On Thu, Jun 28, 2012 at 8:57 AM, Richard Henderson <r...@redhat.com> wrote: > On 2012-06-28 07:05, Jakub Jelinek wrote: >> Unfortunately the addition of the builtin_mul_widen_* hooks on i?86 seems >> to pessimize the generated code for gcc.dg/vect/pr51581-3.c >> testcase (at least with -O3 -mavx) compared to when the hooks aren't >> present, because i?86 has more natural support for widen mult lo/hi >> compoared to widen mult even/odd, but I assume that on powerpc it is the >> other way around. So, how should I find out if both VEC_WIDEN_MULT_*_EXPR >> and builtin_mul_widen_* are possible for the particular vectype which one >> will be cheaper? > > I would assume that if the builtin exists, then it is cheaper. > > I disagree about "x86 has more natural support for hi/lo". The basic sse2 > multiplication is even. One shift per input is needed to generate odd. On > the other hand, one interleave per input is required for both hi/lo. So 4 > setup insns for hi/lo, and 2 setup insns for even/odd. And on top of all > that, XOP includes multiply odd at least for signed V4SI. > > I'll have a look at the test case you mention while I re-look at the > patches... >
The upper 128-bit of 256-bit AVX instructions aren't a good fit with the current vectorizer infrastructure. -- H.J.
