GCC inline assembler, mixing register sizes (x86)

Question

Does anyone know how I can get rid of the following assembler warning?

Code is x86, 32 bit:

int test (int x)
{
  int y;
  // do a bit-rotate by 8 on          


        

Answer 1

While I'm thinking about it ... you should replace the "q" constraint with a capital "Q" constraint in Chris's second solution:

int
test(int x)
{
    int y;
    asm ("xchg %b0, %h0" : "=Q" (y) : "0" (x));
    return y;
}

"q" and "Q" are slightly different in 64-bit mode, where you can get the lowest byte for all of the integer registers (ax, bx, cx, dx, si, di, sp, bp, r8-r15). But you can only get the second-lowest byte (e.g. ah) for the four original 386 registers (ax, bx, cx, dx).