is always true and if INSN is not the last insn, PREV_INSN (NEXT_INSN (INSN)) == INSN is always true. After delay slot scheduling, some of the insns in the chain might be `sequence' expressions, which contain a vector of insns. The value of `NEXT_INSN' in all but the last of these insns is the next insn in the vector; the value of `NEXT_INSN' of the last insn in the vector is the same as the value of `NEXT_INSN' for the `sequence' in which it is contained. Similar rules apply for `PREV_INSN'. This means that the above invariants are not necessarily true for insns inside `sequence' expressions. Specifically, if INSN is the first insn in a `sequence', `NEXT_INSN (PREV_INSN (INSN))' is the insn containing the `sequence' expression, as is the value of `PREV_INSN (NEXT_INSN (INSN))' is INSN is the last insn in the `sequence' expression. You can use these expressions to find the containing `sequence' expression. Every insn has one of the following six expression codes: `insn' The expression code `insn' is used for instructions that do not jump and do not do function calls. `sequence' expressions are always contained in insns with code `insn' even if one of those insns should jump or do function calls. Insns with code `insn' have four additional fields beyond the three mandatory ones listed above. These four are described in a table below. `jump_insn' The expression code `jump_insn' is used for instructions that may jump (or, more generally, may contain `label_ref' expressions). If there is an instruction to return from the current function, it is recorded as a `jump_insn'. `jump_insn' insns have the same extra fields as `insn' insns, accessed in the same way and in addition contain a field `JUMP_LABEL' which is defined once jump optimization has completed. For simple conditional and unconditional jumps, this field contains the `code_label' to which this insn will (possibly conditionally) branch. In a more complex jump, `JUMP_LABEL' records one of the labels that the insn refers to; the only way to find the others is to scan the entire body of the insn. Return insns count as jumps, but since they do not refer to any labels, they have zero in the `JUMP_LABEL' field. `call_insn' The expression code `call_insn' is used for instructions that may do function calls. It is important to distinguish these instructions because they imply that certain registers and memory locations may be altered unpredictably. `call_insn' insns have the same extra fields as `insn' insns, accessed in the same way and in addition contain a field `CALL_INSN_FUNCTION_USAGE', which contains a list (chain of `expr_list' expressions) containing `use' and `clobber' expressions that denote hard registers used or clobbered by the called function. A register specified in a `clobber' in this list is modified *after* the execution of the `call_insn', while a register in a `clobber' in the body of the `call_insn' is clobbered before the insn completes execution. `clobber' expressions in this list augment registers specified in `CALL_USED_REGISTERS' (*note Register Basics::.). `code_label' A `code_label' insn represents a label that a jump insn can jump to. It contains two special fields of data in addition to the three standard ones. `CODE_LABEL_NUMBER' is used to hold the "label number", a number that identifies this label uniquely among all the labels in the compilation (not just in the current function). Ultimately, the label is represented in the assembler output as an assembler label, usually of the form `LN' where N is the label number. When a `code_label' appears in an RTL expression, it normally appears within a `label_ref' which represents the address of the label, as a number. The field `LABEL_NUSES' is only defined once the jump optimization phase is completed and contains the number of times this label is referenced in the current function. `barrier' Barriers are placed in the instruction stream when control cannot flow past them. They are placed after unconditional jump instructions to indicate that the jumps are unconditional and after calls to `volatile' functions, which do not return (e.g., `exit'). They contain no information beyond the three standard fields. `note' `note' insns are used to represent additional debugging and declarative information. They contain two nonstandard fields, an integer which is accessed with the macro `NOTE_LINE_NUMBER' and a string accessed with `NOTE_SOURCE_FILE'. If `NOTE_LINE_NUMBER' is positive, the note represents the position of a source line and `NOTE_SOURCE_FILE' is the source file name that the line came from. These notes control generation of line number data in the assembler output. Otherwise, `NOTE_LINE_NUMBER' is not really a line number but a code with one of the following values (and `NOTE_SOURCE_FILE' must contain a null pointer): `NOTE_INSN_DELETED' Such a note is completely ignorable. Some passes of the