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gcc-1.23
Info file internals, produced by Makeinfo, -*- Text -*- from input
file internals.texinfo.
This file documents the internals of the GNU compiler.
Copyright (C) 1988 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 section entitled ``GNU CC General Public License'' is
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 section entitled ``GNU CC General Public
License'' and this permission notice may be included in translations
approved by the Free Software Foundation instead of in the original
English.
File: internals, 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, remains 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 the function `rest_of_compilation' or
`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
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.
C 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 associative-law simplifications are also
done during this pass.
The source files for parsing are `c-parse.y', `c-decl.c',
`c-typeck.c', `c-convert.c', `stor-layout.c', `fold-const.c',
and `tree.c'. The last three files are intended to be
language-independent. There are also header files `c-parse.h',
`c-tree.h', `tree.h' and `tree.def'. The last two define the
format of the tree representation.
* 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 are `stmt.c', `expr.c',
`explow.c', `expmed.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 files
`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 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.)
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'.
* 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. Its source file is `loop.c'.
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.
* 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.
* 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-alloc.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.
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.
* Jump optimization is repeated, this time including cross-jumping
and deletion of no-op move instructions. Machine-specific
peephole optimizations are performed at the same time.
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.
* Final. This pass outputs the assembler code for the function.
It is also responsible for identifying spurious test and compare
instructions. 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 and `symout.c' for GDB's own symbol table
format.
Some additional files are used by all or many passes:
* Every pass uses `machmode.def', which defines the machine modes.
* 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.
File: internals, 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 bit-fields 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.
File: internals, Node: RTL Objects, Next: Accessors, Prev: RTL, Up: RTL
RTL Object Types
================
RTL uses four kinds of objects: expressions, 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', and a string is a `char *'. Within
RTL code, strings appear only inside `symbol_ref' expressions, but
they appear in other contexts in the RTL expressions that make up
machine descriptions. 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.
A vector contains an arbitrary, specified 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: internals, 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, `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'.
Two other format characters are used occasionally: `u' and `0'. `u'
is equivalent to `e' except that it is printed differently in
debugging dumps, and `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 and the format 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.
Operands of expressions are accessed using the macros `XEXP', `XINT'
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: internals, Node: Flags, Next: Machine Modes, Prev: Accessors, Up: RTL
Flags in an RTL Expression
==========================
RTL expressions contain several flags (one-bit bit-fields) that are
used in certain types of expression.
`used'
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::.).
`volatil'
This flag is used in `mem' 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 `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'
This flag is used 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 RTL dump, this flag is represented as `/s'.
`unchanging'
This flag is used in `reg' and `mem' expressions. 1 means that
the value of the expression never changes (at least within the
current 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: internals, 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.
`QImode'
``Quarter-Integer'' mode represents a single byte treated as an
integer.
`HImode'
``Half-Integer'' mode represents a two-byte integer.
`SImode'
``Single Integer'' mode represents a four-byte integer.
`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.
`TFmode'
``Tetra Floating'' mode represents a quadruple-precision
(sixteen byte) floating point number.
`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.
`EPmode'
``Entry Pointer'' mode is intended to be used for function
variables in Pascal and other block structured languages. Such
values contain both a function address and a static chain
pointer for access to automatic variables of outer levels. This
mode is only partially implemented since C does not use it.
`CSImode, ...'
``Complex Single Integer'' mode stands for a complex number
represented as a pair of `SImode' integers. Any of the integer
and floating modes may have `C' prefixed to its name to obtain a
complex number mode. For example, there are `CQImode',
`CSFmode', and `CDFmode'. Since C does not support complex
numbers, these machine modes are only partially implemented.
`BImode'
This is the machine mode of a bit-field in a structure. It is
used only in the syntax tree, never in RTL, and in the syntax
tree it appears only in declaration nodes. In C, it appears
only in `FIELD_DECL' nodes for structure fields defined with a
bit size.
The machine description defines `Pmode' as a C macro which expands
into the machine mode used for addresses. Normally this is `SImode'.
