On Monday 24 April 2017, Jakub Jelinek wrote:
> On Mon, Apr 24, 2017 at 09:33:09AM +0200, Allan Sandfeld Jensen wrote:
> > --- a/gcc/config/i386/avx2intrin.h
> > +++ b/gcc/config/i386/avx2intrin.h
> > @@ -667,7 +667,7 @@ extern __inline __m256i
> >
> > __attribute__ ((__gnu_inline__, __always_inline__, __artificial__))
> > _mm256_slli_epi16 (__m256i __A, int __B)
> > {
> >
> > - return (__m256i)__builtin_ia32_psllwi256 ((__v16hi)__A, __B);
> > + return ((__B & 0xff) < 16) ? (__m256i)((__v16hi)__A << (__B & 0xff)) :
> > _mm256_setzero_si256();
> >
> > }
>
> What is the advantage of doing that when you replace one operation with
> several (&, <, ?:, <<)?
> I'd say instead we should fold the builtins if in the gimple fold target
> hook we see the shift count constant and can decide based on that.
> Or we could use __builtin_constant_p (__B) to decide whether to use
> the generic vector shifts or builtin, but that means larger IL.
The advantage is that in this builtin, the __B is always a literal (or
constexpr), so the if statement is resolved at compile time.
`Allan
