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This file contains invisible Unicode characters
This file contains invisible Unicode characters that are indistinguishable to humans but may be processed differently by a computer. If you think that this is intentional, you can safely ignore this warning. Use the Escape button to reveal them.
This is Info file gcc.info, produced by Makeinfo-1.47 from the input
file gcc.texi.
This file documents the use and the internals of the GNU compiler.
Copyright (C) 1988, 1989, 1992 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 sections entitled "GNU General Public License" and "Boycott"
are 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 sections entitled "GNU General Public
License" and "Boycott", and this permission notice, may be included in
translations approved by the Free Software Foundation instead of in the
original English.

File: gcc.info, Node: RTL Template, Next: Output Template, Prev: Example, Up: Machine Desc
RTL Template for Generating and Recognizing Insns
=================================================
The RTL template is used to define which insns match the particular
pattern and how to find their operands. For named patterns, the RTL
template also says how to construct an insn from specified operands.
Construction involves substituting specified operands into a copy of
the template. Matching involves determining the values that serve as
the operands in the insn being matched. Both of these activities are
controlled by special expression types that direct matching and
substitution of the operands.
`(match_operand:M N PREDICATE CONSTRAINT)'
This expression is a placeholder for operand number N of the insn.
When constructing an insn, operand number N will be substituted
at this point. When matching an insn, whatever appears at this
position in the insn will be taken as operand number N; but it
must satisfy PREDICATE or this instruction pattern will not match
at all.
Operand numbers must be chosen consecutively counting from zero in
each instruction pattern. There may be only one `match_operand'
expression in the pattern for each operand number. Usually
operands are numbered in the order of appearance in `match_operand'
expressions.
PREDICATE is a string that is the name of a C function that
accepts two arguments, an expression and a machine mode. During
matching, the function will be called with the putative operand as
the expression and M as the mode argument (if M is not specified,
`VOIDmode' will be used, which normally causes PREDICATE to accept
any mode). If it returns zero, this instruction pattern fails to
match. PREDICATE may be an empty string; then it means no test is
to be done on the operand, so anything which occurs in this
position is valid.
Most of the time, PREDICATE will reject modes other than M--but
not always. For example, the predicate `address_operand' uses M
as the mode of memory ref that the address should be valid for.
Many predicates accept `const_int' nodes even though their mode is
`VOIDmode'.
CONSTRAINT controls reloading and the choice of the best register
class to use for a value, as explained later (*note
Constraints::.).
People are often unclear on the difference between the constraint
and the predicate. The predicate helps decide whether a given
insn matches the pattern. The constraint plays no role in this
decision; instead, it controls various decisions in the case of an
insn which does match.
On CISC machines, PREDICATE is most often `"general_operand"'.
This function checks that the putative operand is either a
constant, a register or a memory reference, and that it is valid
for mode M.
For an operand that must be a register, PREDICATE should be
`"register_operand"'. It would be valid to use
`"general_operand"', since the reload pass would copy any
non-register operands through registers, but this would make GNU
CC do extra work, it would prevent invariant operands (such as
constant) from being removed from loops, and it would prevent the
register allocator from doing the best possible job. On RISC
machines, it is usually most efficient to allow PREDICATE to
accept only objects that the constraints allow.
For an operand that must be a constant, either use
`"immediate_operand"' for PREDICATE, or make the instruction
pattern's extra condition require a constant, or both. You cannot
expect the constraints to do this work! If the constraints allow
only constants, but the predicate allows something else, the
compiler will crash when that case arises.
`(match_scratch:M N CONSTRAINT)'
This expression is also a placeholder for operand number N and
indicates that operand must be a `scratch' or `reg' expression.
When matching patterns, this is completely equivalent to
(match_operand:M N "scratch_operand" PRED)
but, when generating RTL, it produces a (`scratch':M) expression.
If the last few expressions in a `parallel' are `clobber'
expressions whose operands are either a hard register or
`match_scratch', the combiner can add them when necessary. *Note
Side Effects::.
