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Constraint Modifier Characters

6.42.3 Constraint Modifier Characters

Here are constraint modifier characters.

‘=’
Means that this operand is write-only for this instruction: the previous value is discarded and replaced by output data.
‘+’
Means that this operand is both read and written by the instruction.

When the compiler fixes up the operands to satisfy the constraints, it needs to know which operands are inputs to the instruction and which are outputs from it. ‘=’ identifies an output; ‘+’ identifies an operand that is both input and output; all other operands are assumed to be input only.

If you specify ‘=’ or ‘+’ in a constraint, you put it in the first character of the constraint string.

‘&’
Means (in a particular alternative) that this operand is an earlyclobber operand, which is modified