rtl.texi

GCC

@code{STORE_FLAG_VALUE} is one, @code{const_true_rtx} and
@code{const1_rtx} will point to the same object.  If
@code{STORE_FLAG_VALUE} is @minus{}1, @code{const_true_rtx} and
@code{constm1_rtx} will point to the same object.@refill

@findex const_double
@item (const_double:@var{m} @var{addr} @var{i0} @var{i1} @dots{})
Represents either a floating-point constant of mode @var{m} or an
integer constant too large to fit into @code{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, @var{m} will be @code{VOIDmode}.

@findex CONST_DOUBLE_MEM
@findex CONST_DOUBLE_CHAIN
@var{addr} is used to contain the @code{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
@code{const_double} expressions in this compilation (maintained using an
undisplayed field), @var{addr} contains @code{const0_rtx}.  If it is not
on the chain, @var{addr} contains @code{cc0_rtx}.  @var{addr} is
customarily accessed with the macro @code{CONST_DOUBLE_MEM} and the
chain field via @code{CONST_DOUBLE_CHAIN}.@refill

@findex CONST_DOUBLE_LOW
If @var{m} is @code{VOIDmode}, the bits of the value are stored in
@var{i0} and @var{i1}.  @var{i0} is customarily accessed with the macro
@code{CONST_DOUBLE_LOW} and @var{i1} with @code{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
@code{REAL_VALUE_TYPE} (@pxref{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
@code{REAL_VALUE_TO_TARGET_DOUBLE} and friends (@pxref{Data Output}).

@findex CONST0_RTX
@findex CONST1_RTX
@findex CONST2_RTX
The macro @code{CONST0_RTX (@var{mode})} refers to an expression with
value 0 in mode @var{mode}.  If mode @var{mode} is of mode class
@code{MODE_INT}, it returns @code{const0_rtx}.  Otherwise, it returns a
@code{CONST_DOUBLE} expression in mode @var{mode}.  Similarly, the macro
@code{CONST1_RTX (@var{mode})} refers to an expression with value 1 in
mode @var{mode} and similarly for @code{CONST2_RTX}.

@findex const_string
@item (const_string @var{str})
Represents a constant string with value @var{str}.  Currently this is
used only for insn attributes (@pxref{Insn Attributes}) since constant
strings in C are placed in memory.

@findex symbol_ref
@item (symbol_ref:@var{mode} @var{symbol})
Represents the value of an assembler label for data.  @var{symbol} is
a string that describes the name of the assembler label.  If it starts
with a @samp{*}, the label is the rest of @var{symbol} not including
the @samp{*}.  Otherwise, the label is @var{symbol}, usually prefixed
with @samp{_}.

The @code{symbol_ref} contains a mode, which is usually @code{Pmode}.
Usually that is the only mode for which a symbol is directly valid.

@findex label_ref
@item (label_ref @var{label})
Represents the value of an assembler label for code.  It contains one
operand, an expression, which must be a @code{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.

@item (const:@var{m} @var{exp})
Represents a constant that is the result of an assembly-time
arithmetic computation.  The operand, @var{exp}, is an expression that
contains only constants (@code{const_int}, @code{symbol_ref} and
@code{label_ref} expressions) combined with @code{plus} and
@code{minus}.  However, not all combinations are valid, since the
assembler cannot do arbitrary arithmetic on relocatable symbols.

@var{m} should be @code{Pmode}.

@findex high
@item (high:@var{m} @var{exp})
Represents the high-order bits of @var{exp}, usually a
@code{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 @code{lo_sum} to
represent the typical two-instruction sequence used in RISC machines to
reference a global memory location.

@var{m} should be @code{Pmode}.
@end table

@node Regs and Memory
@section Registers and Memory
@cindex RTL register expressions
@cindex RTL memory expressions

Here are the RTL expression types for describing access to machine
registers and to main memory.