`(match_dup N)'
This expression is also a placeholder for operand number N. It is
used when the operand needs to appear more than once in the insn.
In construction, `match_dup' behaves exactly like `match_operand':
the operand is substituted into the insn being constructed. But
in matching, `match_dup' behaves differently. It assumes that
operand number N has already been determined by a `match_operand'
appearing earlier in the recognition template, and it matches only
an identical-looking expression.
`(match_operator:M N PREDICATE [OPERANDS...])'
This pattern is a kind of placeholder for a variable RTL expression
code.
When constructing an insn, it stands for an RTL expression whose
expression code is taken from that of operand N, and whose
operands are constructed from the patterns OPERANDS.
When matching an expression, it matches an expression if the
function PREDICATE returns nonzero on that expression *and* the
patterns OPERANDS match the operands of the expression.
Suppose that the function `commutative_operator' is defined as
follows, to match any expression whose operator is one of the
commutative arithmetic operators of RTL and whose mode is MODE:
int
commutative_operator (x, mode)
rtx x;
enum machine_mode mode;
{
enum rtx_code code = GET_CODE (x);
if (GET_MODE (x) != mode)
return 0;
return GET_RTX_CLASS (code) == 'c' || code == EQ || code == NE;
}
Then the following pattern will match any RTL expression consisting
of a commutative operator applied to two general operands:
(match_operator:SI 3 "commutative_operator"
[(match_operand:SI 1 "general_operand" "g")
(match_operand:SI 2 "general_operand" "g")])
Here the vector `[OPERANDS...]' contains two patterns because the
expressions to be matched all contain two operands.
When this pattern does match, the two operands of the commutative
operator are recorded as operands 1 and 2 of the insn. (This is
done by the two instances of `match_operand'.) Operand 3 of the
insn will be the entire commutative expression: use `GET_CODE
(operands[3])' to see which commutative operator was used.
The machine mode M of `match_operator' works like that of
`match_operand': it is passed as the second argument to the
predicate function, and that function is solely responsible for
deciding whether the expression to be matched "has" that mode.
When constructing an insn, argument 3 of the gen-function will
specify the operation (i.e. the expression code) for the
expression to be made. It should be an RTL expression, whose
expression code is copied into a new expression whose operands are
arguments 1 and 2 of the gen-function. The subexpressions of
argument 3 are not used; only its expression code matters.
When `match_operator' is used in a pattern for matching an insn,
it usually best if the operand number of the `match_operator' is
higher than that of the actual operands of the insn. This improves
register allocation because the register allocator often looks at
operands 1 and 2 of insns to see if it can do register tying.
There is no way to specify constraints in `match_operator'. The
operand of the insn which corresponds to the `match_operator'
never has any constraints because it is never reloaded as a whole.
However, if parts of its OPERANDS are matched by `match_operand'
patterns, those parts may have constraints of their own.
`(match_parallel N PREDICATE [SUBPAT...])'
This pattern is a placeholder for an insn that consists of a
`parallel' expression with a variable number of elements. This
expression should only appear at the top level of an insn pattern.
When constructing an insn, operand number N will be substituted at
this point. When matching an insn, it matches if the body of the
insn is a `parallel' expression with at least as many elements as
the vector of SUBPAT expressions in the `match_parallel', if each
SUBPAT matches the corresponding element of the `parallel', *and*
the function PREDICATE returns nonzero on the `parallel' that is
the body of the insn. It is the responsibility of the predicate
to validate elements of the `parallel' beyond those listed in the
`match_parallel'.
A typical use of `match_parallel' is to match load and store
multiple expressions, which can contains a variable number of
elements in a `parallel'. For example,
(define_insn ""
[(match_parallel 0 "load_multiple_operation"
[(set (match_operand:SI 1 "gen_reg_operand" "=r")
(match_operand:SI 2 "memory_operand" "m"))
(use (reg:SI 179))
(clobber (reg:SI 179))])]
""
"loadm 0,0,%1,%2")
This example comes from `a29k.md'. The function
`load_multiple_operations' is defined in `a29k.c' and checks that
subsequent elements in the `parallel' are the same as the `set' in
the pattern, except that they are referencing subsequent registers
and memory locations.