The only modes which a machine description must support are `QImode',
`SImode', `SFmode' and `DFmode'. The compiler will attempt to use
`DImode' for two-word structures and unions, but it would not be hard
to program it to avoid this. Likewise, you can arrange for the C
type `short int' to avoid using `HImode'. In the long term it would
be desirable to make the set of available machine modes
machine-dependent and eliminate all assumptions about specific
machine modes or their uses from the machine-independent code of the
compiler.
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.
`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_UNIT_SIZE (M)'
Returns the size in bits of the subunits of a datum of mode M.
This is the same as `GET_MODE_SIZE' except in the case of
complex modes and `EPmode'. For them, the unit size is the size
of the real or imaginary part, or the size of the function
pointer or the context pointer.
File: internals, 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 `XINT (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'.
Any attempt to create an expression of code `const_int' and
value zero or one will return `const0_rtx' or `const1_rtx' as
appropriate.
`(const_double:M I0 I1)'
Represents a floating point constant value of mode M. The two
integers I0 and I1 together contain the bits of a `double'
value. To convert them to a `double', do
union { double d; int i[2];} u;
u.i[0] = XINT (x, 0);
u.i[1] = XINT (x, 1);
and then refer to `u.d'. The value of the constant is
represented as a double in this fashion even if the value
represented is single-precision.
The global variables `dconst0_rtx' and `fconst0_rtx' hold
`const_double' expressions with value 0, in modes `DFmode' and
`SFmode', respectively.
`(symbol_ref 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, prefixed with `_'.
`(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 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.
File: internals, Node: Regs and Memory, Next: Arithmetic, Prev: Constants, Up: RTL
Registers and Memory
====================
Here are the RTL expression types for describing access to machine
registers and to main memory.
`(reg:M N)'
For small values of the integer N (less than
`FIRST_PSEUDO_REGISTER'), this stands for a reference to machine
register number N: a "hard register". For larger values of N,
it stands for a temporary value or "pseudo register". The
compiler's strategy is to generate code assuming an unlimited
number of such pseudo registers, and later convert them into
hard registers or into memory references.
The symbol `FIRST_PSEUDO_REGISTER' is defined by the machine
description, since the number of hard registers on the machine
is an invariant characteristic of the machine. Note, however,
that not all of the machine registers must be general registers.
All the machine registers that can be used for storage of data
are given hard register numbers, even those that can be used
only in certain instructions or can hold only certain types of
data.
Each pseudo register number used in a function's RTL code is
represented by a unique `reg' expression.
M is the machine mode of the reference. It is necessary because
machines can generally refer to each register in more than one
mode. For example, a register may contain a full word but there
may be instructions to refer to it as a half word or as a single
byte, as well as instructions to refer to it as a floating point
number of various precisions.
Even for a register that the machine can access in only one
mode, the mode must always be specified.
A hard register may be accessed in various modes throughout one
function, but each pseudo register is given a natural mode and
is accessed only in that mode. When it is necessary to describe
an access to a pseudo register using a nonnatural mode, a
`subreg' expression is used.
A `reg' expression with a machine mode that specifies more than
one word of data may actually stand for several consecutive
registers. If in addition the register number specifies a
hardware register, then it actually represents several
consecutive hardware registers starting with the specified one.
Such multi-word hardware register `reg' expressions may not be
live across the boundary of a basic block. The lifetime
analysis pass does not know how to record properly that several
consecutive registers are actually live there, and therefore
register allocation would be confused. The CSE pass must go out
of its way to make sure the situation does not arise.
`(subreg:M REG WORDNUM)'
`subreg' expressions are used to refer to a register in a
machine mode other than its natural one, or to refer to one
register of a multi-word `reg' that actually refers to several
registers.
Each pseudo-register has a natural mode. If it is necessary to
operate on it in a different mode--for example, to perform a
fullword move instruction on a pseudo-register that contains a
single byte-- the pseudo-register must be enclosed in a
`subreg'. In such a case, WORDNUM is zero.
The other use of `subreg' is to extract the individual registers
of a multi-register value. Machine modes such as `DImode' and
`EPmode' indicate values longer than a word, values which
usually require two consecutive registers. To access one of the
registers, use a `subreg' with mode `SImode' and a WORDNUM that
says which register.
The compilation parameter `WORDS_BIG_ENDIAN', if defined, says
that word number zero is the most significant part; otherwise,
it is the least significant part.
