Google Git
Sign in
gnu / gcc / 96e055f719ec2f88fb47185ae05eea5c752bd0fd / . / gcc / gcc.info-14
blob: a21fb4d5f6479c1b6775917c3363b8607e949dbb [file] [log] [blame]
This is Info file gcc.info, produced by Makeinfo version 1.68 from the
input file 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, 1996, 1997, 1998
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: Passes, Next: RTL, Prev: Interface, Up: Top
Passes and Files of the Compiler
********************************
The overall control structure of the compiler is in `toplev.c'. This
file is responsible for initialization, decoding arguments, opening and
closing files, and sequencing the passes.
The parsing pass is invoked only once, to parse the entire input.
The RTL intermediate code for a function is generated as the function
is parsed, a statement at a time. Each statement is read in as a
syntax tree and then converted to RTL; then the storage for the tree
for the statement is reclaimed. Storage for types (and the expressions
for their sizes), declarations, and a representation of the binding
contours and how they nest, remain until the function is finished being
compiled; these are all needed to output the debugging information.
Each time the parsing pass reads a complete function definition or
top-level declaration, it calls either the function
`rest_of_compilation', or the function `rest_of_decl_compilation' in
`toplev.c', which are responsible for all further processing necessary,
ending with output of the assembler language. All other compiler
passes run, in sequence, within `rest_of_compilation'. When that
function returns from compiling a function definition, the storage used
for that function definition's compilation is entirely freed, unless it
is an inline function (*note An Inline Function is As Fast As a Macro:
Inline.).
Here is a list of all the passes of the compiler and their source
files. Also included is a description of where debugging dumps can be
requested with `-d' options.
* Parsing. This pass reads the entire text of a function definition,
constructing partial syntax trees. This and RTL generation are no
longer truly separate passes (formerly they were), but it is
easier to think of them as separate.
The tree representation does not entirely follow C syntax, because
it is intended to support other languages as well.
Language-specific data type analysis is also done in this pass,
and every tree node that represents an expression has a data type
attached. Variables are represented as declaration nodes.
Constant folding and some arithmetic simplifications are also done
during this pass.
The language-independent source files for parsing are
`stor-layout.c', `fold-const.c', and `tree.c'. There are also
header files `tree.h' and `tree.def' which define the format of
the tree representation.
The source files to parse C are `c-parse.in', `c-decl.c',
`c-typeck.c', `c-aux-info.c', `c-convert.c', and `c-lang.c' along
with header files `c-lex.h', and `c-tree.h'.
The source files for parsing C++ are `cp-parse.y', `cp-class.c',
`cp-cvt.c', `cp-decl.c', `cp-decl2.c', `cp-dem.c', `cp-except.c',
`cp-expr.c', `cp-init.c', `cp-lex.c', `cp-method.c', `cp-ptree.c',
`cp-search.c', `cp-tree.c', `cp-type2.c', and `cp-typeck.c', along
with header files `cp-tree.def', `cp-tree.h', and `cp-decl.h'.
The special source files for parsing Objective C are
`objc-parse.y', `objc-actions.c', `objc-tree.def', and
`objc-actions.h'. Certain C-specific files are used for this as
well.
The file `c-common.c' is also used for all of the above languages.
* RTL generation. This is the conversion of syntax tree into RTL
code. It is actually done statement-by-statement during parsing,
but for most purposes it can be thought of as a separate pass.
This is where the bulk of target-parameter-dependent code is found,
since often it is necessary for strategies to apply only when
certain standard kinds of instructions are available. The purpose
of named instruction patterns is to provide this information to
the RTL generation pass.
Optimization is done in this pass for `if'-conditions that are
comparisons, boolean operations or conditional expressions. Tail
recursion is detected at this time also. Decisions are made about
how best to arrange loops and how to output `switch' statements.
The source files for RTL generation include `stmt.c', `calls.c',
`expr.c', `explow.c', `expmed.c', `function.c', `optabs.c' and
`emit-rtl.c'. Also, the file `insn-emit.c', generated from the
machine description by the program `genemit', is used in this
pass. The header file `expr.h' is used for communication within
this pass.
The header files `insn-flags.h' and `insn-codes.h', generated from
the machine description by the programs `genflags' and `gencodes',
tell this pass which standard names are available for use and
which patterns correspond to them.
Aside from debugging information output, none of the following
passes refers to the tree structure representation of the function
(only part of which is saved).
The decision of whether the function can and should be expanded
inline in its subsequent callers is made at the end of rtl
generation. The function must meet certain criteria, currently
related to the size of the function and the types and number of
parameters it has. Note that this function may contain loops,
recursive calls to itself (tail-recursive functions can be
inlined!), gotos, in short, all constructs supported by GNU CC.
The file `integrate.c' contains the code to save a function's rtl
for later inlining and to inline that rtl when the function is
called. The header file `integrate.h' is also used for this
purpose.
The option `-dr' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending `.rtl' to
the input file name.
* Jump optimization. This pass simplifies jumps to the following
instruction, jumps across jumps, and jumps to jumps. It deletes
unreferenced labels and unreachable code, except that unreachable
code that contains a loop is not recognized as unreachable in this
pass. (Such loops are deleted later in the basic block analysis.)