An insn that matches this pattern might look like:
(parallel [(set (reg:SI 20) (mem:SI (reg:SI 100)))
(use (reg:SI 179))
(clobber (reg:SI 179))
(set (reg:SI 21) (mem:SI (plus:SI (reg:SI 100) (const_int 4))))
(set (reg:SI 22) (mem:SI (plus:SI (reg:SI 100) (const_int 8))))])
`(address (match_operand:M N "address_operand" ""))'
This complex of expressions is a placeholder for an operand number
N in a "load address" instruction: an operand which specifies a
memory location in the usual way, but for which the actual operand
value used is the address of the location, not the contents of the
location.
`address' expressions never appear in RTL code, only in machine
descriptions. And they are used only in machine descriptions that
do not use the operand constraint feature. When operand
constraints are in use, the letter `p' in the constraint serves
this purpose.
M is the machine mode of the *memory location being addressed*,
not the machine mode of the address itself. That mode is always
the same on a given target machine (it is `Pmode', which normally
is `SImode'), so there is no point in mentioning it; thus, no
machine mode is written in the `address' expression. If some day
support is added for machines in which addresses of different
kinds of objects appear differently or are used differently (such
as the PDP-10), different formats would perhaps need different
machine modes and these modes might be written in the `address'
expression.

File: gcc.info, Node: Output Template, Next: Output Statement, Prev: RTL Template, Up: Machine Desc
Output Templates and Operand Substitution
=========================================
The "output template" is a string which specifies how to output the
assembler code for an instruction pattern. Most of the template is a
fixed string which is output literally. The character `%' is used to
specify where to substitute an operand; it can also be used to identify
places where different variants of the assembler require different
syntax.
In the simplest case, a `%' followed by a digit N says to output
operand N at that point in the string.
`%' followed by a letter and a digit says to output an operand in an
alternate fashion. Four letters have standard, built-in meanings
described below. The machine description macro `PRINT_OPERAND' can
define additional letters with nonstandard meanings.
`%cDIGIT' can be used to substitute an operand that is a constant
value without the syntax that normally indicates an immediate operand.
`%nDIGIT' is like `%cDIGIT' except that the value of the constant is
negated before printing.
`%aDIGIT' can be used to substitute an operand as if it were a
memory reference, with the actual operand treated as the address. This
may be useful when outputting a "load address" instruction, because
often the assembler syntax for such an instruction requires you to
write the operand as if it were a memory reference.
`%lDIGIT' is used to substitute a `label_ref' into a jump
instruction.
`%' followed by a punctuation character specifies a substitution that
does not use an operand. Only one case is standard: `%%' outputs a `%'
into the assembler code. Other nonstandard cases can be defined in the
`PRINT_OPERAND' macro. You must also define which punctuation
characters are valid with the `PRINT_OPERAND_PUNCT_VALID_P' macro.
The template may generate multiple assembler instructions. Write
the text for the instructions, with `\;' between them.
When the RTL contains two operands which are required by constraint
to match each other, the output template must refer only to the
lower-numbered operand. Matching operands are not always identical, and
the rest of the compiler arranges to put the proper RTL expression for
printing into the lower-numbered operand.
One use of nonstandard letters or punctuation following `%' is to
distinguish between different assembler languages for the same machine;
for example, Motorola syntax versus MIT syntax for the 68000. Motorola
syntax requires periods in most opcode names, while MIT syntax does
not. For example, the opcode `movel' in MIT syntax is `move.l' in
Motorola syntax. The same file of patterns is used for both kinds of
output syntax, but the character sequence `%.' is used in each place
where Motorola syntax wants a period. The `PRINT_OPERAND' macro for
Motorola syntax defines the sequence to output a period; the macro for
MIT syntax defines it to do nothing.

