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

From the file gcc/config/i386/i386.c of the GCC sources:

       b -- print the QImode name of the register for the indicated operand.
        %b0 would print %al if operands[0] is reg 0.
       w --  likewise, print the HImode name of the register.
       k --  likewise, print the SImode name of the register.
       q --  likewise, print the DImode name of the register.
       x --  likewise, print the V4SFmode name of the register.
       t --  likewise, print the V8SFmode name of the register.
       g --  likewise, print the V16SFmode name of the register.
       h -- print the QImode name for a "high" register, either ah, bh, ch or dh.

Similarly from gcc/config/i386/contraints.md:

    ;; We use the Y prefix to denote any number of conditional register sets:
    ;;  z   First SSE register.
    ;;  i   SSE2 inter-unit moves to SSE register enabled
    ;;  j   SSE2 inter-unit moves from SSE register enabled
    ;;  m   MMX inter-unit moves to MMX register enabled
    ;;  n   MMX inter-unit moves from MMX register enabled
    ;;  a   Integer register when zero extensions with AND are disabled
    ;;  p   Integer register when TARGET_PARTIAL_REG_STALL is disabled
    ;;  f   x87 register when 80387 floating point arithmetic is enabled
    ;;  r   SSE regs not requiring REX prefix when prefixes avoidance is enabled
    ;;  and all SSE regs otherwise

This file also defines a "Yk" constraint but I don't know if how well it would work in an asm statement:

    (define_register_constraint "Yk" "TARGET_AVX512F ? MASK_EVEX_REGS : NO_REGS"
    "@internal Any mask register that can be used as predicate, i.e. k1-k7.")

Note this is all copied from the latest SVN revision. I don't know what release of GCC, if any, the particular modifiers and constraints you're interested in were added.

Answer 2

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 <x86intrin.h>
#include <stdint.h>

// 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)