It also converts some code originally written with jumps into
sequences of instructions that directly set values from the
results of comparisons, if the machine has such instructions.
Jump optimization is performed two or three times. The first time
is immediately following RTL generation. The second time is after
CSE, but only if CSE says repeated jump optimization is needed.
The last time is right before the final pass. That time,
cross-jumping and deletion of no-op move instructions are done
together with the optimizations described above.
The source file of this pass is `jump.c'.
The option `-dj' causes a debugging dump of the RTL code after
this pass is run for the first time. This dump file's name is
made by appending `.jump' to the input file name.
* Register scan. This pass finds the first and last use of each
register, as a guide for common subexpression elimination. Its
source is in `regclass.c'.
* Jump threading. This pass detects a condition jump that branches
to an identical or inverse test. Such jumps can be `threaded'
through the second conditional test. The source code for this
pass is in `jump.c'. This optimization is only performed if
`-fthread-jumps' is enabled.
* Common subexpression elimination. This pass also does constant
propagation. Its source file is `cse.c'. If constant propagation
causes conditional jumps to become unconditional or to become
no-ops, jump optimization is run again when CSE is finished.
The option `-ds' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending `.cse' to
the input file name.
* Loop optimization. This pass moves constant expressions out of
loops, and optionally does strength-reduction and loop unrolling
as well. Its source files are `loop.c' and `unroll.c', plus the
header `loop.h' used for communication between them. Loop
unrolling uses some functions in `integrate.c' and the header
`integrate.h'.
The option `-dL' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending `.loop' to
the input file name.
* If `-frerun-cse-after-loop' was enabled, a second common
subexpression elimination pass is performed after the loop
optimization pass. Jump threading is also done again at this time
if it was specified.
The option `-dt' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending `.cse2' to
the input file name.
* Stupid register allocation is performed at this point in a
nonoptimizing compilation. It does a little data flow analysis as
well. When stupid register allocation is in use, the next pass
executed is the reloading pass; the others in between are skipped.
The source file is `stupid.c'.
* Data flow analysis (`flow.c'). This pass divides the program into
basic blocks (and in the process deletes unreachable loops); then
it computes which pseudo-registers are live at each point in the
program, and makes the first instruction that uses a value point at
the instruction that computed the value.
This pass also deletes computations whose results are never used,
and combines memory references with add or subtract instructions
to make autoincrement or autodecrement addressing.
The option `-df' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending `.flow' to
the input file name. If stupid register allocation is in use, this
dump file reflects the full results of such allocation.
* Instruction combination (`combine.c'). This pass attempts to
combine groups of two or three instructions that are related by
data flow into single instructions. It combines the RTL
expressions for the instructions by substitution, simplifies the
result using algebra, and then attempts to match the result
against the machine description.
The option `-dc' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending `.combine'
to the input file name.
* Instruction scheduling (`sched.c'). This pass looks for
instructions whose output will not be available by the time that
it is used in subsequent instructions. (Memory loads and floating
point instructions often have this behavior on RISC machines). It
re-orders instructions within a basic block to try to separate the
definition and use of items that otherwise would cause pipeline
stalls.
Instruction scheduling is performed twice. The first time is
immediately after instruction combination and the second is
immediately after reload.
The option `-dS' causes a debugging dump of the RTL code after this
pass is run for the first time. The dump file's name is made by
appending `.sched' to the input file name.
* Register class preferencing. The RTL code is scanned to find out
which register class is best for each pseudo register. The source
file is `regclass.c'.
* Local register allocation (`local-alloc.c'). This pass allocates
hard registers to pseudo registers that are used only within one
basic block. Because the basic block is linear, it can use fast
and powerful techniques to do a very good job.
The option `-dl' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending `.lreg' to
the input file name.
* Global register allocation (`global.c'). This pass allocates hard
registers for the remaining pseudo registers (those whose life
spans are not contained in one basic block).
* Reloading. This pass renumbers pseudo registers with the hardware
registers numbers they were allocated. Pseudo registers that did
not get hard registers are replaced with stack slots. Then it
finds instructions that are invalid because a value has failed to
end up in a register, or has ended up in a register of the wrong
kind. It fixes up these instructions by reloading the
problematical values temporarily into registers. Additional
instructions are generated to do the copying.
The reload pass also optionally eliminates the frame pointer and
inserts instructions to save and restore call-clobbered registers
around calls.
Source files are `reload.c' and `reload1.c', plus the header
`reload.h' used for communication between them.
The option `-dg' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending `.greg' to
the input file name.
* Instruction scheduling is repeated here to try to avoid pipeline
stalls due to memory loads generated for spilled pseudo registers.
The option `-dR' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending `.sched2'
to the input file name.
* Jump optimization is repeated, this time including cross-jumping
and deletion of no-op move instructions.
The option `-dJ' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending `.jump2' to
the input file name.
* Delayed branch scheduling. This optional pass attempts to find
instructions that can go into the delay slots of other
instructions, usually jumps and calls. The source file name is
`reorg.c'.
The option `-dd' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending `.dbr' to
the input file name.
