rtl.texi

早期freebsd实现

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_DOWNWARDS} 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
It 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
It 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
It 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}.  However, 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.

@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 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
first operand of @code{if_then_else} (in a conditional branch).
@end itemize

@findex cc0_rtx
There is only one expression object of code @code{cc0}; it is the
value of the variable @code{cc0_rtx}.  Any attempt to create an
expression of code @code{cc0} will return @code{cc0_rtx}.

Instructions can set the condition code 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 @code{NOTICE_UPDATE_CC}).  @xref{Condition Code}.  Only
instructions whose sole purpose is to set the condition code, and
instructions that use the condition code, need mention @code{(cc0)}.

On some machines, the condition code register is given a register number
and a @code{reg} is used instead of @code{(cc0)}.  This is usually the
preferable approach if only a small subset of instructions modify the
condition code.  Other machines store condition codes in general
registers; in such cases a pseudo register should be used.

Some machines, such as the Sparc and RS/6000, have two sets of
arithmetic instructions, one that sets and one that does not set the
condition code.  This is best handled by normally generating the
instruction that does not set the condition code, and making a pattern
that both performs the arithmetic and sets the condition code register
(which would not be @code{(cc0)} in this case).  For examples, search
for @samp{addcc} and @samp{andcc} in @file{sparc.md}.

@findex pc
@item (pc)
@cindex program counter
This represents the machine's program counter.  It has no operands and
may not have a machine mode.  @code{(pc)} may be validly used only in
certain specific contexts in jump instructions.

@findex pc_rtx
There is only one expression object of code @code{pc}; it is the value
of the variable @code{pc_rtx}.  Any attempt to create an expression of
code @code{pc} will return @code{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.

@findex mem
@item (mem:@var{m} @var{addr})
This RTX represents a reference to main memory at an address
represented by the expression @var{addr}.  @var{m} specifies how large
a unit of memory is accessed.
@end table

@node Arithmetic, Comparisons, Regs and Memory, RTL
@section RTL Expressions for Arithmetic
@cindex arithmetic, in RTL
@cindex math, in RTL
@cindex RTL expressions for arithmetic

Unless otherwise specified, all the operands of arithmetic expressions
must be valid for mode @var{m}.  An operand is valid for mode @var{m}
if it has mode @var{m}, or if it is a @code{const_int} or
@code{const_double} and @var{m} is a mode of class @code{MODE_INT}.

For commutative binary operations, constants should be placed in the
second operand.

@table @code
@findex plus
@cindex RTL addition
@cindex RTL sum
@item (plus:@var{m} @var{x} @var{y})
Represents the sum of the values represented by @var{x} and @var{y}
carried out in machine mode @var{m}. 

@findex lo_sum
@item (lo_sum:@var{m} @var{x} @var{y})
Like @code{plus}, except that it represents that sum of @var{x} and the
low-order bits of @var{y}.  The number of low order bits is
machine-dependent but is normally the number of bits in a @code{Pmode}
item minus the number of bits set by the @code{high} code
(@pxref{Constants}).

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

@findex minus
@cindex RTL subtraction
@cindex RTL difference
@item (minus:@var{m} @var{x} @var{y})
Like @code{plus} but represents subtraction.

@findex compare
@cindex RTL comparison
@item (compare:@var{m} @var{x} @var{y})
Represents the result of subtracting @var{y} from @var{x} for purposes
of comparison.  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 the condition code.   And that is the only way this kind of expression
may validly be used: as a value to be stored in the condition codes.

The mode @var{m} is not related to the modes of @var{x} and @var{y},
but instead is the mode of the condition code value.  If @code{(cc0)}
is used, it is @code{VOIDmode}.  Otherwise it is some mode in class
@code{MODE_CC}, often @code{CCmode}.  @xref{Condition Code}.

Normally, @var{x} and @var{y} must have the same mode.  Otherwise,
@code{compare} is valid only if the mode of @var{x} is in class
@code{MODE_INT} and @var{y} is a @code{const_int} or
@code{const_double} with mode @code{VOIDmode}.  The mode of @var{x}
determines what mode the comparison is to be done in; thus it must not
be @code{VOIDmode}.

If one of the operands is a constant, it should be placed in the
second operand and the comparison code adjusted as appropriate.  

A @code{compare} specifying two @code{VOIDmode} constants is not valid
since there is no way to know in what mode the comparison is to be
performed; the comparison must either be folded during the compilation
or the first operand must be loaded into a register while its mode is
still known.

@findex neg
@item (neg:@var{m} @var{x})
Represents the negation (subtraction from zero) of the value represented
by @var{x}, carried out in mode @var{m}.

@findex mult
@cindex multiplication
@cindex product
@item (mult:@var{m} @var{x} @var{y})
Represents the signed product of the values represented by @var{x} and
@var{y} carried out in machine mode @var{m}.

Some machines support a multiplication that generates a product wider
than the operands.  Write the pattern for this as

@example
(mult:@var{m} (sign_extend:@var{m} @var{x}) (sign_extend:@var{m} @var{y}))
@end example

where @var{m} is wider than the modes of @var{x} and @var{y}, which need
not be the same.

Write patterns for unsigned widening multiplication similarly using
@code{zero_extend}.

@findex div
@cindex division
@cindex signed division
@cindex quotient
@item (div:@var{m} @var{x} @var{y})
Represents the quotient in signed division of @var{x} by @var{y},
carried out in machine mode @var{m}.  If @var{m} is a floating point
mode, it represents the exact quotient; otherwise, the integerized
quotient.

Some machines have division instructions in which the operands and
quotient widths are not all the same; you should represent 
such instructions using @code{truncate} and @code{sign_extend} as in,

@example
(truncate:@var{m1} (div:@var{m2} @var{x} (sign_extend:@var{m2} @var{y})))
@end example

@findex udiv
@cindex unsigned division
@cindex division
@item (udiv:@var{m} @var{x} @var{y})
Like @code{div} but represents unsigned division.

@findex mod
@findex umod
@cindex remainder
@cindex division
@item (mod:@var{m} @var{x} @var{y})