Between the combiner pass and the reload pass, it is possible to
have a `subreg' which contains a `mem' instead of a `reg' as its
first operand. The reload pass eliminates these cases by
reloading the `mem' into a suitable register.
Note that it is not valid to access a `DFmode' value in `SFmode'
using a `subreg'. On some machines the most significant part of
a `DFmode' value does not have the same format as a
single-precision floating value.
`(cc0)'
This refers to the machine's condition code register. It has no
operands and may not have a machine mode. It may be validly
used in only two contexts: as the destination of an assignment
(in test and compare instructions) and in comparison operators
comparing against zero (`const_int' with value zero; that is to
say, `const0_rtx').
There is only one expression object of code `cc0'; it is the
value of the variable `cc0_rtx'. Any attempt to create an
expression of code `cc0' will return `cc0_rtx'.
One special thing about the condition code register is that
instructions can set it implicitly. On many machines, nearly
all instructions set the condition code based on the value that
they compute or store. It is not necessary to record these
actions explicitly in the RTL because the machine description
includes a prescription for recognizing the instructions that do
so (by means of the macro `NOTICE_UPDATE_CC'). Only
instructions whose sole purpose is to set the condition code,
and instructions that use the condition code, need mention
`(cc0)'.
`(pc)'
This represents the machine's program counter. It has no
operands and may not have a machine mode. `(pc)' may be validly
used only in certain specific contexts in jump instructions.
There is only one expression object of code `pc'; it is the
value of the variable `pc_rtx'. Any attempt to create an
expression of code `pc' will return `pc_rtx'.
All instructions that do not jump alter the program counter
implicitly by incrementing it, but there is no need to mention
this in the RTL.
`(mem:M ADDR)'
This RTX represents a reference to main memory at an address
represented by the expression ADDR. M specifies how large a
unit of memory is accessed.
File: internals, Node: Arithmetic, Next: Comparisons, Prev: Regs and Memory, Up: RTL
RTL Expressions for Arithmetic
==============================
`(plus:M X Y)'
Represents the sum of the values represented by X and Y carried
out in machine mode M. This is valid only if X and Y both are
valid for mode M.
`(minus:M X Y)'
Like `plus' but represents subtraction.
`(minus X Y)'
Represents the result of subtracting Y from X for purposes of
comparison. The absence of a machine mode in the `minus'
expression indicates that the result is computed without
overflow, as if with infinite precision.
Of course, machines can't really subtract with infinite precision.
However, they can pretend to do so when only the sign of the
result will be used, which is the case when the result is stored
in `(cc0)'. And that is the only way this kind of expression
may validly be used: as a value to be stored in the condition
codes.
`(neg:M X)'
Represents the negation (subtraction from zero) of the value
represented by X, carried out in mode M. X must be valid for
mode M.
`(mult:M X Y)'
Represents the signed product of the values represented by X and
Y carried out in machine mode M. If X and Y are both valid for
mode M, this is ordinary size-preserving multiplication.
Alternatively, both X and Y may be valid for a different,
narrower mode. This represents the kind of multiplication that
generates a product wider than the operands. Widening
multiplication and same-size multiplication are completely
distinct and supported by different machine instructions;
machines may support one but not the other.
`mult' may be used for floating point division as well. Then M
is a floating point machine mode.
`(umult:M X Y)'
Like `mult' but represents unsigned multiplication. It may be
used in both same-size and widening forms, like `mult'. `umult'
is used only for fixed-point multiplication.
`(div:M X Y)'
Represents the quotient in signed division of X by Y, carried
out in machine mode M. If M is a floating-point mode, it
represents the exact quotient; otherwise, the integerized
quotient. If X and Y are both valid for mode M, this is
ordinary size-preserving division. Some machines have division
instructions in which the operands and quotient widths are not
all the same; such instructions are represented by `div'
expressions in which the machine modes are not all the same.
`(udiv:M X Y)'
Like `div' but represents unsigned division.
`(mod:M X Y)'
`(umod:M X Y)'
Like `div' and `udiv' but represent the remainder instead of the
quotient.