* Conversion from usage of some hard registers to usage of a register
stack may be done at this point. Currently, this is supported only
for the floating-point registers of the Intel 80387 coprocessor.
The source file name is `reg-stack.c'.
The options `-dk' causes a debugging dump of the RTL code after
this pass. This dump file's name is made by appending `.stack' to
the input file name.
* Final. This pass outputs the assembler code for the function. It
is also responsible for identifying spurious test and compare
instructions. Machine-specific peephole optimizations are
performed at the same time. The function entry and exit sequences
are generated directly as assembler code in this pass; they never
exist as RTL.
The source files are `final.c' plus `insn-output.c'; the latter is
generated automatically from the machine description by the tool
`genoutput'. The header file `conditions.h' is used for
communication between these files.
* Debugging information output. This is run after final because it
must output the stack slot offsets for pseudo registers that did
not get hard registers. Source files are `dbxout.c' for DBX
symbol table format, `sdbout.c' for SDB symbol table format, and
`dwarfout.c' for DWARF symbol table format.
Some additional files are used by all or many passes:
* Every pass uses `machmode.def' and `machmode.h' which define the
machine modes.
* Several passes use `real.h', which defines the default
representation of floating point constants and how to operate on
them.
* All the passes that work with RTL use the header files `rtl.h' and
`rtl.def', and subroutines in file `rtl.c'. The tools `gen*' also
use these files to read and work with the machine description RTL.
* Several passes refer to the header file `insn-config.h' which
contains a few parameters (C macro definitions) generated
automatically from the machine description RTL by the tool
`genconfig'.
* Several passes use the instruction recognizer, which consists of
`recog.c' and `recog.h', plus the files `insn-recog.c' and
`insn-extract.c' that are generated automatically from the machine
description by the tools `genrecog' and `genextract'.
* Several passes use the header files `regs.h' which defines the
information recorded about pseudo register usage, and
`basic-block.h' which defines the information recorded about basic
blocks.
* `hard-reg-set.h' defines the type `HARD_REG_SET', a bit-vector
with a bit for each hard register, and some macros to manipulate
it. This type is just `int' if the machine has few enough hard
registers; otherwise it is an array of `int' and some of the
macros expand into loops.
* Several passes use instruction attributes. A definition of the
attributes defined for a particular machine is in file
`insn-attr.h', which is generated from the machine description by
the program `genattr'. The file `insn-attrtab.c' contains
subroutines to obtain the attribute values for insns. It is
generated from the machine description by the program `genattrtab'.

File: gcc.info, Node: RTL, Next: Machine Desc, Prev: Passes, Up: Top
RTL Representation
******************
Most of the work of the compiler is done on an intermediate
representation called register transfer language. In this language,
the instructions to be output are described, pretty much one by one, in
an algebraic form that describes what the instruction does.
RTL is inspired by Lisp lists. It has both an internal form, made
up of structures that point at other structures, and a textual form
that is used in the machine description and in printed debugging dumps.
The textual form uses nested parentheses to indicate the pointers in
the internal form.
* Menu:
* RTL Objects:: Expressions vs vectors vs strings vs integers.
* Accessors:: Macros to access expression operands or vector elts.
* Flags:: Other flags in an RTL expression.
* Machine Modes:: Describing the size and format of a datum.
* Constants:: Expressions with constant values.
* Regs and Memory:: Expressions representing register contents or memory.
* Arithmetic:: Expressions representing arithmetic on other expressions.
* Comparisons:: Expressions representing comparison of expressions.
* Bit Fields:: Expressions representing bitfields in memory or reg.
* Conversions:: Extending, truncating, floating or fixing.
* RTL Declarations:: Declaring volatility, constancy, etc.
* Side Effects:: Expressions for storing in registers, etc.
* Incdec:: Embedded side-effects for autoincrement addressing.
* Assembler:: Representing `asm' with operands.
* Insns:: Expression types for entire insns.
* Calls:: RTL representation of function call insns.
* Sharing:: Some expressions are unique; others *must* be copied.
* Reading RTL:: Reading textual RTL from a file.

File: gcc.info, Node: RTL Objects, Next: Accessors, Prev: RTL, Up: RTL
RTL Object Types
================
RTL uses five kinds of objects: expressions, integers, wide integers,
strings and vectors. Expressions are the most important ones. An RTL
expression ("RTX", for short) is a C structure, but it is usually
referred to with a pointer; a type that is given the typedef name `rtx'.
An integer is simply an `int'; their written form uses decimal
digits. A wide integer is an integral object whose type is
`HOST_WIDE_INT' (*note Config::.); their written form uses decimal
digits.
A string is a sequence of characters. In core it is represented as a
`char *' in usual C fashion, and it is written in C syntax as well.
However, strings in RTL may never be null. If you write an empty
string in a machine description, it is represented in core as a null
pointer rather than as a pointer to a null character. In certain
contexts, these null pointers instead of strings are valid. Within RTL
code, strings are most commonly found inside `symbol_ref' expressions,
but they appear in other contexts in the RTL expressions that make up
machine descriptions.
A vector contains an arbitrary number of pointers to expressions.
