md.texi

理解和实践操作系统的一本好书

The program @command{genrecog} scans @code{define_predicate} and
@code{define_special_predicate} expressions to determine which RTX
codes are possibly allowed.  You should always make this explicit in
the RTL predicate expression, using @code{MATCH_OPERAND} and
@code{MATCH_CODE}.

Here is an example of a simple predicate definition, from the IA64
machine description:

@smallexample
@group
;; @r{True if @var{op} is a @code{SYMBOL_REF} which refers to the sdata section.}
(define_predicate "small_addr_symbolic_operand"
  (and (match_code "symbol_ref")
       (match_test "SYMBOL_REF_SMALL_ADDR_P (op)")))
@end group
@end smallexample

@noindent
And here is another, showing the use of the C block.

@smallexample
@group
;; @r{True if @var{op} is a register operand that is (or could be) a GR reg.}
(define_predicate "gr_register_operand"
  (match_operand 0 "register_operand")
@{
  unsigned int regno;
  if (GET_CODE (op) == SUBREG)
    op = SUBREG_REG (op);

  regno = REGNO (op);
  return (regno >= FIRST_PSEUDO_REGISTER || GENERAL_REGNO_P (regno));
@})
@end group
@end smallexample

Predicates written with @code{define_predicate} automatically include
a test that @var{mode} is @code{VOIDmode}, or @var{op} has the same
mode as @var{mode}, or @var{op} is a @code{CONST_INT} or
@code{CONST_DOUBLE}.  They do @emph{not} check specifically for
integer @code{CONST_DOUBLE}, nor do they test that the value of either
kind of constant fits in the requested mode.  This is because
target-specific predicates that take constants usually have to do more
stringent value checks anyway.  If you need the exact same treatment
of @code{CONST_INT} or @code{CONST_DOUBLE} that the generic predicates
provide, use a @code{MATCH_OPERAND} subexpression to call
@code{const_int_operand}, @code{const_double_operand}, or
@code{immediate_operand}.

Predicates written with @code{define_special_predicate} do not get any
automatic mode checks, and are treated as having special mode handling
by @command{genrecog}.

The program @command{genpreds} is responsible for generating code to
test predicates.  It also writes a header file containing function
declarations for all machine-specific predicates.  It is not necessary
to declare these predicates in @file{@var{cpu}-protos.h}.
@end ifset

@c Most of this node appears by itself (in a different place) even
@c when the INTERNALS flag is clear.  Passages that require the internals
@c manual's context are conditionalized to appear only in the internals manual.
@ifset INTERNALS
@node Constraints
@section Operand Constraints
@cindex operand constraints
@cindex constraints

Each @code{match_operand} in an instruction pattern can specify
constraints for the operands allowed.  The constraints allow you to
fine-tune matching within the set of operands allowed by the
predicate.

@end ifset
@ifclear INTERNALS
@node Constraints
@section Constraints for @code{asm} Operands
@cindex operand constraints, @code{asm}
@cindex constraints, @code{asm}
@cindex @code{asm} constraints

Here are specific details on what constraint letters you can use with
@code{asm} operands.
@end ifclear
Constraints can say whether
an operand may be in a register, and which kinds of register; whether the
operand can be a memory reference, and which kinds of address; whether the
operand may be an immediate constant, and which possible values it may
have.  Constraints can also require two operands to match.

@ifset INTERNALS
@menu
* Simple Constraints::  Basic use of constraints.
* Multi-Alternative::   When an insn has two alternative constraint-patterns.
* Class Preferences::   Constraints guide which hard register to put things in.
* Modifiers::           More precise control over effects of constraints.
* Machine Constraints:: Existing constraints for some particular machines.
* Define Constraints::  How to define machine-specific constraints.
* C Constraint Interface:: How to test constraints from C code.
@end menu
@end ifset

@ifclear INTERNALS
@menu
* Simple Constraints::  Basic use of constraints.
* Multi-Alternative::   When an insn has two alternative constraint-patterns.
* Modifiers::           More precise control over effects of constraints.
* Machine Constraints:: Special constraints for some particular machines.
@end menu
@end ifclear

@node Simple Constraints
@subsection Simple Constraints
@cindex simple constraints

The simplest kind of constraint is a string full of letters, each of
which describes one kind of operand that is permitted.  Here are
the letters that are allowed:

@table @asis
@item whitespace
Whitespace characters are ignored and can be inserted at any position
except the first.  This enables each alternative for different operands to
be visually aligned in the machine description even if they have different
number of constraints and modifiers.

@cindex @samp{m} in constraint
@cindex memory references in constraints
@item @samp{m}
A memory operand is allowed, with any kind of address that the machine
supports in general.

@cindex offsettable address
@cindex @samp{o} in constraint
@item @samp{o}
A memory operand is allowed, but only if the address is
@dfn{offsettable}.  This means that adding a small integer (actually,
the width in bytes of the operand, as determined by its machine mode)
may be added to the address and the result is also a valid memory
address.

@cindex autoincrement/decrement addressing
For example, an address which is constant is offsettable; so is an
address that is the sum of a register and a constant (as long as a
slightly larger constant is also within the range of address-offsets
supported by the machine); but an autoincrement or autodecrement
address is not offsettable.  More complicated indirect/indexed
addresses may or may not be offsettable depending on the other
addressing modes that the machine supports.

Note that in an output operand which can be matched by another
operand, the constraint letter @samp{o} is valid only when accompanied
by both @samp{<} (if the target machine has predecrement addressing)
and @samp{>} (if the target machine has preincrement addressing).

