[Bug target/68655] SSE2 cannot vec_perm of low and high part

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68655

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 36897
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36897&action=edit
gcc6-pr68655.patch

Initial untested patch.  Unfortunately, it doesn't seem to be always a win,
when looking at the differences between old and new compiler.
I'm looking at
cd /usr/src/gcc/gcc/testsuite/gcc.dg/torture; for i in vshuf-v*[hqs]i.c; do for
j in -msse2 -msse4 -mavx -mavx2 -mavx512f -mavx512bw; do
/usr/src/gcc/obj/gcc/cc1.v246 -quiet -O2 $j $i -DEXPENSIVE -o /tmp/1.s;
/usr/src/gcc/obj/gcc/cc1 -quiet -O2 $j $i -DEXPENSIVE -o /tmp/2.s; echo ===$i
$j===; diff -up /tmp/1.s /tmp/2.s; done; done
output now (where cc1.v246 is vanilla cc1, cc1 is one with this patch applied).
In some cases the patch helps, but I've seen so far some cases where for
AVX512* it resulted in more instructions.