GNU Compiler Collection (GCC) Internals

10.18 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)) if 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. In an addr_vec, JUMP_LABEL is NULL_RTX.

Return insns count as jumps, but since they do not refer to any labels, their JUMP_LABEL is NULL_RTX.

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 and MEMs used or clobbered by the called function.

A MEM generally points to a stack slots in which arguments passed to the libcall by reference (see TARGET_PASS_BY_REFERENCE) are stored. If the argument is caller-copied (see TARGET_CALLEE_COPIES), the stack slot will be mentioned in CLOBBER and USE entries; if it's callee-copied, only a USE will appear, and the MEM may point to addresses that are not stack slots. These MEMs are used only in libcalls, because, unlike regular function calls, CONST_CALLs (which libcalls generally are, see CONST_CALL_P) aren't assumed to read and write all memory, so flow would consider the stores dead and remove them. Note that, since a libcall must never return values in memory (see RETURN_IN_MEMORY), there will never be a CLOBBER for a memory address holding a return value.

CLOBBERed registers in this list augment registers specified in CALL_USED_REGISTERS (see 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.

Besides as a code_label, a label can also be represented as a note of type NOTE_INSN_DELETED_LABEL.

The field LABEL_NUSES is only defined once the jump optimization phase is completed. It contains the number of times this label is referenced in the current function.

The field LABEL_KIND differentiates four different types of labels: LABEL_NORMAL, LABEL_STATIC_ENTRY, LABEL_GLOBAL_ENTRY, and LABEL_WEAK_ENTRY. The only labels that do not have type LABEL_NORMAL are alternate entry points to the current function. These may be static (visible only in the containing translation unit), global (exposed to all translation units), or weak (global, but can be overridden by another symbol with the same name).

Much of the compiler treats all four kinds of label identically. Some of it needs to know whether or not a label is an alternate entry point; for this purpose, the macro LABEL_ALT_ENTRY_P is provided. It is equivalent to testing whether `LABEL_KIND (label) == LABEL_NORMAL'. The only place that cares about the distinction between static, global, and weak alternate entry points, besides the front-end code that creates them, is the function output_alternate_entry_point, in final.c.

To set the kind of a label, use the SET_LABEL_KIND macro.

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_DELETED_LABEL

This marks what used to be a code_label, but was not used for other purposes than taking its address and was transformed to mark that no code jumps to it.

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_EH_REGION_BEG

NOTE_INSN_EH_REGION_END

These types of notes indicate the position of the beginning and end of a level of scoping for exception handling. NOTE_BLOCK_NUMBER identifies which CODE_LABEL or note of type NOTE_INSN_DELETED_LABEL is associated with the given region.

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 at the start of the function body, after the function prologue.

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.

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.

The second Haifa scheduling pass, for targets that can multiple issue, sets the mode of an insn to TImode when it is believed that the instruction begins an issue group. That is, when the instruction cannot issue simultaneously with the previous. This may be relied on by later passes, in particular machine-dependent reorg.

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, cond_exec, 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.

It does not follow that the register op has no useful value after this insn since op is not necessarily modified by this insn. Rather, no subsequent instruction uses the contents of op.

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_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 or a note of type NOTE_INSN_DELETED_LABEL, but is not a jump_insn, or it is a jump_insn that required the label to be held in a register. The presence of this note allows jump optimization to be aware that op is, in fact, being used, and flow optimization to build an accurate flow graph.

REG_CROSSING_JUMP

This insn is an branching instruction (either an unconditional jump or an indirect jump) which crosses between hot and cold sections, which could potentially be very far apart in the executable. The presence of this note indicates to other optimizations that this this branching instruction should not be “collapsed” into a simpler branching construct. It is used when the optimization to partition basic blocks into hot and cold sections is turned on.

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.

A REG_EQUIV note is also used on an instruction which copies a register parameter into a pseudo-register at entry to a function, if there is a stack slot where that parameter could be stored. Although other insns may set the pseudo-register, it is valid for the compiler to replace the pseudo-register by stack slot throughout the function, provided the compiler ensures that the stack slot is properly initialized by making the replacement in the initial copy instruction as well. This is used on machines for which the calling convention allocates stack space for register parameters. See REG_PARM_STACK_SPACE in Stack Arguments.

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 their 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.

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.

These notes will be deleted after reload, since they are no longer accurate or useful.

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.

These notes are deleted after reload, since they are no longer useful or accurate.

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).

These notes describe information gathered from gcov profile data. They are stored in the REG_NOTES field of an insn as an expr_list.

REG_BR_PROB

This is used to specify the ratio of branches to non-branches of a branch insn according to the profile data. The value is stored as a value between 0 and REG_BR_PROB_BASE; larger values indicate a higher probability that the branch will be taken.

REG_BR_PRED

These notes are found in JUMP insns after delayed branch scheduling has taken place. They indicate both the direction and the likelihood of the JUMP. The format is a bitmask of ATTR_FLAG_* values.

REG_FRAME_RELATED_EXPR

This is used on an RTX_FRAME_RELATED_P insn wherein the attached expression is used in place of the actual insn pattern. This is done in cases where the pattern is either complex or misleading.

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.