@cindex @samp{V} in constraint
@item @samp{V}
A memory operand that is not offsettable.  In other words, anything that
would fit the @samp{m} constraint but not the @samp{o} constraint.

@cindex @samp{<} in constraint
@item @samp{<}
A memory operand with autodecrement addressing (either predecrement or
postdecrement) is allowed.

@cindex @samp{>} in constraint
@item @samp{>}
A memory operand with autoincrement addressing (either preincrement or
postincrement) is allowed.

@cindex @samp{r} in constraint
@cindex registers in constraints
@item @samp{r}
A register operand is allowed provided that it is in a general
register.

@cindex constants in constraints
@cindex @samp{i} in constraint
@item @samp{i}
An immediate integer operand (one with constant value) is allowed.
This includes symbolic constants whose values will be known only at
assembly time or later.

@cindex @samp{n} in constraint
@item @samp{n}
An immediate integer operand with a known numeric value is allowed.
Many systems cannot support assembly-time constants for operands less
than a word wide.  Constraints for these operands should use @samp{n}
rather than @samp{i}.

@cindex @samp{I} in constraint
@item @samp{I}, @samp{J}, @samp{K}, @dots{} @samp{P}
Other letters in the range @samp{I} through @samp{P} may be defined in
a machine-dependent fashion to permit immediate integer operands with
explicit integer values in specified ranges.  For example, on the
68000, @samp{I} is defined to stand for the range of values 1 to 8.
This is the range permitted as a shift count in the shift
instructions.

@cindex @samp{E} in constraint
@item @samp{E}
An immediate floating operand (expression code @code{const_double}) is
allowed, but only if the target floating point format is the same as
that of the host machine (on which the compiler is running).

@cindex @samp{F} in constraint
@item @samp{F}
An immediate floating operand (expression code @code{const_double} or
@code{const_vector}) is allowed.

@cindex @samp{G} in constraint
@cindex @samp{H} in constraint
@item @samp{G}, @samp{H}
@samp{G} and @samp{H} may be defined in a machine-dependent fashion to
permit immediate floating operands in particular ranges of values.

@cindex @samp{s} in constraint
@item @samp{s}
An immediate integer operand whose value is not an explicit integer is
allowed.

This might appear strange; if an insn allows a constant operand with a
value not known at compile time, it certainly must allow any known
value.  So why use @samp{s} instead of @samp{i}?  Sometimes it allows
better code to be generated.

For example, on the 68000 in a fullword instruction it is possible to
use an immediate operand; but if the immediate value is between @minus{}128
and 127, better code results from loading the value into a register and
using the register.  This is because the load into the register can be
done with a @samp{moveq} instruction.  We arrange for this to happen
by defining the letter @samp{K} to mean ``any integer outside the
range @minus{}128 to 127'', and then specifying @samp{Ks} in the operand
constraints.

@cindex @samp{g} in constraint
@item @samp{g}
Any register, memory or immediate integer operand is allowed, except for
registers that are not general registers.

@cindex @samp{X} in constraint
@item @samp{X}
@ifset INTERNALS
Any operand whatsoever is allowed, even if it does not satisfy
@code{general_operand}.  This is normally used in the constraint of
a @code{match_scratch} when certain alternatives will not actually
require a scratch register.
@end ifset
@ifclear INTERNALS
Any operand whatsoever is allowed.
@end ifclear

@cindex @samp{0} in constraint
@cindex digits in constraint
@item @samp{0}, @samp{1}, @samp{2}, @dots{} @samp{9}
An operand that matches the specified operand number is allowed.  If a
digit is used together with letters within the same alternative, the
digit should come last.

This number is allowed to be more than a single digit.  If multiple
digits are encountered consecutively, they are interpreted as a single
decimal integer.  There is scant chance for ambiguity, since to-date
it has never been desirable that @samp{10} be interpreted as matching
either operand 1 @emph{or} operand 0.  Should this be desired, one
can use multiple alternatives instead.

@cindex matching constraint
@cindex constraint, matching
This is called a @dfn{matching constraint} and what it really means is
that the assembler has only a single operand that fills two roles
@ifset INTERNALS
considered separate in the RTL insn.  For example, an add insn has two
input operands and one output operand in the RTL, but on most CISC
@end ifset
@ifclear INTERNALS
which @code{asm} distinguishes.  For example, an add instruction uses
two input operands and an output operand, but on most CISC
@end ifclear
machines an add instruction really has only two operands, one of them an
input-output operand:

@smallexample
addl #35,r12
@end smallexample

Matching constraints are used in these circumstances.
More precisely, the two operands that match must include one input-only
operand and one output-only operand.  Moreover, the digit must be a
smaller number than the number of the operand that uses it in the
constraint.

@ifset INTERNALS
For operands to match in a particular case usually means that they
are identical-looking RTL expressions.  But in a few special cases
specific kinds of dissimilarity are allowed.  For example, @code{*x}
as an input operand will match @code{*x++} as an output operand.
For proper results in such cases, the output template should always
use the output-operand's number when printing the operand.
@end ifset

@cindex load address instruction
@cindex push address instruction
@cindex address constraints
@cindex @samp{p} in constraint
@item @samp{p}
An operand that is a valid memory address is allowed.  This is
for ``load address'' and ``push address'' instructions.

@findex address_operand
@samp{p} in the constraint must be accompanied by @code{address_operand}
as the predicate in the @code{match_operand}.  This predicate interprets
the mode specified in the @code{match_operand} as the mode of the memory
reference for which the address would be valid.

@cindex other register constraints
@cindex extensible constraints