Old Constraints - GNU Compiler Collection (GCC) Internals
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### 17.9 Obsolete Macros for Defining Constraints

Machine-specific constraints can be defined with these macros instead of the machine description constructs described in Define Constraints. This mechanism is obsolete. New ports should not use it; old ports should convert to the new mechanism.

— Macro: CONSTRAINT_LEN (char, str)

> For the constraint at the start of str, which starts with the letter c, return the length. This allows you to have register class / constant / extra constraints that are longer than a single letter; you don't need to define this macro if you can do with single-letter constraints only. The definition of this macro should use DEFAULT_CONSTRAINT_LEN for all the characters that you don't want to handle specially. There are some sanity checks in genoutput.c that check the constraint lengths for the md file, so you can also use this macro to help you while you are transitioning from a byzantine single-letter-constraint scheme: when you return a negative length for a constraint you want to re-use, genoutput will complain about every instance where it is used in the md file.

— Macro: REG_CLASS_FROM_LETTER (char)

> A C expression which defines the machine-dependent operand constraint letters for register classes. If char is such a letter, the value should be the register class corresponding to it. Otherwise, the value should be `NO_REGS`. The register letter ‘`r`’, corresponding to class `GENERAL_REGS`, will not be passed to this macro; you do not need to handle it.

— Macro: REG_CLASS_FROM_CONSTRAINT (char, str)

> Like `REG_CLASS_FROM_LETTER`, but you also get the constraint string passed in str, so that you can use suffixes to distinguish between different variants.

— Macro: CONST_OK_FOR_LETTER_P (value, c)

> A C expression that defines the machine-dependent operand constraint letters (‘`I`’, ‘`J`’, ‘`K`’, ... ‘`P`’) that specify particular ranges of integer values. If c is one of those letters, the expression should check that value, an integer, is in the appropriate range and return 1 if so, 0 otherwise. If c is not one of those letters, the value should be 0 regardless of value.

— Macro: CONST_OK_FOR_CONSTRAINT_P (value, c, str)

> Like `CONST_OK_FOR_LETTER_P`, but you also get the constraint string passed in str, so that you can use suffixes to distinguish between different variants.

— Macro: CONST_DOUBLE_OK_FOR_LETTER_P (value, c)

> A C expression that defines the machine-dependent operand constraint letters that specify particular ranges of `const_double` values (‘`G`’ or ‘`H`’).

> If c is one of those letters, the expression should check that value, an RTX of code `const_double`, is in the appropriate range and return 1 if so, 0 otherwise. If c is not one of those letters, the value should be 0 regardless of value.

> `const_double` is used for all floating-point constants and for `DImode` fixed-point constants. A given letter can accept either or both kinds of values. It can use `GET_MODE` to distinguish between these kinds.

— Macro: CONST_DOUBLE_OK_FOR_CONSTRAINT_P (value, c, str)

> Like `CONST_DOUBLE_OK_FOR_LETTER_P`, but you also get the constraint string passed in str, so that you can use suffixes to distinguish between different variants.

— Macro: EXTRA_CONSTRAINT (value, c)

> A C expression that defines the optional machine-dependent constraint letters that can be used to segregate specific types of operands, usually memory references, for the target machine. Any letter that is not elsewhere defined and not matched by `REG_CLASS_FROM_LETTER` / `REG_CLASS_FROM_CONSTRAINT` may be used. Normally this macro will not be defined.

> If it is required for a particular target machine, it should return 1 if value corresponds to the operand type represented by the constraint letter c. If c is not defined as an extra constraint, the value returned should be 0 regardless of value.

> For example, on the ROMP, load instructions cannot have their output in r0 if the memory reference contains a symbolic address. Constraint letter ‘`Q`’ is defined as representing a memory address that does not contain a symbolic address. An alternative is specified with a ‘`Q`’ constraint on the input and ‘`r`’ on the output. The next alternative specifies ‘`m`’ on the input and a register class that does not include r0 on the output.

— Macro: EXTRA_CONSTRAINT_STR (value, c, str)

> Like `EXTRA_CONSTRAINT`, but you also get the constraint string passed in str, so that you can use suffixes to distinguish between different variants.

— Macro: EXTRA_MEMORY_CONSTRAINT (c, str)

> A C expression that defines the optional machine-dependent constraint letters, amongst those accepted by `EXTRA_CONSTRAINT`, that should be treated like memory constraints by the reload pass.

> It should return 1 if the operand type represented by the constraint at the start of str, the first letter of which is the letter c, comprises a subset of all memory references including all those whose address is simply a base register. This allows the reload pass to reload an operand, if it does not directly correspond to the operand type of c, by copying its address into a base register.

> 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 via `EXTRA_CONSTRAINT` as representing a memory address of this type. If the letter ‘`Q`’ is marked as `EXTRA_MEMORY_CONSTRAINT`, a ‘`Q`’ constraint can handle any memory operand, because the reload pass knows it can be reloaded by copying the memory address into a base register if required. This is analogous to the way an ‘`o`’ constraint can handle any memory operand.

— Macro: EXTRA_ADDRESS_CONSTRAINT (c, str)

> A C expression that defines the optional machine-dependent constraint letters, amongst those accepted by `EXTRA_CONSTRAINT` / `EXTRA_CONSTRAINT_STR`, that should be treated like address constraints by the reload pass.

> It should return 1 if the operand type represented by the constraint at the start of str, which starts with the letter c, comprises a subset of all memory addresses including all those that consist of just a base register. This allows the reload pass to reload an operand, if it does not directly correspond to the operand type of str, by copying it into a base register.

> Any constraint marked as `EXTRA_ADDRESS_CONSTRAINT` can only be used with the `address_operand` predicate. It is treated analogously to the ‘`p`’ constraint.