6.43.3.3. Constraint Modifier Characters

6.43.3.3 Constraint Modifier Characters

Here are constraint modifier characters.

=
Means that this operand is written to by this instruction: the previous value is discarded and replaced by new 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 read by the instruction and which are written by it. ‘=’ identifies an operand which is only written; ‘+’ identifies an operand that is both read and written; all other operands are assumed to only be read.

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,