Can I modify input operands in gcc inline assembly

Question

We know, if I set %eax as the input, then I cannot include it into list of clobbered registers. So my question is that, is that legal to modify the value of %eax in the assembly code without any declaration in clobbered list?

__asm__ __volatile__("inc %0" :: "a"(num) : "%eax"); // illegal
__asm__ __volatile__("inc %0" :: "a"(num));       // Can I modify %eax?

Answer 1

No, if the assembly code changes an input register, it must also be listed as an output register, or as an in-out register, by using "+" instead of "=" in the constraint.

Examples:

__asm__ __volatile__("..." : "+r"(num));
__asm__ __volatile__("..." : "=a"(dummy) : "a"(num));
__asm__ __volatile__("..." : "=r"(dummy) : "0"(num));

The first example specifies num as both an input and an output. This will overwrite the prior value of num , which may be undesirable if the operand is clobbered rather being set to a useful value.

The second example explicitly uses eax for both input and output. It directs the output to a dummy variable so as to not corrupt the actual input variable num .

The third example uses a dummy output, to avoid modifying num , and also avoids specifying eax explicitly, allowing the compiler to choose which register to use. The "0" constraint tells the compiler to use the same register as operand 0 for this input operand.