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
statements; others represent labels to jump to or various sorts of
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
XINT, but instead three special macros are
- Accesses the unique id of insn i.
- Accesses the chain pointer to the insn preceding i. If i is the first insn, this is a null pointer.
- 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
last insn is the result of calling
get_last_insn. Within the
chain delimited by these insns, the
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
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
which it is contained. Similar rules apply for
This means that the above invariants are not necessarily true for insns
sequence expressions. Specifically, if insn is the
first insn in a
NEXT_INSN (PREV_INSN (insn
is the insn containing the
sequence expression, as is the value
PREV_INSN (NEXT_INSN (insn
)) if insn is the last
insn in the
sequence expression. You can use these expressions
to find the containing
- The expression code
insnis used for instructions that do not jump and do not do function calls.
sequenceexpressions are always contained in insns with code
insneven if one of those insns should jump or do function calls.
Insns with code
insnhave four additional fields beyond the three mandatory ones listed above. These four are described in a table below.
- The expression code
jump_insnis used for instructions that may jump (or, more generally, may contain
label_refexpressions to which
pccan be set in that instruction). If there is an instruction to return from the current function, it is recorded as a
For simple conditional and unconditional jumps, this field contains the
code_labelto which this insn will (possibly conditionally) branch. In a more complex jump,
JUMP_LABELrecords one of the labels that the insn refers to; other jump target labels are recorded as
REG_LABEL_TARGETnotes. The exception is
NULL_RTXand the only way to find the labels is to scan the entire body of the insn.
Return insns count as jumps, but since they do not refer to any labels, their
- The expression code
call_insnis 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_insninsns have the same extra fields as
insninsns, accessed in the same way and in addition contain a field
CALL_INSN_FUNCTION_USAGE, which contains a list (chain of
clobberexpressions that denote hard registers and
MEMs used or clobbered by the called function.
MEMgenerally 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
USEentries; if it's callee-copied, only a
USEwill appear, and the
MEMmay point to addresses that are not stack slots.
CLOBBERed registers in this list augment registers specified in
CALL_USED_REGISTERS(see Register Basics).
code_labelinsn 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_NUMBERis 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.
code_labelappears in an RTL expression, it normally appears within a
label_refwhich represents the address of the label, as a number.
Besides as a
code_label, a label can also be represented as a
LABEL_KINDdifferentiates four different types of labels:
LABEL_WEAK_ENTRY. The only labels that do not have type
LABEL_NORMALare 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_Pis 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
- 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
volatilefunctions, which do not return (e.g.,
exit). They contain no information beyond the three standard fields.
noteinsns are used to represent additional debugging and declarative information. They contain two nonstandard fields, an integer which is accessed with the macro
NOTE_LINE_NUMBERand a string accessed with
NOTE_LINE_NUMBERis positive, the note represents the position of a source line and
NOTE_SOURCE_FILEis the source file name that the line came from. These notes control generation of line number data in the assembler output.
- Such a note is completely ignorable. Some passes of the compiler delete insns by altering them into notes of this kind.
- 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.
- 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.
- These types of notes indicate the position of the beginning and end of a
level of scoping for exception handling.
NOTE_INSN_DELETED_LABELis associated with the given region.
- These types of notes indicate the position of the beginning and end
forloop. They enable the loop optimizer to find loops quickly.
- Appears at the place in a loop that
continuestatements jump to.
- 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.
- Appears at the start of the function body, after the function prologue.
These codes are printed symbolically when they appear in debugging dumps.
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
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.
- An expression for the side effect performed by this insn. This must be
one of the following codes:
sequence. If it is a
parallel, each element of the
parallelmust be one these codes, except that
parallelexpressions cannot be nested and
addr_diff_vecare not permitted inside a
- 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
Matching is also never attempted on insns that result from an
asmstatement. These contain at least one
asm_operandsexpression. The function
asm_noperandsreturns 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.
- A list (chain of
insn_listexpressions) giving information about dependencies between instructions within a basic block. Neither a jump nor a label may come between the related insns. These are only used by the schedulers and by combine. This is a deprecated data structure. Def-use and use-def chains are now preferred.
- A list (chain of
insn_listexpressions) giving miscellaneous information about the insn. It is often information pertaining to the registers used in this insn.
LOG_LINKS field of an insn is a chain of
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
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.
REG_NOTES field of an insn is a chain similar to the
LOG_LINKS field but it includes
expr_list expressions in
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
The first operand op of the note is data whose meaning depends on
the kind of note.
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
- 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.
- The register op being set by this insn will not be used in a
subsequent insn. This differs from a
REG_DEADnote, 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.
- 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
- 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.
REG_NONNEGnote is added to insns only if the machine description has a decrement_and_branch_until_zero pattern.
- This insn uses op, a
NOTE_INSN_DELETED_LABEL, but is not a
jump_insn, or it is a
jump_insnthat refers to the operand as an ordinary operand. The label may still eventually be a jump target, but if so in an indirect jump in a subsequent insn. 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.
- This insn is a
jump_insnbut not a
addr_diff_vec. It uses op, a
code_labelas a direct or indirect jump target. Its purpose is similar to that of
REG_LABEL_OPERAND. This note is only present if the insn has multiple targets; the last label in the insn (in the highest numbered insn-field) goes into the
JUMP_LABELfield and does not have a
REG_LABEL_TARGETnote. See JUMP_LABEL.
- 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 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.
- Appears attached to each
setjmpor a related function.
- 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
strict_low_partexpression, the note refers to the register that is contained in
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.
REG_EQUIVnote 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_SPACEin 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_EQUALis 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_EQUIVnotes are used by register allocation to indicate that there is an available substitute expression (either a constant or a
memexpression 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_EQUIVnote 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_EQUALnote. In the early stages of register allocation, a
REG_EQUALnote is changed into a
REG_EQUIVnote 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_EQUALnotes and passes subsequent to register allocation need only check for
- On machines that use
cc0, the insns which set and use
cc0set and use
cc0are adjacent. However, when branch delay slot filling is done, this may no longer be true. In this case a
REG_CC_USERnote will be placed on the insn setting
cc0to point to the insn using
REG_CC_SETTERnote will be placed on the insn using
cc0to point to the insn setting
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
- This indicates a true dependence (a read after write dependence).
- This indicates an output dependence (a write after write dependence).
- This indicates an anti dependence (a write after read dependence).
- 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.
- 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.
- 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
expr_list is printed using these symbolic codes in debugging dumps.
The only difference between the expression codes
expr_list is that the first operand of an
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.