| Simple Constraints

| Operand Constraints | Next

The simplest kind of constraint is a string full of letters, each of which describes one kind of operand that is permitted. These 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 is allowed.
`>`
  A memory operand with autoincrement addressing is allowed.
`r`
  A register operand is allowed provided that it is in a general register.
`d`
  A data register is allowed. This is a Motorola-specific constraint.
`a`
  An address register is allowed. This is a Motorola-specific constraint.
`f`
  A 68881 floating-point register is allowed, if available (of course, it is not available on the TI-89 and TI-92 Plus). This is a Motorola-specific constraint.
`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`
  An integer in the range 1 to 8 is allowed. This is a Motorola-specific constraint, and this is for example the range permitted as a shift count in the shift instructions.
`J`
  A 16 bit signed number is allowed. This is a Motorola-specific constraint.
`K`
  A signed number whose magnitude is greater than 0x80 is allowed. This is a Motorola-specific constraint.
`L`
  An integer in the range -8 to -1 is allowed. This is a Motorola-specific constraint.
`M`
  A signed number whose magnitude is greater than 0x100 is allowed. This is a Motorola-specific constraint.
`s`
  An immediate integer operand whose value is not an explicit integer is allowed.
  This might appear strange; if an instruction 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. The GNU team arranges 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. (This constraint is very unlikely to be useful for inline assembly, since you cannot use constraints to make GCC choose between 2 different inline assembly statements.)
`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. 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.
`p`
  An operand that is a valid memory address is allowed.

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 instruction pattern. 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.