The number of elements in the vector is explicitly present in the
vector. The written form of a vector consists of square brackets
(`[...]') surrounding the elements, in sequence and with whitespace
separating them. Vectors of length zero are not created; null pointers
are used instead.
Expressions are classified by "expression codes" (also called RTX
codes). The expression code is a name defined in `rtl.def', which is
also (in upper case) a C enumeration constant. The possible expression
codes and their meanings are machine-independent. The code of an RTX
can be extracted with the macro `GET_CODE (X)' and altered with
`PUT_CODE (X, NEWCODE)'.
The expression code determines how many operands the expression
contains, and what kinds of objects they are. In RTL, unlike Lisp, you
cannot tell by looking at an operand what kind of object it is.
Instead, you must know from its context--from the expression code of
the containing expression. For example, in an expression of code
`subreg', the first operand is to be regarded as an expression and the
second operand as an integer. In an expression of code `plus', there
are two operands, both of which are to be regarded as expressions. In
a `symbol_ref' expression, there is one operand, which is to be
regarded as a string.
Expressions are written as parentheses containing the name of the
expression type, its flags and machine mode if any, and then the
operands of the expression (separated by spaces).
Expression code names in the `md' file are written in lower case,
but when they appear in C code they are written in upper case. In this
manual, they are shown as follows: `const_int'.
In a few contexts a null pointer is valid where an expression is
normally wanted. The written form of this is `(nil)'.

File: gcc.info, Node: Accessors, Next: Flags, Prev: RTL Objects, Up: RTL
Access to Operands
==================
For each expression type `rtl.def' specifies the number of contained
objects and their kinds, with four possibilities: `e' for expression
(actually a pointer to an expression), `i' for integer, `w' for wide
integer, `s' for string, and `E' for vector of expressions. The
sequence of letters for an expression code is called its "format".
Thus, the format of `subreg' is `ei'.
A few other format characters are used occasionally:
`u'
`u' is equivalent to `e' except that it is printed differently in
debugging dumps. It is used for pointers to insns.
`n'
`n' is equivalent to `i' except that it is printed differently in
debugging dumps. It is used for the line number or code number of
a `note' insn.
`S'
`S' indicates a string which is optional. In the RTL objects in
core, `S' is equivalent to `s', but when the object is read, from
an `md' file, the string value of this operand may be omitted. An
omitted string is taken to be the null string.
`V'
`V' indicates a vector which is optional. In the RTL objects in
core, `V' is equivalent to `E', but when the object is read from
an `md' file, the vector value of this operand may be omitted. An
omitted vector is effectively the same as a vector of no elements.
`0'
`0' means a slot whose contents do not fit any normal category.
`0' slots are not printed at all in dumps, and are often used in
special ways by small parts of the compiler.
There are macros to get the number of operands, the format, and the
class of an expression code:
`GET_RTX_LENGTH (CODE)'
Number of operands of an RTX of code CODE.
`GET_RTX_FORMAT (CODE)'
The format of an RTX of code CODE, as a C string.
`GET_RTX_CLASS (CODE)'
A single character representing the type of RTX operation that code
CODE performs.
The following classes are defined:
`o'
An RTX code that represents an actual object, such as `reg' or
`mem'. `subreg' is not in this class.
`<'
An RTX code for a comparison. The codes in this class are
`NE', `EQ', `LE', `LT', `GE', `GT', `LEU', `LTU', `GEU',
`GTU'.
`1'
An RTX code for a unary arithmetic operation, such as `neg'.
`c'
An RTX code for a commutative binary operation, other than
`NE' and `EQ' (which have class `<').
`2'
An RTX code for a noncommutative binary operation, such as
`MINUS'.
`b'
An RTX code for a bitfield operation, either `ZERO_EXTRACT' or
`SIGN_EXTRACT'.
`3'
An RTX code for other three input operations, such as
`IF_THEN_ELSE'.
`i'
An RTX code for a machine insn (`INSN', `JUMP_INSN', and
`CALL_INSN').
`m'
An RTX code for something that matches in insns, such as
`MATCH_DUP'.
`x'
All other RTX codes.
Operands of expressions are accessed using the macros `XEXP',
`XINT', `XWINT' and `XSTR'. Each of these macros takes two arguments:
an expression-pointer (RTX) and an operand number (counting from zero).
Thus,
XEXP (X, 2)
accesses operand 2 of expression X, as an expression.
XINT (X, 2)
accesses the same operand as an integer. `XSTR', used in the same
fashion, would access it as a string.
Any operand can be accessed as an integer, as an expression or as a
string. You must choose the correct method of access for the kind of
value actually stored in the operand. You would do this based on the
expression code of the containing expression. That is also how you
would know how many operands there are.
For example, if X is a `subreg' expression, you know that it has two
operands which can be correctly accessed as `XEXP (X, 0)' and `XINT (X,
1)'. If you did `XINT (X, 0)', you would get the address of the
expression operand but cast as an integer; that might occasionally be
useful, but it would be cleaner to write `(int) XEXP (X, 0)'. `XEXP
(X, 1)' would also compile without error, and would return the second,
integer operand cast as an expression pointer, which would probably
result in a crash when accessed. Nothing stops you from writing `XEXP
(X, 28)' either, but this will access memory past the end of the
expression with unpredictable results.
