16.8.7 Defining Machine-Specific Constraints
Machine-specific constraints fall into two categories: register and
non-register constraints. Within the latter category, constraints
which allow subsets of all possible memory or address operands should
be specially marked, to give
reload more information.
Machine-specific constraints can be given names of arbitrary length, but they must be entirely composed of letters, digits, underscores (_), and angle brackets (< >). Like C identifiers, they must begin with a letter or underscore.
In order to avoid ambiguity in operand constraint strings, no
constraint can have a name that begins with any other constraint's
name. For example, if
x is defined as a constraint name,
xy may not be, and vice versa. As a consequence of this rule,
no constraint may begin with one of the generic constraint letters:
E F V X g i m n o p r s.
Register constraints correspond directly to register classes. See Register Classes. There is thus not much flexibility in their definitions.
All three arguments are string constants. name is the name of the constraint, as it will appear in
match_operandexpressions. If name is a multi-letter constraint its length shall be the same for all constraints starting with the same letter. regclass can be either the name of the corresponding register class (see Register Classes), or a C expression which evaluates to the appropriate register class. If it is an expression, it must have no side effects, and it cannot look at the operand. The usual use of expressions is to map some register constraints to
NO_REGSwhen the register class is not available on a given subarchitecture.
docstring is a sentence documenting the meaning of the constraint. Docstrings are explained further below.
Non-register constraints are more like predicates: the constraint definition gives a Boolean expression which indicates whether the constraint matches.
The name and docstring arguments are the same as for
define_register_constraint, but note that the docstring comes immediately after the name for these expressions. exp is an RTL expression, obeying the same rules as the RTL expressions in predicate definitions. See Defining Predicates, for details. If it evaluates true, the constraint matches; if it evaluates false, it doesn't. Constraint expressions should indicate which RTL codes they might match, just like predicate expressions.
match_testC expressions have access to the following variables:
- The RTL object defining the operand.
- The machine mode of op.
- INTVAL (op), if op is a
- CONST_DOUBLE_HIGH (op), if op is an integer
- CONST_DOUBLE_LOW (op), if op is an integer
- CONST_DOUBLE_REAL_VALUE (op), if op is a floating-point
The *val variables should only be used once another piece of the expression has verified that op is the appropriate kind of RTL object.
Most non-register constraints should be defined with
define_constraint. The remaining two definition expressions
are only appropriate for constraints that should be handled specially
reload if they fail to match.
Use this expression for constraints that match a subset of all memory operands: that is,
reloadcan make them match by converting the operand to the form (mem (reg X)), where X is a base register (from the register class specified by
BASE_REG_CLASS, see Register Classes).
For example, on the S/390, some instructions do not accept arbitrary memory references, but only those that do not make use of an index register. The constraint letter Q is defined to represent a memory address of this type. If Q is defined with
define_memory_constraint, a Q constraint can handle any memory operand, because
reloadknows it can simply copy the memory address into a base register if required. This is analogous to the way a o constraint can handle any memory operand.
The syntax and semantics are otherwise identical to
Use this expression for constraints that match a subset of all address operands: that is,
reloadcan make the constraint match by converting the operand to the form (reg X), again with X a base register.
Constraints defined with
define_address_constraintcan only be used with the
address_operandpredicate, or machine-specific predicates that work the same way. They are treated analogously to the generic p constraint.
The syntax and semantics are otherwise identical to
For historical reasons, names beginning with the letters G H
are reserved for constraints that match only
names beginning with the letters I J K L M N O P are reserved
for constraints that match only
const_ints. This may change in
the future. For the time being, constraints with these names must be
written in a stylized form, so that
genpreds can tell you did
(define_constraint "[GHIJKLMNOP]..." "doc..." (and (match_code "const_int") ;
const_doublefor G/H condition...)) ; usually a
It is fine to use names beginning with other letters for constraints
Each docstring in a constraint definition should be one or more complete
sentences, marked up in Texinfo format. They are currently unused.
In the future they will be copied into the GCC manual, in Machine Constraints, replacing the hand-maintained tables currently found in
that section. Also, in the future the compiler may use this to give
more helpful diagnostics when poor choice of
causes a reload failure.
If you put the pseudo-Texinfo directive @internal at the
beginning of a docstring, then (in the future) it will appear only in
the internals manual's version of the machine-specific constraint tables.
Use this for constraints that should not appear in