GCC inline assembler, mixing register sizes (x86)

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 the lower word. leave upper word intact.
  asm ("rorw $8, %0\n\t": "=q"(y) :"0"(x));
  return y;
}

If I compile it I get the following (very valid) warning:

Warning: using `%ax' instead of `%eax' due to `w' suffix

What I'm looking for is a way to tell the compiler/assembler that I want to access the lower 16 bit sub-register of %0. Accessing the byte sub-registers (in this case AL and AH) would be nice to know as well.

I've already chosen the "q" modifier, so the compiler is forced to use EAX, EBX, ECX or EDX. I've made sure the compiler has to pick a register that has sub-registers.

I know that I can force the asm-code to use a specific register (and its sub-registers), but I want to leave the register-allocation job up to the compiler.

Answers


You can use %w0 if I remember right. I just tested it, too. :-)

int
test(int x)
{
    int y;
    asm ("rorw $8, %w0" : "=q" (y) : "0" (x));
    return y;
}

Edit: In response to the OP, yes, you can do the following too:

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

At present, the only place (that I know of) it's documented in is gcc/config/i386/i386.md, not in any of the standard documentation.


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