`(not:M X)'
Represents the bitwise complement of the value represented by X,
carried out in mode M, which must be a fixed-point machine mode.
x must be valid for mode M, which must be a fixed-point mode.
`(and:M X Y)'
Represents the bitwise logical-and of the values represented by
X and Y, carried out in machine mode M. This is valid only if X
and Y both are valid for mode M, which must be a fixed-point mode.
`(ior:M X Y)'
Represents the bitwise inclusive-or of the values represented by
X and Y, carried out in machine mode M. This is valid only if X
and Y both are valid for mode M, which must be a fixed-point mode.
`(xor:M X Y)'
Represents the bitwise exclusive-or of the values represented by
X and Y, carried out in machine mode M. This is valid only if X
and Y both are valid for mode M, which must be a fixed-point mode.
`(lshift:M X C)'
Represents the result of logically shifting X left by C places.
X must be valid for the mode M, a fixed-point machine mode. C
must be valid for a fixed-point mode; which mode is determined
by the mode called for in the machine description entry for the
left-shift instruction. For example, on the Vax, the mode of C
is `QImode' regardless of M.
On some machines, negative values of C may be meaningful; this
is why logical left shift and arithmetic left shift are
distinguished. For example, Vaxes have no right-shift
instructions, and right shifts are represented as left-shift
instructions whose counts happen to be negative constants or
else computed (in a previous instruction) by negation.
`(ashift:M X C)'
Like `lshift' but for arithmetic left shift.
`(lshiftrt:M X C)'
`(ashiftrt:M X C)'
Like `lshift' and `ashift' but for right shift.
`(rotate:M X C)'
`(rotatert:M X C)'
Similar but represent left and right rotate.
`(abs:M X)'
Represents the absolute value of X, computed in mode M. X must
be valid for M.
`(sqrt:M X)'
Represents the square root of X, computed in mode M. X must be
valid for M. Most often M will be a floating point mode.
`(ffs:M X)'
Represents the one plus the index of the least significant 1-bit
in X, represented as an integer of mode M. (The value is zero
if X is zero.) The mode of X need not be M; depending on the
target machine, various mode combinations may be valid.
File: internals, Node: Comparisons, Next: Bit Fields, Prev: Arithmetic, Up: RTL
Comparison Operations
=====================
Comparison operators test a relation on two operands and are
considered to represent the value 1 if the relation holds, or zero if
it does not. The mode of the comparison is determined by the
operands; they must both be valid for a common machine mode. A
comparison with both operands constant would be invalid as the
machine mode could not be deduced from it, but such a comparison
should never exist in RTL due to constant folding.
Inequality comparisons come in two flavors, signed and unsigned.
Thus, there are distinct expression codes `gt' and `gtu' for signed
and unsigned greater-than. These can produce different results for
the same pair of integer values: for example, 1 is signed
greater-than -1 but not unsigned greater-than, because -1 when
regarded as unsigned is actually `0xffffffff' which is greater than 1.
The signed comparisons are also used for floating point values.
Floating point comparisons are distinguished by the machine modes of
the operands.
The comparison operators may be used to compare the condition codes
`(cc0)' against zero, as in `(eq (cc0) (const_int 0))'. Such a
construct actually refers to the result of the preceding instruction
in which the condition codes were set. The above example stands for
1 if the condition codes were set to say ``zero'' or ``equal'', 0
otherwise. Although the same comparison operators are used for this
as may be used in other contexts on actual data, no confusion can
result since the machine description would never allow both kinds of
uses in the same context.
`(eq X Y)'
1 if the values represented by X and Y are equal, otherwise 0.
`(ne X Y)'
1 if the values represented by X and Y are not equal, otherwise 0.
`(gt X Y)'
1 if the X is greater than Y. If they are fixed-point, the
comparison is done in a signed sense.
`(gtu X Y)'
Like `gt' but does unsigned comparison, on fixed-point numbers
only.
`(lt X Y)'
`(ltu X Y)'
Like `gt' and `gtu' but test for ``less than''.
`(ge X Y)'
`(geu X Y)'
Like `gt' and `gtu' but test for ``greater than or equal''.
`(le X Y)'
`(leu X Y)'
Like `gt' and `gtu' but test for ``less than or equal''.