@table @code
@findex reg
@cindex hard registers
@cindex pseudo registers
@item (reg:@var{m} @var{n})
For small values of the integer @var{n} (those that are less than
@code{FIRST_PSEUDO_REGISTER}), this stands for a reference to machine
register number @var{n}: a @dfn{hard register}.  For larger values of
@var{n}, it stands for a temporary value or @dfn{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.

@var{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.

The symbol @code{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.

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 @code{subreg}
expression is used.

A @code{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.

Each pseudo register number used in a function's RTL code is
represented by a unique @code{reg} expression.

@findex FIRST_VIRTUAL_REGISTER
@findex LAST_VIRTUAL_REGISTER
Some pseudo register numbers, those within the range of
@code{FIRST_VIRTUAL_REGISTER} to @code{LAST_VIRTUAL_REGISTER} only
appear during the RTL generation phase and are eliminated before the
optimization phases.  These represent locations in the stack frame that
cannot be determined until RTL generation for the function has been
completed.  The following virtual register numbers are defined:

@table @code
@findex VIRTUAL_INCOMING_ARGS_REGNUM
@item VIRTUAL_INCOMING_ARGS_REGNUM
This points to the first word of the incoming arguments passed on the
stack.  Normally these arguments are placed there by the caller, but the
callee may have pushed some arguments that were previously passed in
registers.

@cindex @code{FIRST_PARM_OFFSET} and virtual registers
@cindex @code{ARG_POINTER_REGNUM} and virtual registers
When RTL generation is complete, this virtual register is replaced
by the sum of the register given by @code{ARG_POINTER_REGNUM} and the
value of @code{FIRST_PARM_OFFSET}.

@findex VIRTUAL_STACK_VARS_REGNUM
@cindex @code{FRAME_GROWS_DOWNWARD} and virtual registers
@item VIRTUAL_STACK_VARS_REGNUM
If @code{FRAME_GROWS_DOWNWARD} is defined, this points to immediately
above the first variable on the stack.  Otherwise, it points to the
first variable on the stack.

@cindex @code{STARTING_FRAME_OFFSET} and virtual registers
@cindex @code{FRAME_POINTER_REGNUM} and virtual registers
@code{VIRTUAL_STACK_VARS_REGNUM} is replaced with the sum of the
register given by @code{FRAME_POINTER_REGNUM} and the value
@code{STARTING_FRAME_OFFSET}.

@findex VIRTUAL_STACK_DYNAMIC_REGNUM
@item VIRTUAL_STACK_DYNAMIC_REGNUM
This points to the location of dynamically allocated memory on the stack
immediately after the stack pointer has been adjusted by the amount of
memory desired.

@cindex @code{STACK_DYNAMIC_OFFSET} and virtual registers
@cindex @code{STACK_POINTER_REGNUM} and virtual registers
This virtual register is replaced by the sum of the register given by
@code{STACK_POINTER_REGNUM} and the value @code{STACK_DYNAMIC_OFFSET}.

@findex VIRTUAL_OUTGOING_ARGS_REGNUM
@item VIRTUAL_OUTGOING_ARGS_REGNUM
This points to the location in the stack at which outgoing arguments
should be written when the stack is pre-pushed (arguments pushed using
push insns should always use @code{STACK_POINTER_REGNUM}).

@cindex @code{STACK_POINTER_OFFSET} and virtual registers
This virtual register is replaced by the sum of the register given by
@code{STACK_POINTER_REGNUM} and the value @code{STACK_POINTER_OFFSET}.
@end table

@findex subreg
@item (subreg:@var{m} @var{reg} @var{wordnum})
@code{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 @code{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 @code{subreg}.  In
such a case, @var{wordnum} is zero.

Usually @var{m} is at least as narrow as the mode of @var{reg}, in which
case it is restricting consideration to only the bits of @var{reg} that
are in @var{m}.