Access to operands which are vectors is more complicated. You can
use the macro `XVEC' to get the vector-pointer itself, or the macros
`XVECEXP' and `XVECLEN' to access the elements and length of a vector.
`XVEC (EXP, IDX)'
Access the vector-pointer which is operand number IDX in EXP.
`XVECLEN (EXP, IDX)'
Access the length (number of elements) in the vector which is in
operand number IDX in EXP. This value is an `int'.
`XVECEXP (EXP, IDX, ELTNUM)'
Access element number ELTNUM in the vector which is in operand
number IDX in EXP. This value is an RTX.
It is up to you to make sure that ELTNUM is not negative and is
less than `XVECLEN (EXP, IDX)'.
All the macros defined in this section expand into lvalues and
therefore can be used to assign the operands, lengths and vector
elements as well as to access them.

File: gcc.info, Node: Flags, Next: Machine Modes, Prev: Accessors, Up: RTL
Flags in an RTL Expression
==========================
RTL expressions contain several flags (one-bit bitfields) that are
used in certain types of expression. Most often they are accessed with
the following macros:
`MEM_VOLATILE_P (X)'
In `mem' expressions, nonzero for volatile memory references.
Stored in the `volatil' field and printed as `/v'.
`MEM_IN_STRUCT_P (X)'
In `mem' expressions, nonzero for reference to an entire
structure, union or array, or to a component of one. Zero for
references to a scalar variable or through a pointer to a scalar.
Stored in the `in_struct' field and printed as `/s'.
`REG_LOOP_TEST_P'
In `reg' expressions, nonzero if this register's entire life is
contained in the exit test code for some loop. Stored in the
`in_struct' field and printed as `/s'.
`REG_USERVAR_P (X)'
In a `reg', nonzero if it corresponds to a variable present in the
user's source code. Zero for temporaries generated internally by
the compiler. Stored in the `volatil' field and printed as `/v'.
`REG_FUNCTION_VALUE_P (X)'
Nonzero in a `reg' if it is the place in which this function's
value is going to be returned. (This happens only in a hard
register.) Stored in the `integrated' field and printed as `/i'.
The same hard register may be used also for collecting the values
of functions called by this one, but `REG_FUNCTION_VALUE_P' is zero
in this kind of use.
`SUBREG_PROMOTED_VAR_P'
Nonzero in a `subreg' if it was made when accessing an object that
was promoted to a wider mode in accord with the `PROMOTED_MODE'
machine description macro (*note Storage Layout::.). In this
case, the mode of the `subreg' is the declared mode of the object
and the mode of `SUBREG_REG' is the mode of the register that
holds the object. Promoted variables are always either sign- or
zero-extended to the wider mode on every assignment. Stored in
the `in_struct' field and printed as `/s'.
`SUBREG_PROMOTED_UNSIGNED_P'
Nonzero in a `subreg' that has `SUBREG_PROMOTED_VAR_P' nonzero if
the object being referenced is kept zero-extended and zero if it
is kept sign-extended. Stored in the `unchanging' field and
printed as `/u'.
`RTX_UNCHANGING_P (X)'
Nonzero in a `reg' or `mem' if the value is not changed. (This
flag is not set for memory references via pointers to constants.
Such pointers only guarantee that the object will not be changed
explicitly by the current function. The object might be changed by
other functions or by aliasing.) Stored in the `unchanging' field
and printed as `/u'.
`RTX_INTEGRATED_P (INSN)'
Nonzero in an insn if it resulted from an in-line function call.
Stored in the `integrated' field and printed as `/i'. This may be
deleted; nothing currently depends on it.
`SYMBOL_REF_USED (X)'
In a `symbol_ref', indicates that X has been used. This is
normally only used to ensure that X is only declared external
once. Stored in the `used' field.
`SYMBOL_REF_FLAG (X)'
In a `symbol_ref', this is used as a flag for machine-specific
purposes. Stored in the `volatil' field and printed as `/v'.
`LABEL_OUTSIDE_LOOP_P'
In `label_ref' expressions, nonzero if this is a reference to a
label that is outside the innermost loop containing the reference
to the label. Stored in the `in_struct' field and printed as `/s'.
`INSN_DELETED_P (INSN)'
In an insn, nonzero if the insn has been deleted. Stored in the
`volatil' field and printed as `/v'.
`INSN_ANNULLED_BRANCH_P (INSN)'
In an `insn' in the delay slot of a branch insn, indicates that an
annulling branch should be used. See the discussion under
`sequence' below. Stored in the `unchanging' field and printed as
`/u'.
`INSN_FROM_TARGET_P (INSN)'
In an `insn' in a delay slot of a branch, indicates that the insn
is from the target of the branch. If the branch insn has
`INSN_ANNULLED_BRANCH_P' set, this insn should only be executed if
the branch is taken. For annulled branches with this bit clear,
the insn should be executed only if the branch is not taken.
Stored in the `in_struct' field and printed as `/s'.