File: gcc.info, Node: Output Statement, Next: Constraints, Prev: Output Template, Up: Machine Desc
C Statements for Generating Assembler Output
============================================
Often a single fixed template string cannot produce correct and
efficient assembler code for all the cases that are recognized by a
single instruction pattern. For example, the opcodes may depend on the
kinds of operands; or some unfortunate combinations of operands may
require extra machine instructions.
If the output control string starts with a `@', then it is actually
a series of templates, each on a separate line. (Blank lines and
leading spaces and tabs are ignored.) The templates correspond to the
pattern's constraint alternatives (*note Multi-Alternative::.). For
example, if a target machine has a two-address add instruction `addr'
to add into a register and another `addm' to add a register to memory,
you might write this pattern:
(define_insn "addsi3"
[(set (match_operand:SI 0 "general_operand" "=r,m")
(plus:SI (match_operand:SI 1 "general_operand" "0,0")
(match_operand:SI 2 "general_operand" "g,r")))]
""
"@
addr %1,%0
addm %1,%0")
If the output control string starts with a `*', then it is not an
output template but rather a piece of C program that should compute a
template. It should execute a `return' statement to return the
template-string you want. Most such templates use C string literals,
which require doublequote characters to delimit them. To include these
doublequote characters in the string, prefix each one with `\'.
The operands may be found in the array `operands', whose C data type
is `rtx []'.
It is very common to select different ways of generating assembler
code based on whether an immediate operand is within a certain range.
Be careful when doing this, because the result of `INTVAL' is an
integer on the host machine. If the host machine has more bits in an
`int' than the target machine has in the mode in which the constant
will be used, then some of the bits you get from `INTVAL' will be
superfluous. For proper results, you must carefully disregard the
values of those bits.
It is possible to output an assembler instruction and then go on to
output or compute more of them, using the subroutine `output_asm_insn'.
This receives two arguments: a template-string and a vector of
operands. The vector may be `operands', or it may be another array of
`rtx' that you declare locally and initialize yourself.
When an insn pattern has multiple alternatives in its constraints,
often the appearance of the assembler code is determined mostly by
which alternative was matched. When this is so, the C code can test
the variable `which_alternative', which is the ordinal number of the
alternative that was actually satisfied (0 for the first, 1 for the
second alternative, etc.).
For example, suppose there are two opcodes for storing zero, `clrreg'
for registers and `clrmem' for memory locations. Here is how a pattern
could use `which_alternative' to choose between them:
(define_insn ""
[(set (match_operand:SI 0 "general_operand" "=r,m")
(const_int 0))]
""
"*
return (which_alternative == 0
? \"clrreg %0\" : \"clrmem %0\");
")
The example above, where the assembler code to generate was *solely*
determined by the alternative, could also have been specified as
follows, having the output control string start with a `@':
(define_insn ""
[(set (match_operand:SI 0 "general_operand" "=r,m")
(const_int 0))]
""
"@
clrreg %0
clrmem %0")

File: gcc.info, Node: Constraints, Next: Standard Names, Prev: Output Statement, Up: Machine Desc
Operand Constraints
===================
Each `match_operand' in an instruction pattern can specify a
constraint for the type of operands allowed. 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.
* 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.
* No Constraints:: Describing a clean machine without constraints.

File: gcc.info, Node: Simple Constraints, Next: Multi-Alternative, Prev: Constraints, Up: Constraints
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:
`m'
A memory operand is allowed, with any kind of address that the
machine supports in general.
`o'
A memory operand is allowed, but only if the address is
"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.
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 `o' is valid only when accompanied
by both `<' (if the target machine has predecrement addressing)
and `>' (if the target machine has preincrement addressing).
`V'
A memory operand that is not offsettable. In other words,
anything that would fit the `m' constraint but not the `o'
constraint.
`<'
A memory operand with autodecrement addressing (either
predecrement or postdecrement) is allowed.
`>'
A memory operand with autoincrement addressing (either
preincrement or postincrement) is allowed.
