Inline Assembly Error: suffix or operands invalid for 'shr'

[Rodrigo Dominguez]

Hi,

While debugging a problem with Open64, I ran into a similar problem with
GCC. I created the following unit test program:

#include <stdint.h>

int main(void)
{
    uint32_t    a = 7;
    int8_t      s = -1;

    __asm__ ("shrl %1, %0\n\t"
            : "+r" (a)
            : "c" (-s)
            );

    return a;
}

When assembling this program, 'cc1' emits a 'shrl %ecx, %eax' instruction.
The 'shr' instruction can only take an 8-bit register as the first operand.
The emitted instruction should have been 'shrl %cl, %eax'. Therefore, the
compilation fails with a 'suffix or operands invalid for shr' message.

I have 2 questions:

1. AFAIK, by default, __asm__ chooses a register according to the size of
the operand (int8_t in this case). Is this correct? Where can I find
documentation of this?

2. If #1 is true, why does this fail?

Thank you,

Rodrigo