`CONSTANT_POOL_ADDRESS_P (X)'
Nonzero in a `symbol_ref' if it refers to part of the current
function's "constants pool". These are addresses close to the
beginning of the function, and GNU CC assumes they can be addressed
directly (perhaps with the help of base registers). Stored in the
`unchanging' field and printed as `/u'.
`CONST_CALL_P (X)'
In a `call_insn', indicates that the insn represents a call to a
const function. Stored in the `unchanging' field and printed as
`/u'.
`LABEL_PRESERVE_P (X)'
In a `code_label', indicates that the label can never be deleted.
Labels referenced by a non-local goto will have this bit set.
Stored in the `in_struct' field and printed as `/s'.
`SCHED_GROUP_P (INSN)'
During instruction scheduling, in an insn, indicates that the
previous insn must be scheduled together with this insn. This is
used to ensure that certain groups of instructions will not be
split up by the instruction scheduling pass, for example, `use'
insns before a `call_insn' may not be separated from the
`call_insn'. Stored in the `in_struct' field and printed as `/s'.
These are the fields which the above macros refer to:
`used'
Normally, this flag is used only momentarily, at the end of RTL
generation for a function, to count the number of times an
expression appears in insns. Expressions that appear more than
once are copied, according to the rules for shared structure
(*note Sharing::.).
In a `symbol_ref', it indicates that an external declaration for
the symbol has already been written.
In a `reg', it is used by the leaf register renumbering code to
ensure that each register is only renumbered once.
`volatil'
This flag is used in `mem', `symbol_ref' and `reg' expressions and
in insns. In RTL dump files, it is printed as `/v'.
In a `mem' expression, it is 1 if the memory reference is volatile.
Volatile memory references may not be deleted, reordered or
combined.
In a `symbol_ref' expression, it is used for machine-specific
purposes.
In a `reg' expression, it is 1 if the value is a user-level
variable. 0 indicates an internal compiler temporary.
In an insn, 1 means the insn has been deleted.
`in_struct'
In `mem' expressions, it is 1 if the memory datum referred to is
all or part of a structure or array; 0 if it is (or might be) a
scalar variable. A reference through a C pointer has 0 because
the pointer might point to a scalar variable. This information
allows the compiler to determine something about possible cases of
aliasing.
In an insn in the delay slot of a branch, 1 means that this insn
is from the target of the branch.
During instruction scheduling, in an insn, 1 means that this insn
must be scheduled as part of a group together with the previous
insn.
In `reg' expressions, it is 1 if the register has its entire life
contained within the test expression of some loop.
In `subreg' expressions, 1 means that the `subreg' is accessing an
object that has had its mode promoted from a wider mode.
In `label_ref' expressions, 1 means that the referenced label is
outside the innermost loop containing the insn in which the
`label_ref' was found.
In `code_label' expressions, it is 1 if the label may never be
deleted. This is used for labels which are the target of
non-local gotos.
In an RTL dump, this flag is represented as `/s'.
`unchanging'
In `reg' and `mem' expressions, 1 means that the value of the
expression never changes.
In `subreg' expressions, it is 1 if the `subreg' references an
unsigned object whose mode has been promoted to a wider mode.
In an insn, 1 means that this is an annulling branch.
In a `symbol_ref' expression, 1 means that this symbol addresses
something in the per-function constants pool.
In a `call_insn', 1 means that this instruction is a call to a
const function.
In an RTL dump, this flag is represented as `/u'.
`integrated'
In some kinds of expressions, including insns, this flag means the
rtl was produced by procedure integration.
In a `reg' expression, this flag indicates the register containing
the value to be returned by the current function. On machines
that pass parameters in registers, the same register number may be
used for parameters as well, but this flag is not set on such uses.

File: gcc.info, Node: Machine Modes, Next: Constants, Prev: Flags, Up: RTL
Machine Modes
=============
A machine mode describes a size of data object and the
representation used for it. In the C code, machine modes are
represented by an enumeration type, `enum machine_mode', defined in
`machmode.def'. Each RTL expression has room for a machine mode and so
do certain kinds of tree expressions (declarations and types, to be
precise).
In debugging dumps and machine descriptions, the machine mode of an
RTL expression is written after the expression code with a colon to
separate them. The letters `mode' which appear at the end of each
machine mode name are omitted. For example, `(reg:SI 38)' is a `reg'
expression with machine mode `SImode'. If the mode is `VOIDmode', it
is not written at all.
Here is a table of machine modes. The term "byte" below refers to an
object of `BITS_PER_UNIT' bits (*note Storage Layout::.).
`QImode'
"Quarter-Integer" mode represents a single byte treated as an
integer.
`HImode'
"Half-Integer" mode represents a two-byte integer.
`PSImode'
"Partial Single Integer" mode represents an integer which occupies
four bytes but which doesn't really use all four. On some
machines, this is the right mode to use for pointers.
`SImode'
"Single Integer" mode represents a four-byte integer.
`PDImode'
"Partial Double Integer" mode represents an integer which occupies
eight bytes but which doesn't really use all eight. On some
machines, this is the right mode to use for certain pointers.
`DImode'
"Double Integer" mode represents an eight-byte integer.
`TImode'
"Tetra Integer" (?) mode represents a sixteen-byte integer.