`r'
A register operand is allowed provided that it is in a general
register.
`d', `a', `f', ...
Other letters can be defined in machine-dependent fashion to stand
for particular classes of registers. `d', `a' and `f' are defined
on the 68000/68020 to stand for data, address and floating point
registers.
`i'
An immediate integer operand (one with constant value) is allowed.
This includes symbolic constants whose values will be known only at
assembly time.
`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
`n' rather than `i'.
`I', `J', `K', ... `P'
Other letters in the range `I' through `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, `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.
`E'
An immediate floating operand (expression 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).
`F'
An immediate floating operand (expression code `const_double') is
allowed.
`G', `H'
`G' and `H' may be defined in a machine-dependent fashion to
permit immediate floating operands in particular ranges of values.
`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 `s' instead of `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
-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 `moveq' instruction. We arrange
for this to happen by defining the letter `K' to mean "any integer
outside the range -128 to 127", and then specifying `Ks' in the
operand constraints.
`g'
Any register, memory or immediate integer operand is allowed,
except for registers that are not general registers.
`X'
Any operand whatsoever is allowed, even if it does not satisfy
`general_operand'. This is normally used in the constraint of a
`match_scratch' when certain alternatives will not actually
require a scratch register.
`0', `1', `2', ... `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 is called a "matching constraint" and what it really means is
that the assembler has only a single operand that fills two roles
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 machines an add instruction really has only two operands, one
of them an input-output operand:
addl #35,r12
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.
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, `*x' as
an input operand will match `*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.
`p'
An operand that is a valid memory address is allowed. This is for
"load address" and "push address" instructions.
`p' in the constraint must be accompanied by `address_operand' as
the predicate in the `match_operand'. This predicate interprets
the mode specified in the `match_operand' as the mode of the memory
reference for which the address would be valid.
`Q', `R', `S', ... `U'
Letters in the range `Q' through `U' may be defined in a
machine-dependent fashion to stand for arbitrary operand types.
The machine description macro `EXTRA_CONSTRAINT' is passed the
operand as its first argument and the constraint letter as its
second operand.
A typical use for this would be to distinguish certain types of
memory references that affect other insn operands.
Do not define these constraint letters to accept register
references (`reg'); the reload pass does not expect this and would
not handle it properly.
In order to have valid assembler code, each operand must satisfy its
constraint. But a failure to do so does not prevent the pattern from
applying to an insn. Instead, it directs the compiler to modify the
code so that the constraint will be satisfied. Usually this is done by
copying an operand into a register.
Contrast, therefore, the two instruction patterns that follow:
(define_insn ""
[(set (match_operand:SI 0 "general_operand" "=r")
(plus:SI (match_dup 0)
(match_operand:SI 1 "general_operand" "r")))]
""
"...")
which has two operands, one of which must appear in two places, and
(define_insn ""
[(set (match_operand:SI 0 "general_operand" "=r")
(plus:SI (match_operand:SI 1 "general_operand" "0")
(match_operand:SI 2 "general_operand" "r")))]
""
"...")
which has three operands, two of which are required by a constraint to
be identical. If we are considering an insn of the form
(insn N PREV NEXT
(set (reg:SI 3)
(plus:SI (reg:SI 6) (reg:SI 109)))
...)
the first pattern would not apply at all, because this insn does not
contain two identical subexpressions in the right place. The pattern
would say, "That does not look like an add instruction; try other
patterns." The second pattern would say, "Yes, that's an add
instruction, but there is something wrong with it." It would direct
the reload pass of the compiler to generate additional insns to make
the constraint true. The results might look like this:
(insn N2 PREV N
(set (reg:SI 3) (reg:SI 6))
...)
(insn N N2 NEXT
(set (reg:SI 3)
(plus:SI (reg:SI 3) (reg:SI 109)))
...)
It is up to you to make sure that each operand, in each pattern, has
constraints that can handle any RTL expression that could be present for
that operand. (When multiple alternatives are in use, each pattern
must, for each possible combination of operand expressions, have at
least one alternative which can handle that combination of operands.)