compiler delete insns by altering them into notes of this kind. `NOTE_INSN_BLOCK_BEG' `NOTE_INSN_BLOCK_END' These types of notes indicate the position of the beginning and end of a level of scoping of variable names. They control the output of debugging information. `NOTE_INSN_LOOP_BEG' `NOTE_INSN_LOOP_END' These types of notes indicate the position of the beginning and end of a `while' or `for' loop. They enable the loop optimizer to find loops quickly. `NOTE_INSN_LOOP_CONT' Appears at the place in a loop that `continue' statements jump to. `NOTE_INSN_LOOP_VTOP' This note indicates the place in a loop where the exit test begins for those loops in which the exit test has been duplicated. This position becomes another virtual start of the loop when considering loop invariants. `NOTE_INSN_FUNCTION_END' Appears near the end of the function body, just before the label that `return' statements jump to (on machine where a single instruction does not suffice for returning). This note may be deleted by jump optimization. `NOTE_INSN_SETJMP' Appears following each call to `setjmp' or a related function. These codes are printed symbolically when they appear in debugging dumps. The machine mode of an insn is normally `VOIDmode', but some phases use the mode for various purposes; for example, the reload pass sets it to `HImode' if the insn needs reloading but not register elimination and `QImode' if both are required. The common subexpression elimination pass sets the mode of an insn to `QImode' when it is the first insn in a block that has already been processed. Here is a table of the extra fields of `insn', `jump_insn' and `call_insn' insns: `PATTERN (I)' An expression for the side effect performed by this insn. This must be one of the following codes: `set', `call', `use', `clobber', `return', `asm_input', `asm_output', `addr_vec', `addr_diff_vec', `trap_if', `unspec', `unspec_volatile', `parallel', or `sequence'. If it is a `parallel', each element of the `parallel' must be one these codes, except that `parallel' expressions cannot be nested and `addr_vec' and `addr_diff_vec' are not permitted inside a `parallel' expression. `INSN_CODE (I)' An integer that says which pattern in the machine description matches this insn, or -1 if the matching has not yet been attempted. Such matching is never attempted and this field remains -1 on an insn whose pattern consists of a single `use', `clobber', `asm_input', `addr_vec' or `addr_diff_vec' expression. Matching is also never attempted on insns that result from an `asm' statement. These contain at least one `asm_operands' expression. The function `asm_noperands' returns a non-negative value for such insns. In the debugging output, this field is printed as a number followed by a symbolic representation that locates the pattern in the `md' file as some small positive or negative offset from a named pattern. `LOG_LINKS (I)' A list (chain of `insn_list' expressions) giving information about dependencies between instructions within a basic block. Neither a jump nor a label may come between the related insns. `REG_NOTES (I)' A list (chain of `expr_list' and `insn_list' expressions) giving miscellaneous information about the insn. It is often information pertaining to the registers used in this insn. The `LOG_LINKS' field of an insn is a chain of `insn_list' expressions. Each of these has two operands: the first is an insn, and the second is another `insn_list' expression (the next one in the chain). The last `insn_list' in the chain has a null pointer as second operand. The significant thing about the chain is which insns appear in it (as first operands of `insn_list' expressions). Their order is not significant. This list is originally set up by the flow analysis pass; it is a null pointer until then. Flow only adds links for those data dependencies which can be used for instruction combination. For each insn, the flow analysis pass adds a link to insns which store into registers values that are used for the first time in this insn. The instruction scheduling pass adds extra links so that every dependence will be represented. Links represent data dependencies, antidependencies and output dependencies; the machine mode of the link distinguishes these three types: antidependencies have mode `REG_DEP_ANTI', output dependencies have mode `REG_DEP_OUTPUT', and data dependencies have mode `VOIDmode'. The `REG_NOTES' field of an insn is a chain similar to the `LOG_LINKS' field but it includes `expr_list' expressions in addition to `insn_list' expressions. There are several kinds of register notes, which are distinguished by the machine mode, which in a register note is really understood as being an `enum reg_note'. The first operand OP of the note is data whose meaning depends on the kind of note. The macro `REG_NOTE_KIND (X)' returns the kind of register note. Its counterpart, the macro `PUT_REG_NOTE_KIND (X, NEWKIND)' sets the register note type of X to be NEWKIND. Register notes are of three classes: They may say something about an input to an insn, they may say something about an output of an insn, or they may create a linkage between two insns. There are also a set of values that are only used in `LOG_LINKS'. These register notes annotate inputs to an insn: `REG_DEAD' The value in OP dies in this insn; that is to say, altering the value immediately after this insn would not affect the future behavior of the program. This does not necessarily mean that the register OP has no useful value after this insn since it may also be an output of the insn. In such a case, however, a `REG_DEAD' note would be redundant and is usually not present until after the reload pass, but no code relies on this fact. `REG_INC' The register OP is incremented (or decremented; at this level there is no distinction) by an embedded side effect inside this insn. This means it appears in a `post_inc', `pre_inc', `post_dec' or `pre_dec' expression. `REG_NONNEG' The register OP is known to have a nonnegative value when this insn is reached. This is used so that decrement and branch until zero instructions, such as the m68k dbra, can be matched. The `REG_NONNEG' note is added to insns only if the machine description has a `decrement_and_branch_until_zero' pattern. `REG_NO_CONFLICT' This insn does not cause a conflict between OP and the item being set by this insn even though it might appear that it does. In other words, if the destination register and OP could otherwise be assigned the same register, this insn does not prevent that assignment. Insns with this note are usually part of a block that begins with a `clobber' insn specifying a multi-word pseudo register (which will be the output of the block), a group of insns that each set one word of the value and have the `REG_NO_CONFLICT' note attached, and a final insn that copies the output to itself with an attached `REG_EQUAL' note giving the expression being computed. This block is encapsulated with `REG_LIBCALL' and `REG_RETVAL' notes on the first and last insns, respectively. `REG_LABEL' This insn uses OP, a `code_label', but is not a `jump_insn'. The presence of this note allows jump optimization to be aware that OP is, in fact, being used. The following notes describe attributes of outputs of an insn: `REG_EQUIV' `REG_EQUAL' This note is only valid on an insn that sets only one register and indicates that that register will be equal to OP at run time; the scope of this equivalence differs between the two types of notes. The value which the insn explicitly copies into the register may look different from OP, but they will be equal at run time. If the output of the single `set' is a `strict_low_part' expression, the note refers to the register that is contained in `SUBREG_REG' of the `subreg' expression. For `REG_EQUIV', the register is equivalent to OP throughout the entire function, and could validly be replaced in all its occurrences by OP. ("Validly" here refers to the data flow of the program; simple replacement may make some insns invalid.) For example, when a constant is loaded into a register that is never assigned any other value, this kind of note is used. When a parameter is copied into a pseudo-register at entry to a function, a note of this kind records that the register is equivalent to the stack slot where the parameter was passed. Although in this case the register may be set by other insns, it is still valid to replace the register by the stack slot throughout the function. In the case of `REG_EQUAL', the register that is set by this insn will be equal to OP at run time at the end of this insn but not necessarily elsewhere in the function. In this case, OP is typically an arithmetic expression. For example, when a sequence of insns such as a library call is used to perform an arithmetic operation, this kind of note is attached to the insn that produces or copies the final value. These two notes are used in different ways by the compiler passes. `REG_EQUAL' is used by passes prior to register allocation (such as common subexpression elimination and loop optimization) to tell them how to think of that value. `REG_EQUIV' notes are used by register allocation to indicate that there is an available substitute expression (either a constant or a `mem' expression for the location of a parameter on the stack) that may be used in place of a register if insufficient registers are available. Except for stack homes for parameters, which are indicated by a `REG_EQUIV' note and are not useful to the early optimization passes and pseudo registers that are equivalent to a memory location throughout there entire life, which is not detected until later in the compilation, all equivalences are initially indicated by an attached `REG_EQUAL' note. In the early stages of register allocation, a `REG_EQUAL' note is changed into a `REG_EQUIV' note if OP is a constant and the insn represents the only set of its destination register. Thus, compiler passes prior to register allocation need only check for `REG_EQUAL' notes and passes subsequent to register allocation need only check for `REG_EQUIV' notes. `REG_UNUSED' The register OP being set by this insn will not be used in a subsequent insn. This differs from a `REG_DEAD' note, which indicates that the value in an input will not be used subsequently. These two notes are independent; both may be present for the same register. `REG_WAS_0' The single output of this insn contained zero before this insn. OP is the insn that set it to zero. You can rely on this note if it is present and OP has not been deleted or turned into a `note'; its absence implies nothing. These notes describe linkages between insns. They occur in pairs: one insn has one of a pair of notes that points to a second insn, which has the inverse note pointing back to the first insn. `REG_RETVAL' This insn copies the value of a multi-insn sequence (for example, a library call), and OP is the first insn of the sequence (for a library call, the first insn that was generated to set up the arguments for the library call). Loop optimization uses this note to treat such a sequence as a single operation for code motion purposes and flow analysis uses this note to delete such sequences whose results are dead. A `REG_EQUAL' note will also usually be attached to this insn to provide the expression being computed by the sequence. `REG_LIBCALL' This is the inverse of `REG_RETVAL': it is placed on the first insn of a multi-insn sequence, and it points to the last one. `REG_CC_SETTER' `REG_CC_USER' On machines that use `cc0', the insns which set and use `cc0' set and use `cc0' are adjacent. However, when branch delay slot filling is done, this may no longer be true. In this case a `REG_CC_USER' note will be placed on the insn setting `cc0' to point to the insn using `cc0' and a `REG_CC_SETTER' note will be placed on the insn using `cc0' to point to the insn setting `cc0'. These values are only used in the `LOG_LINKS' field, and indicate the type of dependency that each link represents. Links which indicate a data dependence (a read after write dependence) do not use any code, they simply have mode `VOIDmode', and are printed without any descriptive text. `REG_DEP_ANTI' This indicates an anti dependence (a write after read dependence). `REG_DEP_OUTPUT' This indicates an output dependence (a write after write dependence). For convenience, the machine mode in an `insn_list' or `expr_list' is printed using these symbolic codes in debugging dumps. The only difference between the expression codes `insn_list' and `expr_list' is that the first operand of an `insn_list' is assumed to be an insn and is printed in debugging dumps as the insn's unique id; the first operand of an `expr_list' is printed in the ordinary way as an expression. File: gcc.info, Node: Calls, Next: Sharing, Prev: Insns, Up: RTL RTL Representation of Function-Call Insns ========================================= Insns that call subroutines have the RTL expression code `call_insn'. These insns must satisfy special rules, and their bodies must use a special RTL expression code, `call'. A `call' expression has two operands, as follows: (call (mem:FM ADDR) NBYTES) Here NBYTES is an operand that represents the number of bytes of argument data being passed to the subroutine, FM is a machine mode (which must equal as the definition of the `FUNCTION_MODE' macro in the machine description) and ADDR represents the address of the subroutine. For a subroutine that returns no value, the `call' expression as shown above is the entire body of the insn, except that the insn might also contain `use' or `clobber' expressions. For a subroutine that returns a value whose mode is not `BLKmode', the value is returned in a hard register. If this register's number is R, then the body of the call insn looks like this: (set (reg:M R) (call (mem:FM ADDR) NBYTES)) This RTL expression makes it clear (to the optimizer passes) that the appropriate register receives a useful value in this insn. When a subroutine returns a `BLKmode' value, it is handled by passing to the subroutine the address of a place to store the value. So the call insn itself does not "return" any value, and it has the same RTL form as a call that returns nothing. On some machines, the call instruction itself clobbers some register, for example to contain the return address. `call_insn' insns on these machines should have a body which is a `parallel' that contains both the `call' expression and `clobber' expressions that indicate which registers are destroyed. Similarly, if the call instruction requires some register other than the stack pointer that is not explicitly mentioned it its RTL, a `use' subexpression should