md.texi

理解和实践操作系统的一本好书

for registers and @samp{clrmem} for memory locations.  Here is how
a pattern could use @code{which_alternative} to choose between them:

@smallexample
(define_insn ""
  [(set (match_operand:SI 0 "general_operand" "=r,m")
        (const_int 0))]
  ""
  @{
  return (which_alternative == 0
          ? "clrreg %0" : "clrmem %0");
  @})
@end smallexample

The example above, where the assembler code to generate was
@emph{solely} determined by the alternative, could also have been specified
as follows, having the output control string start with a @samp{@@}:

@smallexample
@group
(define_insn ""
  [(set (match_operand:SI 0 "general_operand" "=r,m")
        (const_int 0))]
  ""
  "@@
   clrreg %0
   clrmem %0")
@end group
@end smallexample

@node Predicates
@section Predicates
@cindex predicates
@cindex operand predicates
@cindex operator predicates

A predicate determines whether a @code{match_operand} or
@code{match_operator} expression matches, and therefore whether the
surrounding instruction pattern will be used for that combination of
operands.  GCC has a number of machine-independent predicates, and you
can define machine-specific predicates as needed.  By convention,
predicates used with @code{match_operand} have names that end in
@samp{_operand}, and those used with @code{match_operator} have names
that end in @samp{_operator}.

All predicates are Boolean functions (in the mathematical sense) of
two arguments: the RTL expression that is being considered at that
position in the instruction pattern, and the machine mode that the
@code{match_operand} or @code{match_operator} specifies.  In this
section, the first argument is called @var{op} and the second argument
@var{mode}.  Predicates can be called from C as ordinary two-argument
functions; this can be useful in output templates or other
machine-specific code.

Operand predicates can allow operands that are not actually acceptable
to the hardware, as long as the constraints give reload the ability to
fix them up (@pxref{Constraints}).  However, GCC will usually generate
better code if the predicates specify the requirements of the machine
instructions as closely as possible.  Reload cannot fix up operands
that must be constants (``immediate operands''); you must use a
predicate that allows only constants, or else enforce the requirement
in the extra condition.

@cindex predicates and machine modes
@cindex normal predicates
@cindex special predicates
Most predicates handle their @var{mode} argument in a uniform manner.
If @var{mode} is @code{VOIDmode} (unspecified), then @var{op} can have
any mode.  If @var{mode} is anything else, then @var{op} must have the
same mode, unless @var{op} is a @code{CONST_INT} or integer
@code{CONST_DOUBLE}.  These RTL expressions always have
@code{VOIDmode}, so it would be counterproductive to check that their
mode matches.  Instead, predicates that accept @code{CONST_INT} and/or
integer @code{CONST_DOUBLE} check that the value stored in the
constant will fit in the requested mode.

Predicates with this behavior are called @dfn{normal}.
@command{genrecog} can optimize the instruction recognizer based on
knowledge of how normal predicates treat modes.  It can also diagnose
certain kinds of common errors in the use of normal predicates; for
instance, it is almost always an error to use a normal predicate
without specifying a mode.

Predicates that do something different with their @var{mode} argument
are called @dfn{special}.  The generic predicates
@code{address_operand} and @code{pmode_register_operand} are special
predicates.  @command{genrecog} does not do any optimizations or
diagnosis when special predicates are used.

@menu
* Machine-Independent Predicates::  Predicates available to all back ends.
* Defining Predicates::             How to write machine-specific predicate
                                    functions.
@end menu

@node Machine-Independent Predicates
@subsection Machine-Independent Predicates
@cindex machine-independent predicates
@cindex generic predicates

These are the generic predicates available to all back ends.  They are
defined in @file{recog.c}.  The first category of predicates allow
only constant, or @dfn{immediate}, operands.

@defun immediate_operand
This predicate allows any sort of constant that fits in @var{mode}.
It is an appropriate choice for instructions that take operands that
must be constant.
@end defun

@defun const_int_operand
This predicate allows any @code{CONST_INT} expression that fits in
@var{mode}.  It is an appropriate choice for an immediate operand that
does not allow a symbol or label.
@end defun

@defun const_double_operand
This predicate accepts any @code{CONST_DOUBLE} expression that has
exactly @var{mode}.  If @var{mode} is @code{VOIDmode}, it will also
accept @code{CONST_INT}.  It is intended for immediate floating point
constants.
@end defun

@noindent
The second category of predicates allow only some kind of machine
register.

@defun register_operand
This predicate allows any @code{REG} or @code{SUBREG} expression that
is valid for @var{mode}.  It is often suitable for arithmetic
instruction operands on a RISC machine.
@end defun

@defun pmode_register_operand
This is a slight variant on @code{register_operand} which works around
a limitation in the machine-description reader.

@smallexample
(match_operand @var{n} "pmode_register_operand" @var{constraint})
@end smallexample

@noindent
means exactly what

@smallexample
(match_operand:P @var{n} "register_operand" @var{constraint})
@end smallexample

@noindent
would mean, if the machine-description reader accepted @samp{:P}
mode suffixes.  Unfortunately, it cannot, because @code{Pmode} is an
alias for some other mode, and might vary with machine-specific
options.  @xref{Misc}.
@end defun

@defun scratch_operand
This predicate allows hard registers and @code{SCRATCH} expressions,
but not pseudo-registers.  It is used internally by @code{match_scratch};
it should not be used directly.
@end defun

@noindent
The third category of predicates allow only some kind of memory reference.