The constraints don't need to *allow* any possible operand--when this is
the case, they do not constrain--but they must at least point the way to
reloading any possible operand so that it will fit.
* If the constraint accepts whatever operands the predicate permits,
there is no problem: reloading is never necessary for this operand.
For example, an operand whose constraints permit everything except
registers is safe provided its predicate rejects registers.
An operand whose predicate accepts only constant values is safe
provided its constraints include the letter `i'. If any possible
constant value is accepted, then nothing less than `i' will do; if
the predicate is more selective, then the constraints may also be
more selective.
* Any operand expression can be reloaded by copying it into a
register. So if an operand's constraints allow some kind of
register, it is certain to be safe. It need not permit all
classes of registers; the compiler knows how to copy a register
into another register of the proper class in order to make an
instruction valid.
* A nonoffsettable memory reference can be reloaded by copying the
address into a register. So if the constraint uses the letter
`o', all memory references are taken care of.
* A constant operand can be reloaded by allocating space in memory to
hold it as preinitialized data. Then the memory reference can be
used in place of the constant. So if the constraint uses the
letters `o' or `m', constant operands are not a problem.
* If the constraint permits a constant and a pseudo register used in
an insn was not allocated to a hard register and is equivalent to
a constant, the register will be replaced with the constant. If
the predicate does not permit a constant and the insn is
re-recognized for some reason, the compiler will crash. Thus the
predicate must always recognize any objects allowed by the
constraint.
If the operand's predicate can recognize registers, but the
constraint does not permit them, it can make the compiler crash. When
this operand happens to be a register, the reload pass will be stymied,
because it does not know how to copy a register temporarily into memory.

File: gcc.info, Node: Multi-Alternative, Next: Class Preferences, Prev: Simple Constraints, Up: Constraints
Multiple Alternative Constraints
--------------------------------
Sometimes a single instruction has multiple alternative sets of
possible operands. For example, on the 68000, a logical-or instruction
can combine register or an immediate value into memory, or it can
combine any kind of operand into a register; but it cannot combine one
memory location into another.
These constraints are represented as multiple alternatives. An
alternative can be described by a series of letters for each operand.
The overall constraint for an operand is made from the letters for this
operand from the first alternative, a comma, the letters for this
operand from the second alternative, a comma, and so on until the last
alternative. Here is how it is done for fullword logical-or on the
68000:
(define_insn "iorsi3"
[(set (match_operand:SI 0 "general_operand" "=m,d")
(ior:SI (match_operand:SI 1 "general_operand" "%0,0")
(match_operand:SI 2 "general_operand" "dKs,dmKs")))]
...)
The first alternative has `m' (memory) for operand 0, `0' for
operand 1 (meaning it must match operand 0), and `dKs' for operand 2.
The second alternative has `d' (data register) for operand 0, `0' for
operand 1, and `dmKs' for operand 2. The `=' and `%' in the
constraints apply to all the alternatives; their meaning is explained
in the next section (*note Class Preferences::.).
If all the operands fit any one alternative, the instruction is
valid. Otherwise, for each alternative, the compiler counts how many
instructions must be added to copy the operands so that that
alternative applies. The alternative requiring the least copying is
chosen. If two alternatives need the same amount of copying, the one
that comes first is chosen. These choices can be altered with the `?'
and `!' characters:
`?'
Disparage slightly the alternative that the `?' appears in, as a
choice when no alternative applies exactly. The compiler regards
this alternative as one unit more costly for each `?' that appears
in it.
`!'
Disparage severely the alternative that the `!' appears in. This
alternative can still be used if it fits without reloading, but if
reloading is needed, some other alternative will be used.
When an insn pattern has multiple alternatives in its constraints,
often the appearance of the assembler code is determined mostly by which
alternative was matched. When this is so, the C code for writing the
assembler code can use the variable `which_alternative', which is the
ordinal number of the alternative that was actually satisfied (0 for
the first, 1 for the second alternative, etc.). *Note Output
Statement::.

