Define Constraints - GNU Compiler Collection (GCC) Internals
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#### 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.

— MD Expression: define_register_constraint name regclass docstring

> All three arguments are string constants. name is the name of the constraint, as it will appear in `match_operand` expressions. 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_REGS` when 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.

— MD Expression: define_constraint name docstring exp

> 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_test` C expressions have access to the following variables:

> op
>   The RTL object defining the operand.
> mode
>   The machine mode of op.
> ival
>   ‘`INTVAL (op)`’, if op is a `const_int`.
> hval
>   ‘`CONST_DOUBLE_HIGH (op)`’, if op is an integer `const_double`.
> lval
>   ‘`CONST_DOUBLE_LOW (op)`’, if op is an integer `const_double`.
> rval
>   ‘`CONST_DOUBLE_REAL_VALUE (op)`’, if op is a floating-point `const_double`.

> 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 by `reload` if they fail to match.

— MD Expression: define_memory_constraint name docstring exp

> Use this expression for constraints that match a subset of all memory operands: that is, `reload` can 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 `reload` knows it can simply copy the memory address into a base register if required. This is analogous to the way an ‘`o`’ constraint can handle any memory operand.

> The syntax and semantics are otherwise identical to `define_constraint`.

— MD Expression: define_address_constraint name docstring exp

> Use this expression for constraints that match a subset of all address operands: that is, `reload` can make the constraint match by converting the operand to the form ‘`(reg X)`’, again with X a base register.

> Constraints defined with `define_address_constraint` can only be used with the `address_operand` predicate, 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 `define_constraint`.

For historical reasons, names beginning with the letters ‘`G H`’ are reserved for constraints that match only `const_double`s, and names beginning with the letters ‘`I J K L M N O P`’ are reserved for constraints that match only `const_int`s. 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 it correctly:

         (define_constraint "[GHIJKLMNOP]..."
           "doc..."
           (and (match_code "const_int")  ; const_double for G/H
                condition...))            ; usually a match_test

It is fine to use names beginning with other letters for constraints that match `const_double`s or `const_int`s.

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 `asm` constraints 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 `asm` statements.