`SFmode'
"Single Floating" mode represents a single-precision (four byte)
floating point number.
`DFmode'
"Double Floating" mode represents a double-precision (eight byte)
floating point number.
`XFmode'
"Extended Floating" mode represents a triple-precision (twelve
byte) floating point number. This mode is used for IEEE extended
floating point. On some systems not all bits within these bytes
will actually be used.
`TFmode'
"Tetra Floating" mode represents a quadruple-precision (sixteen
byte) floating point number.
`CCmode'
"Condition Code" mode represents the value of a condition code,
which is a machine-specific set of bits used to represent the
result of a comparison operation. Other machine-specific modes
may also be used for the condition code. These modes are not used
on machines that use `cc0' (see *note Condition Code::.).
`BLKmode'
"Block" mode represents values that are aggregates to which none of
the other modes apply. In RTL, only memory references can have
this mode, and only if they appear in string-move or vector
instructions. On machines which have no such instructions,
`BLKmode' will not appear in RTL.
`VOIDmode'
Void mode means the absence of a mode or an unspecified mode. For
example, RTL expressions of code `const_int' have mode `VOIDmode'
because they can be taken to have whatever mode the context
requires. In debugging dumps of RTL, `VOIDmode' is expressed by
the absence of any mode.
`SCmode, DCmode, XCmode, TCmode'
These modes stand for a complex number represented as a pair of
floating point values. The floating point values are in `SFmode',
`DFmode', `XFmode', and `TFmode', respectively.
`CQImode, CHImode, CSImode, CDImode, CTImode, COImode'
These modes stand for a complex number represented as a pair of
integer values. The integer values are in `QImode', `HImode',
`SImode', `DImode', `TImode', and `OImode', respectively.
The machine description defines `Pmode' as a C macro which expands
into the machine mode used for addresses. Normally this is the mode
whose size is `BITS_PER_WORD', `SImode' on 32-bit machines.
The only modes which a machine description must support are
`QImode', and the modes corresponding to `BITS_PER_WORD',
`FLOAT_TYPE_SIZE' and `DOUBLE_TYPE_SIZE'. The compiler will attempt to
use `DImode' for 8-byte structures and unions, but this can be
prevented by overriding the definition of `MAX_FIXED_MODE_SIZE'.
Alternatively, you can have the compiler use `TImode' for 16-byte
structures and unions. Likewise, you can arrange for the C type `short
int' to avoid using `HImode'.
Very few explicit references to machine modes remain in the compiler
and these few references will soon be removed. Instead, the machine
modes are divided into mode classes. These are represented by the
enumeration type `enum mode_class' defined in `machmode.h'. The
possible mode classes are:
`MODE_INT'
Integer modes. By default these are `QImode', `HImode', `SImode',
`DImode', and `TImode'.
`MODE_PARTIAL_INT'
The "partial integer" modes, `PSImode' and `PDImode'.
`MODE_FLOAT'
floating point modes. By default these are `SFmode', `DFmode',
`XFmode' and `TFmode'.
`MODE_COMPLEX_INT'
Complex integer modes. (These are not currently implemented).
`MODE_COMPLEX_FLOAT'
Complex floating point modes. By default these are `SCmode',
`DCmode', `XCmode', and `TCmode'.
`MODE_FUNCTION'
Algol or Pascal function variables including a static chain.
(These are not currently implemented).
`MODE_CC'
Modes representing condition code values. These are `CCmode' plus
any modes listed in the `EXTRA_CC_MODES' macro. *Note Jump
Patterns::, also see *Note Condition Code::.
`MODE_RANDOM'
This is a catchall mode class for modes which don't fit into the
above classes. Currently `VOIDmode' and `BLKmode' are in
`MODE_RANDOM'.
Here are some C macros that relate to machine modes:
`GET_MODE (X)'
Returns the machine mode of the RTX X.
`PUT_MODE (X, NEWMODE)'
Alters the machine mode of the RTX X to be NEWMODE.
`NUM_MACHINE_MODES'
Stands for the number of machine modes available on the target
machine. This is one greater than the largest numeric value of any
machine mode.
`GET_MODE_NAME (M)'
Returns the name of mode M as a string.
`GET_MODE_CLASS (M)'
Returns the mode class of mode M.
`GET_MODE_WIDER_MODE (M)'
Returns the next wider natural mode. For example, the expression
`GET_MODE_WIDER_MODE (QImode)' returns `HImode'.
`GET_MODE_SIZE (M)'
Returns the size in bytes of a datum of mode M.
`GET_MODE_BITSIZE (M)'
Returns the size in bits of a datum of mode M.
`GET_MODE_MASK (M)'
Returns a bitmask containing 1 for all bits in a word that fit
within mode M. This macro can only be used for modes whose
bitsize is less than or equal to `HOST_BITS_PER_INT'.
`GET_MODE_ALIGNMENT (M))'
Return the required alignment, in bits, for an object of mode M.
`GET_MODE_UNIT_SIZE (M)'
Returns the size in bytes of the subunits of a datum of mode M.
This is the same as `GET_MODE_SIZE' except in the case of complex
modes. For them, the unit size is the size of the real or
imaginary part.
