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:
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).
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).
const_vector) 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.
This number is allowed to be more than a single digit. If multiple digits are encountered consecutively, they are interpreted as a single decimal integer. There is scant chance for ambiguity, since to-date it has never been desirable that `10' be interpreted as matching either operand 1 or operand 0. Should this be desired, one can use multiple alternatives instead.
This is called a matching constraint and what it really means is
that the assembler has only a single operand that fills two roles
asm distinguishes. For example, an add instruction uses
two input operands and an output operand, but on most CISC
machines an add instruction really has only two operands, one of them an
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.
`p' in the constraint must be accompanied by
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.