@defun memory_operand
This predicate allows any valid reference to a quantity of mode
@var{mode} in memory, as determined by the weak form of
@code{GO_IF_LEGITIMATE_ADDRESS} (@pxref{Addressing Modes}).
@end defun

@defun address_operand
This predicate is a little unusual; it allows any operand that is a
valid expression for the @emph{address} of a quantity of mode
@var{mode}, again determined by the weak form of
@code{GO_IF_LEGITIMATE_ADDRESS}.  To first order, if
@samp{@w{(mem:@var{mode} (@var{exp}))}} is acceptable to
@code{memory_operand}, then @var{exp} is acceptable to
@code{address_operand}.  Note that @var{exp} does not necessarily have
the mode @var{mode}.
@end defun

@defun indirect_operand
This is a stricter form of @code{memory_operand} which allows only
memory references with a @code{general_operand} as the address
expression.  New uses of this predicate are discouraged, because
@code{general_operand} is very permissive, so it's hard to tell what
an @code{indirect_operand} does or does not allow.  If a target has
different requirements for memory operands for different instructions,
it is better to define target-specific predicates which enforce the
hardware's requirements explicitly.
@end defun

@defun push_operand
This predicate allows a memory reference suitable for pushing a value
onto the stack.  This will be a @code{MEM} which refers to
@code{stack_pointer_rtx}, with a side-effect in its address expression
(@pxref{Incdec}); which one is determined by the
@code{STACK_PUSH_CODE} macro (@pxref{Frame Layout}).
@end defun

@defun pop_operand
This predicate allows a memory reference suitable for popping a value
off the stack.  Again, this will be a @code{MEM} referring to
@code{stack_pointer_rtx}, with a side-effect in its address
expression.  However, this time @code{STACK_POP_CODE} is expected.
@end defun

@noindent
The fourth category of predicates allow some combination of the above
operands.

@defun nonmemory_operand
This predicate allows any immediate or register operand valid for @var{mode}.
@end defun

@defun nonimmediate_operand
This predicate allows any register or memory operand valid for @var{mode}.
@end defun

@defun general_operand
This predicate allows any immediate, register, or memory operand
valid for @var{mode}.
@end defun

@noindent
Finally, there is one generic operator predicate.

@defun comparison_operator
This predicate matches any expression which performs an arithmetic
comparison in @var{mode}; that is, @code{COMPARISON_P} is true for the
expression code.
@end defun

@node Defining Predicates
@subsection Defining Machine-Specific Predicates
@cindex defining predicates
@findex define_predicate
@findex define_special_predicate

Many machines have requirements for their operands that cannot be
expressed precisely using the generic predicates.  You can define
additional predicates using @code{define_predicate} and
@code{define_special_predicate} expressions.  These expressions have
three operands:

@itemize @bullet
@item
The name of the predicate, as it will be referred to in
@code{match_operand} or @code{match_operator} expressions.

@item
An RTL expression which evaluates to true if the predicate allows the
operand @var{op}, false if it does not.  This expression can only use
the following RTL codes:

@table @code
@item MATCH_OPERAND
When written inside a predicate expression, a @code{MATCH_OPERAND}
expression evaluates to true if the predicate it names would allow
@var{op}.  The operand number and constraint are ignored.  Due to
limitations in @command{genrecog}, you can only refer to generic
predicates and predicates that have already been defined.

@item MATCH_CODE
This expression evaluates to true if @var{op} or a specified
subexpression of @var{op} has one of a given list of RTX codes.

The first operand of this expression is a string constant containing a
comma-separated list of RTX code names (in lower case).  These are the
codes for which the @code{MATCH_CODE} will be true.

The second operand is a string constant which indicates what
subexpression of @var{op} to examine.  If it is absent or the empty
string, @var{op} itself is examined.  Otherwise, the string constant
must be a sequence of digits and/or lowercase letters.  Each character
indicates a subexpression to extract from the current expression; for
the first character this is @var{op}, for the second and subsequent
characters it is the result of the previous character.  A digit
@var{n} extracts @samp{@w{XEXP (@var{e}, @var{n})}}; a letter @var{l}
extracts @samp{@w{XVECEXP (@var{e}, 0, @var{n})}} where @var{n} is the
alphabetic ordinal of @var{l} (0 for `a', 1 for 'b', and so on).  The
@code{MATCH_CODE} then examines the RTX code of the subexpression
extracted by the complete string.  It is not possible to extract
components of an @code{rtvec} that is not at position 0 within its RTX
object.

@item MATCH_TEST
This expression has one operand, a string constant containing a C
expression.  The predicate's arguments, @var{op} and @var{mode}, are
available with those names in the C expression.  The @code{MATCH_TEST}
evaluates to true if the C expression evaluates to a nonzero value.
@code{MATCH_TEST} expressions must not have side effects.

@item  AND
@itemx IOR
@itemx NOT
@itemx IF_THEN_ELSE
The basic @samp{MATCH_} expressions can be combined using these
logical operators, which have the semantics of the C operators
@samp{&&}, @samp{||}, @samp{!}, and @samp{@w{? :}} respectively.  As
in Common Lisp, you may give an @code{AND} or @code{IOR} expression an
arbitrary number of arguments; this has exactly the same effect as
writing a chain of two-argument @code{AND} or @code{IOR} expressions.
@end table

@item
An optional block of C code, which should execute
@samp{@w{return true}} if the predicate is found to match and
@samp{@w{return false}} if it does not.  It must not have any side
effects.  The predicate arguments, @var{op} and @var{mode}, are
available with those names.

If a code block is present in a predicate definition, then the RTL
expression must evaluate to true @emph{and} the code block must
execute @samp{@w{return true}} for the predicate to allow the operand.
The RTL expression is evaluated first; do not re-check anything in the
code block that was checked in the RTL expression.
@end itemize