Re: [PATCH] x86: Define _mm*_undefined_*

[Ilya Tocar]

On 17 Mar 22:18, Ulrich Drepper wrote:
> On Mon, Mar 17, 2014 at 7:39 AM, Ilya Tocar <tocarip.in...@gmail.com> wrote:
> 
> > undefined is similar in behavior to setzero, but it also clobbers
> > flags. Maybe just define it to setzero for now?
> >
> >
> What do you mean by "clobbers flags"?  Do you have an example?

I've used follwing example:

#include <x86intrin.h>

extern __inline __m512
__attribute__ ((__gnu_inline__, __always_inline__, __artificial__))
_mm512_undefined_ps (void)
{
  __m512 __Y;
  __asm__ ("" : "=x" (__Y));
  return __Y;
}


__m512 foo1(__m512 __A)
{
return (__m512) __builtin_ia32_rcp14ps512_mask ((__v16sf) __A,
                                                  (__v16sf)
                                                 _mm512_undefined_ps (),
                                                  (__mmask16) -1);
}

__m512 foo2(__m512 __A)
{
return (__m512) __builtin_ia32_rcp14ps512_mask ((__v16sf) __A,
                                                  (__v16sf)
 _mm512_setzero_ps (),
                                                  (__mmask16) -1);
}


In foo1 asm statement is expanded into following rtl:

(insn 6 3 7 2 (parallel [
            (set (reg:V16SF 87 [ __Y ])
                (asm_operands:V16SF ("") ("=x") 0 []
                     []
                     [] foo.c:8))
            (clobber (reg:QI 18 fpsr))
            (clobber (reg:QI 17 flags))
        ]) foo.c:8 -1

As you can see flags are clobbered by asm statement, while in setzero
case (foo2) i have just:
(insn 7 6 8 2 (set (reg:V16SF 88)
        (const_vector:V16SF [
                (const_double:SF 0.0 [0x0.0p+0])
                (const_double:SF 0.0 [0x0.0p+0])
//rest of zeroes skipped.