GNU Info
(gcc-295.info)Insns
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_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' 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 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-dependant 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', 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.
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 Note: 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 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.
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_EXEC_COUNT'
This is used to indicate the number of times a basic block was
executed according to the profile data. The note is attached to
the first insn in the basic block.
`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
likelyhood 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.
automatically generated by info2www version 1.2.2.9