Sometimes @var{m} is wider than the mode of @var{reg}.  These
@code{subreg} expressions are often called @dfn{paradoxical}.  They are
used in cases where we want to refer to an object in a wider mode but do
not care what value the additional bits have.  The reload pass ensures
that paradoxical references are only made to hard registers.

The other use of @code{subreg} is to extract the individual registers of
a multi-register value.  Machine modes such as @code{DImode} and
@code{TImode} can indicate values longer than a word, values which
usually require two or more consecutive registers.  To access one of the
registers, use a @code{subreg} with mode @code{SImode} and a
@var{wordnum} that says which register.

Storing in a non-paradoxical @code{subreg} has undefined results for
bits belonging to the same word as the @code{subreg}.  This laxity makes
it easier to generate efficient code for such instructions.  To
represent an instruction that preserves all the bits outside of those in
the @code{subreg}, use @code{strict_low_part} around the @code{subreg}.

@cindex @code{WORDS_BIG_ENDIAN}, effect on @code{subreg}
The compilation parameter @code{WORDS_BIG_ENDIAN}, if set to 1, says
that word number zero is the most significant part; otherwise, it is
the least significant part.

@cindex @code{FLOAT_WORDS_BIG_ENDIAN}, (lack of) effect on @code{subreg}
On a few targets, @code{FLOAT_WORDS_BIG_ENDIAN} disagrees with
@code{WORDS_BIG_ENDIAN}.
However, most parts of the compiler treat floating point values as if
they had the same endianness as integer values.  This works because
they handle them solely as a collection of integer values, with no
particular numerical value.  Only real.c and the runtime libraries
care about @code{FLOAT_WORDS_BIG_ENDIAN}.

@cindex combiner pass
@cindex reload pass
@cindex @code{subreg}, special reload handling
Between the combiner pass and the reload pass, it is possible to have a
paradoxical @code{subreg} which contains a @code{mem} instead of a
@code{reg} as its first operand.  After the reload pass, it is also
possible to have a non-paradoxical @code{subreg} which contains a
@code{mem}; this usually occurs when the @code{mem} is a stack slot
which replaced a pseudo register.

Note that it is not valid to access a @code{DFmode} value in @code{SFmode}
using a @code{subreg}.  On some machines the most significant part of a
@code{DFmode} value does not have the same format as a single-precision
floating value.

It is also not valid to access a single word of a multi-word value in a
hard register when less registers can hold the value than would be
expected from its size.  For example, some 32-bit machines have
floating-point registers that can hold an entire @code{DFmode} value.
If register 10 were such a register @code{(subreg:SI (reg:DF 10) 1)}
would be invalid because there is no way to convert that reference to
a single machine register.  The reload pass prevents @code{subreg}
expressions such as these from being formed.

@findex SUBREG_REG
@findex SUBREG_WORD
The first operand of a @code{subreg} expression is customarily accessed
with the @code{SUBREG_REG} macro and the second operand is customarily
accessed with the @code{SUBREG_WORD} macro.

@findex scratch
@cindex scratch operands
@item (scratch:@var{m})
This represents a scratch register that will be required for the
execution of a single instruction and not used subsequently.  It is
converted into a @code{reg} by either the local register allocator or
the reload pass.

@code{scratch} is usually present inside a @code{clobber} operation
(@pxref{Side Effects}).

@findex cc0
@cindex condition code register
@item (cc0)
This refers to the machine's condition code register.  It has no
operands and may not have a machine mode.  There are two ways to use it:

@itemize @bullet
@item
To stand for a complete set of condition code flags.  This is best on
most machines, where each comparison sets the entire series of flags.

With this technique, @code{(cc0)} 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
(@code{const_int} with value zero; that is to say, @code{const0_rtx}).

@item
To stand for a single flag that is the result of a single condition.
This is useful on machines that have only a single flag bit, and in
which comparison instructions must specify the condition to test.

With this technique, @code{(cc0)} may be validly used in only two
contexts: as the destination of an assignment (in test and compare
instructions) where the source is a comparison operator, and as the