mention that register. Functions that are called are assumed to modify all registers listed in the configuration macro `CALL_USED_REGISTERS' (*note Register Basics::.) and, with the exception of `const' functions and library calls, to modify all of memory. Insns containing just `use' expressions directly precede the `call_insn' insn to indicate which registers contain inputs to the function. Similarly, if registers other than those in `CALL_USED_REGISTERS' are clobbered by the called function, insns containing a single `clobber' follow immediately after the call to indicate which registers. File: gcc.info, Node: Sharing, Next: Reading RTL, Prev: Calls, Up: RTL Structure Sharing Assumptions ============================= The compiler assumes that certain kinds of RTL expressions are unique; there do not exist two distinct objects representing the same value. In other cases, it makes an opposite assumption: that no RTL expression object of a certain kind appears in more than one place in the containing structure. These assumptions refer to a single function; except for the RTL objects that describe global variables and external functions, and a few standard objects such as small integer constants, no RTL objects are common to two functions. * Each pseudo-register has only a single `reg' object to represent it, and therefore only a single machine mode. * For any symbolic label, there is only one `symbol_ref' object referring to it. * There is only one `const_int' expression with value 0, only one with value 1, and only one with value -1. Some other integer values are also stored uniquely. * There is only one `pc' expression. * There is only one `cc0' expression. * There is only one `const_double' expression with value 0 for each floating point mode. Likewise for values 1 and 2. * No `label_ref' or `scratch' appears in more than one place in the RTL structure; in other words, it is safe to do a tree-walk of all the insns in the function and assume that each time a `label_ref' or `scratch' is seen it is distinct from all others that are seen. * Only one `mem' object is normally created for each static variable or stack slot, so these objects are frequently shared in all the places they appear. However, separate but equal objects for these variables are occasionally made. * When a single `asm' statement has multiple output operands, a distinct `asm_operands' expression is made for each output operand. However, these all share the vector which contains the sequence of input operands. This sharing is used later on to test whether two `asm_operands' expressions come from the same statement, so all optimizations must carefully preserve the sharing if they copy the vector at all. * No RTL object appears in more than one place in the RTL structure except as described above. Many passes of the compiler rely on this by assuming that they can modify RTL objects in place without unwanted side-effects on other insns. * During initial RTL generation, shared structure is freely introduced. After all the RTL for a function has been generated, all shared structure is copied by `unshare_all_rtl' in `emit-rtl.c', after which the above rules are guaranteed to be followed. * During the combiner pass, shared structure within an insn can exist temporarily. However, the shared structure is copied before the combiner is finished with the insn. This is done by calling `copy_rtx_if_shared', which is a subroutine of `unshare_all_rtl'. File: gcc.info, Node: Reading RTL, Prev: Sharing, Up: RTL Reading RTL =========== To read an RTL object from a file, call `read_rtx'. It takes one argument, a stdio stream, and returns a single RTL object. Reading RTL from a file is very slow. This is not currently a problem since reading RTL occurs only as part of building the compiler. People frequently have the idea of using RTL stored as text in a file as an interface between a language front end and the bulk of GNU CC. This idea is not feasible. GNU CC was designed to use RTL internally only. Correct RTL for a given program is very dependent on the particular target machine. And the RTL does not contain all the information about the program. The proper way to interface GNU CC to a new language front end is with the "tree" data structure. There is no manual for this data structure, but it is described in the files `tree.h' and `tree.def'. File: gcc.info, Node: Machine Desc, Next: Target Macros, Prev: RTL, Up: Top Machine Descriptions ******************** A machine description has two parts: a file of instruction patterns (`.md' file) and a C header file of macro definitions. The `.md' file for a target machine contains a pattern for each instruction that the target machine supports (or at least each instruction that is worth telling the compiler about). It may also contain comments. A semicolon causes the rest of the line to be a comment, unless the semicolon is inside