`GET_MODE_NUNITS (M)'
Returns the number of units contained in a mode, i.e.,
`GET_MODE_SIZE' divided by `GET_MODE_UNIT_SIZE'.
`GET_CLASS_NARROWEST_MODE (C)'
Returns the narrowest mode in mode class C.
The global variables `byte_mode' and `word_mode' contain modes whose
classes are `MODE_INT' and whose bitsizes are either `BITS_PER_UNIT' or
`BITS_PER_WORD', respectively. On 32-bit machines, these are `QImode'
and `SImode', respectively.

File: gcc.info, Node: Constants, Next: Regs and Memory, Prev: Machine Modes, Up: RTL
Constant Expression Types
=========================
The simplest RTL expressions are those that represent constant
values.
`(const_int I)'
This type of expression represents the integer value I. I is
customarily accessed with the macro `INTVAL' as in `INTVAL (EXP)',
which is equivalent to `XWINT (EXP, 0)'.
There is only one expression object for the integer value zero; it
is the value of the variable `const0_rtx'. Likewise, the only
expression for integer value one is found in `const1_rtx', the only
expression for integer value two is found in `const2_rtx', and the
only expression for integer value negative one is found in
`constm1_rtx'. Any attempt to create an expression of code
`const_int' and value zero, one, two or negative one will return
`const0_rtx', `const1_rtx', `const2_rtx' or `constm1_rtx' as
appropriate.
Similarly, there is only one object for the integer whose value is
`STORE_FLAG_VALUE'. It is found in `const_true_rtx'. If
`STORE_FLAG_VALUE' is one, `const_true_rtx' and `const1_rtx' will
point to the same object. If `STORE_FLAG_VALUE' is -1,
`const_true_rtx' and `constm1_rtx' will point to the same object.
`(const_double:M ADDR I0 I1 ...)'
Represents either a floating-point constant of mode M or an
integer constant too large to fit into `HOST_BITS_PER_WIDE_INT'
bits but small enough to fit within twice that number of bits (GNU
CC does not provide a mechanism to represent even larger
constants). In the latter case, M will be `VOIDmode'.
ADDR is used to contain the `mem' expression that corresponds to
the location in memory that at which the constant can be found. If
it has not been allocated a memory location, but is on the chain
of all `const_double' expressions in this compilation (maintained
using an undisplayed field), ADDR contains `const0_rtx'. If it is
not on the chain, ADDR contains `cc0_rtx'. ADDR is customarily
accessed with the macro `CONST_DOUBLE_MEM' and the chain field via
`CONST_DOUBLE_CHAIN'.
If M is `VOIDmode', the bits of the value are stored in I0 and I1.
I0 is customarily accessed with the macro `CONST_DOUBLE_LOW' and
I1 with `CONST_DOUBLE_HIGH'.
If the constant is floating point (regardless of its precision),
then the number of integers used to store the value depends on the
size of `REAL_VALUE_TYPE' (*note Cross-compilation::.). The
integers represent a floating point number, but not precisely in
the target machine's or host machine's floating point format. To
convert them to the precise bit pattern used by the target
machine, use the macro `REAL_VALUE_TO_TARGET_DOUBLE' and friends
(*note Data Output::.).
The macro `CONST0_RTX (MODE)' refers to an expression with value 0
in mode MODE. If mode MODE is of mode class `MODE_INT', it
returns `const0_rtx'. Otherwise, it returns a `CONST_DOUBLE'
expression in mode MODE. Similarly, the macro `CONST1_RTX (MODE)'
refers to an expression with value 1 in mode MODE and similarly
for `CONST2_RTX'.
`(const_string STR)'
Represents a constant string with value STR. Currently this is
used only for insn attributes (*note Insn Attributes::.) since
constant strings in C are placed in memory.
`(symbol_ref:MODE SYMBOL)'
Represents the value of an assembler label for data. SYMBOL is a
string that describes the name of the assembler label. If it
starts with a `*', the label is the rest of SYMBOL not including
the `*'. Otherwise, the label is SYMBOL, usually prefixed with
`_'.
The `symbol_ref' contains a mode, which is usually `Pmode'.
Usually that is the only mode for which a symbol is directly valid.
`(label_ref LABEL)'
Represents the value of an assembler label for code. It contains
one operand, an expression, which must be a `code_label' that
appears in the instruction sequence to identify the place where
the label should go.
The reason for using a distinct expression type for code label
references is so that jump optimization can distinguish them.
`(const:M EXP)'
Represents a constant that is the result of an assembly-time
arithmetic computation. The operand, EXP, is an expression that
contains only constants (`const_int', `symbol_ref' and `label_ref'
expressions) combined with `plus' and `minus'. However, not all
combinations are valid, since the assembler cannot do arbitrary
arithmetic on relocatable symbols.
M should be `Pmode'.
`(high:M EXP)'
Represents the high-order bits of EXP, usually a `symbol_ref'.
The number of bits is machine-dependent and is normally the number
of bits specified in an instruction that initializes the high
order bits of a register. It is used with `lo_sum' to represent
the typical two-instruction sequence used in RISC machines to
reference a global memory location.
M should be `Pmode'.