[Bug tree-optimization/60451] New: X86 vectorization improve: pack instead of pshufb

[evstupac at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60451

            Bug ID: 60451
           Summary: X86 vectorization improve: pack instead of pshufb
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: evstupac at gmail dot com

Created attachment 32294
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=32294&action=edit
test case

Currently vectorizer use 2 "pshufb" and "or" for even/odd permutation.
(odd case)
 pshufb  %xmm5, %xmm1
 pshufb  %xmm4, %xmm0
 por     %xmm0, %xmm1

 where
 %xmm4 0 2 4 6 8 a c e -1 -1 -1 -1 -1 -1 -1 -1
 %xmm5 -1 -1 -1 -1 -1 -1 -1 -1 0 2 4 6 8 a c e

gcc/config/i386/i386.c (expand_vec_perm_even_odd_1):

    case V16QImode: 
      if (TARGET_SSSE3) 
        return expand_vec_perm_pshufb2 (d);

However in case of even/odd we can use:

2 "pand" and 1 "packuswb"

  pand     %xmm6, %xmm0
  pand     %xmm6, %xmm1
  packuswb %xmm1, %xmm0

  where
  %xmm6 is 0x00ff00ff00ff00ff


This will improve performance for architectures with slow pshufb instructions
and reduce code size on 1 constant.

For attached test Silvermont performance improve is 30%.