`(if_then_else COND THEN ELSE)'
This is not a comparison operation but is listed here because it
is always used in conjunction with a comparison operation. To
be precise, COND is a comparison expression. This expression
represents a choice, according to COND, between the value
represented by THEN and the one represented by ELSE.
On most machines, `if_then_else' expressions are valid only to
express conditional jumps.
File: internals, Node: Bit Fields, Next: Conversions, Prev: Comparisons, Up: RTL
Bit-fields
==========
Special expression codes exist to represent bit-field instructions.
These types of expressions are lvalues in RTL; they may appear on the
left side of a assignment, indicating insertion of a value into the
specified bit field.
`(sign_extract:SI LOC SIZE POS)'
This represents a reference to a sign-extended bit-field
contained or starting in LOC (a memory or register reference).
The bit field is SIZE bits wide and starts at bit POS. The
compilation option `BITS_BIG_ENDIAN' says which end of the
memory unit POS counts from.
Which machine modes are valid for LOC depends on the machine,
but typically LOC should be a single byte when in memory or a
full word in a register.
`(zero_extract:SI LOC SIZE POS)'
Like `sign_extract' but refers to an unsigned or zero-extended
bit field. The same sequence of bits are extracted, but they
are filled to an entire word with zeros instead of by
sign-extension.
File: internals, Node: Conversions, Next: RTL Declarations, Prev: Bit Fields, Up: RTL
Conversions
===========
All conversions between machine modes must be represented by explicit
conversion operations. For example, an expression which is the sum
of a byte and a full word cannot be written as `(plus:SI (reg:QI 34)
(reg:SI 80))' because the `plus' operation requires two operands of
the same machine mode. Therefore, the byte-sized operand is enclosed
in a conversion operation, as in
(plus:SI (sign_extend:SI (reg:QI 34)) (reg:SI 80))
The conversion operation is not a mere placeholder, because there may
be more than one way of converting from a given starting mode to the
desired final mode. The conversion operation code says how to do it.
`(sign_extend:M X)'
Represents the result of sign-extending the value X to machine
mode M. M must be a fixed-point mode and X a fixed-point value
of a mode narrower than M.
`(zero_extend:M X)'
Represents the result of zero-extending the value X to machine
mode M. M must be a fixed-point mode and X a fixed-point value
of a mode narrower than M.
`(float_extend:M X)'
Represents the result of extending the value X to machine mode
M. M must be a floating point mode and X a floating point value
of a mode narrower than M.
`(truncate:M X)'
Represents the result of truncating the value X to machine mode
M. M must be a fixed-point mode and X a fixed-point value of a
mode wider than M.
`(float_truncate:M X)'
Represents the result of truncating the value X to machine mode
M. M must be a floating point mode and X a floating point value
of a mode wider than M.
`(float:M X)'
Represents the result of converting fixed point value X,
regarded as signed, to floating point mode M.
`(unsigned_float:M X)'
Represents the result of converting fixed point value X,
regarded as unsigned, to floating point mode M.
`(fix:M X)'
When M is a fixed point mode, represents the result of
converting floating point value X to mode M, regarded as signed.
How rounding is done is not specified, so this operation may be
used validly in compiling C code only for integer-valued operands.
`(unsigned_fix:M X)'
Represents the result of converting floating point value X to
fixed point mode M, regarded as unsigned. How rounding is done
is not specified.
`(fix:M X)'
When M is a floating point mode, represents the result of
converting floating point value X (valid for mode M) to an
integer, still represented in floating point mode M, by rounding
towards zero.
File: internals, Node: RTL Declarations, Next: Side Effects, Prev: Conversions, Up: RTL
Declarations
============
Declaration expression codes do not represent arithmetic operations
but rather state assertions about their operands.
`(strict_low_part (subreg:M (reg:N R) 0))'
This expression code is used in only one context: operand 0 of a
`set' expression. In addition, the operand of this expression
must be a `subreg' expression.
The presence of `strict_low_part' says that the part of the
register which is meaningful in mode N, but is not part of mode
M, is not to be altered. Normally, an assignment to such a
subreg is allowed to have undefined effects on the rest of the
register when M is less than a word.
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