File: gcc.info, Node: Class Preferences, Next: Modifiers, Prev: Multi-Alternative, Up: Constraints
Register Class Preferences
--------------------------
The operand constraints have another function: they enable the
compiler to decide which kind of hardware register a pseudo register is
best allocated to. The compiler examines the constraints that apply to
the insns that use the pseudo register, looking for the
machine-dependent letters such as `d' and `a' that specify classes of
registers. The pseudo register is put in whichever class gets the most
"votes". The constraint letters `g' and `r' also vote: they vote in
favor of a general register. The machine description says which
registers are considered general.
Of course, on some machines all registers are equivalent, and no
register classes are defined. Then none of this complexity is relevant.

File: gcc.info, Node: Modifiers, Next: No Constraints, Prev: Class Preferences, Up: Constraints
Constraint Modifier Characters
------------------------------
`='
Means that this operand is write-only for this instruction: the
previous value is discarded and replaced by output data.
`+'
Means that this operand is both read and written by the
instruction.
When the compiler fixes up the operands to satisfy the constraints,
it needs to know which operands are inputs to the instruction and
which are outputs from it. `=' identifies an output; `+'
identifies an operand that is both input and output; all other
operands are assumed to be input only.
`&'
Means (in a particular alternative) that this operand is written
before the instruction is finished using the input operands.
Therefore, this operand may not lie in a register that is used as
an input operand or as part of any memory address.
`&' applies only to the alternative in which it is written. In
constraints with multiple alternatives, sometimes one alternative
requires `&' while others do not. See, for example, the `movdf'
insn of the 68000.
`&' does not obviate the need to write `='.
`%'
Declares the instruction to be commutative for this operand and the
following operand. This means that the compiler may interchange
the two operands if that is the cheapest way to make all operands
fit the constraints. This is often used in patterns for addition
instructions that really have only two operands: the result must
go in one of the arguments. Here for example, is how the 68000
halfword-add instruction is defined:
(define_insn "addhi3"
[(set (match_operand:HI 0 "general_operand" "=m,r")
(plus:HI (match_operand:HI 1 "general_operand" "%0,0")
(match_operand:HI 2 "general_operand" "di,g")))]
...)
`#'
Says that all following characters, up to the next comma, are to be
ignored as a constraint. They are significant only for choosing
register preferences.
`*'
Says that the following character should be ignored when choosing
register preferences. `*' has no effect on the meaning of the
constraint as a constraint, and no effect on reloading.
Here is an example: the 68000 has an instruction to sign-extend a
halfword in a data register, and can also sign-extend a value by
copying it into an address register. While either kind of
register is acceptable, the constraints on an address-register
destination are less strict, so it is best if register allocation
makes an address register its goal. Therefore, `*' is used so
that the `d' constraint letter (for data register) is ignored when
computing register preferences.
(define_insn "extendhisi2"
[(set (match_operand:SI 0 "general_operand" "=*d,a")
(sign_extend:SI
(match_operand:HI 1 "general_operand" "0,g")))]
...)

File: gcc.info, Node: No Constraints, Prev: Modifiers, Up: Constraints
Not Using Constraints
---------------------
Some machines are so clean that operand constraints are not
required. For example, on the Vax, an operand valid in one context is
valid in any other context. On such a machine, every operand
constraint would be `g', excepting only operands of "load address"
instructions which are written as if they referred to a memory
location's contents but actual refer to its address. They would have
constraint `p'.
For such machines, instead of writing `g' and `p' for all the
constraints, you can choose to write a description with empty
constraints. Then you write `""' for the constraint in every
`match_operand'. Address operands are identified by writing an
`address' expression around the `match_operand', not by their
constraints.
When the machine description has just empty constraints, certain
parts of compilation are skipped, making the compiler faster. However,
few machines actually do not need constraints; all machine descriptions
now in existence use constraints.

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