In GNU C inline asm, what are the size-override modifiers for xmm/ymm/zmm for a single operand?

Question

While trying to answer Embedded broadcasts with intrinsics and assembly, I was trying to do something like this:

__m512 mul_bcast(__m512 a, float b) {
    asm         


        

Answer 1

It seems like all recent versions of GCC will accept both 'q' and 'x' as modifiers to print the XMM version of a YMM register.

Intel's icc looks to accept 'q', but not 'x' (at least through version 13.0.1).

[Edit: Well, it worked in this small example below, but in a real test case, I'm having problems with icc 14.0.3 accepting the 'q' but writing a 'ymm'.]

[Edit: Testing with more recent versions of icc, I'm finding that neither icc 15 nor icc 16 work with either 'q' or 'x'.]

But Clang 3.6 and earlier accept neither syntax. And at least on Godbolt, Clang 3.7 crashes with both!

// inline assembly modifiers to convert ymm to xmm

#include 
#include 

// gcc also accepts "%q1" as "%x1" 
// icc accepts "%q1" but not "%x1"
// clang-3.6 accepts neither
// clang-3.7 crashes with both!

#define ASM_MOVD(vec, reg)       \
__asm volatile("vmovd %q1, %0" : \
               "=r" (reg) :      \
               "x" (vec)         \
    );          

uint32_t movd_ymm(__m256i ymm) {
   uint32_t low;
   ASM_MOVD(ymm, low);
   return low;
}

uint32_t movd_xmm(__m128i xmm) {
   uint32_t low;
   ASM_MOVD(xmm, low);
   return low;
}

Link to test on Godbolt: http://goo.gl/bOkjNu

(Sorry that this isn't full answer to your question, but it seemed like useful information to share and was too long for a comment)