a quoted string. See the next chapter for information on the C header file. * Menu: * Patterns:: How to write instruction patterns. * Example:: An explained example of a `define_insn' pattern. * RTL Template:: The RTL template defines what insns match a pattern. * Output Template:: The output template says how to make assembler code from such an insn. * Output Statement:: For more generality, write C code to output the assembler code. * Constraints:: When not all operands are general operands. * Standard Names:: Names mark patterns to use for code generation. * Pattern Ordering:: When the order of patterns makes a difference. * Dependent Patterns:: Having one pattern may make you need another. * Jump Patterns:: Special considerations for patterns for jump insns. * Insn Canonicalizations::Canonicalization of Instructions * Peephole Definitions::Defining machine-specific peephole optimizations. * Expander Definitions::Generating a sequence of several RTL insns for a standard operation. * Insn Splitting:: Splitting Instructions into Multiple Instructions * Insn Attributes:: Specifying the value of attributes for generated insns. File: gcc.info, Node: Patterns, Next: Example, Up: Machine Desc Everything about Instruction Patterns ===================================== Each instruction pattern contains an incomplete RTL expression, with pieces to be filled in later, operand constraints that restrict how the pieces can be filled in, and an output pattern or C code to generate the assembler output, all wrapped up in a `define_insn' expression. A `define_insn' is an RTL expression containing four or five operands: 1. An optional name. The presence of a name indicate that this instruction pattern can perform a certain standard job for the RTL-generation pass of the compiler. This pass knows certain names and will use the instruction patterns with those names, if the names are defined in the machine description. The absence of a name is indicated by writing an empty string where the name should go. Nameless instruction patterns are never used for generating RTL code, but they may permit several simpler insns to be combined later on. Names that are not thus known and used in RTL-generation have no effect; they are equivalent to no name at all. 2. The "RTL template" (*note RTL Template::.) is a vector of incomplete RTL expressions which show what the instruction should look like. It is incomplete because it may contain `match_operand', `match_operator', and `match_dup' expressions that stand for operands of the instruction. If the vector has only one element, that element is the template for the instruction pattern. If the vector has multiple elements, then the instruction pattern is a `parallel' expression containing the elements described. 3. A condition. This is a string which contains a C expression that is the final test to decide whether an insn body matches this pattern. For a named pattern, the condition (if present) may not depend on the data in the insn being matched, but only the target-machine-type flags. The compiler needs to test these conditions during initialization in order to learn exactly which named instructions are available in a particular run. For nameless patterns, the condition is applied only when matching an individual insn, and only after the insn has matched the pattern's recognition template. The insn's operands may be found in the vector `operands'. 4. The "output template": a string that says how to output matching insns as assembler code. `%' in this string specifies where to substitute the value of an operand. *Note Output Template::. When simple substitution isn't general enough, you can specify a piece of C code to compute the output. *Note Output Statement::. 5. Optionally, a vector containing the values of attributes for insns matching this pattern. *Note Insn Attributes::. File: gcc.info, Node: Example, Next: RTL Template, Prev: Patterns, Up: Machine Desc Example of `define_insn' ======================== Here is an actual example of an instruction pattern, for the 68000/68020. (define_insn "tstsi" [(set (cc0) (match_operand:SI 0 "general_operand" "rm"))] "" "* { if (TARGET_68020 || ! ADDRESS_REG_P (operands[0])) return \"tstl %0\"; return \"cmpl #0,%0\"; }") This is an instruction that sets the condition codes based on the value of a general operand. It has no condition, so any insn whose RTL description has the form shown may be handled according to this pattern. The name `tstsi' means "test a `SImode' value" and tells the RTL generation pass that, when it is necessary to test such a value, an insn to do so can be constructed using this pattern. The output control string is a piece of C code which chooses which output template to return based on the kind of operand and the specific type of CPU for which code is being generated. `"rm"' is an operand constraint. Its meaning is explained below.