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Subject: [Bug tree-optimization/104582] [11/12 Regression] Unoptimal code for
 __negdi2 (and others) from libgcc2 due to unwanted vectorization
Date: Fri, 18 Feb 2022 13:45:35 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104582

--- Comment #17 from Richard Biener <rguenth at gcc dot gnu.org> ---
For

FAIL: gcc.target/i386/pr91446.c scan-assembler-times vmovdqa[^\\n\\r]*xmm[0=
-9]
2

we used to produce

0000000000000000 <foo>:
   0:   48 83 ec 28             sub    $0x28,%rsp
   4:   c4 e1 f9 6e d7          vmovq  %rdi,%xmm2
   9:   c4 e1 f9 6e da          vmovq  %rdx,%xmm3
   e:   c4 e3 e9 22 ce 01       vpinsrq $0x1,%rsi,%xmm2,%xmm1
  14:   c4 e3 e1 22 c1 01       vpinsrq $0x1,%rcx,%xmm3,%xmm0
  1a:   48 89 e7                mov    %rsp,%rdi
  1d:   c5 f9 7f 0c 24          vmovdqa %xmm1,(%rsp)
  22:   c5 f9 7f 44 24 10       vmovdqa %xmm0,0x10(%rsp)
  28:   e8 00 00 00 00          call   2d <foo+0x2d>
  2d:   48 83 c4 28             add    $0x28,%rsp
  31:   c3                      ret=20=20=20=20

but now reject this on costing grounds.  The scalar code is

0000000000000000 <foo>:
   0:   48 83 ec 28             sub    $0x28,%rsp
   4:   48 89 3c 24             mov    %rdi,(%rsp)
   8:   48 89 e7                mov    %rsp,%rdi
   b:   48 89 74 24 08          mov    %rsi,0x8(%rsp)
  10:   48 89 54 24 10          mov    %rdx,0x10(%rsp)
  15:   48 89 4c 24 18          mov    %rcx,0x18(%rsp)
  1a:   e8 00 00 00 00          call   1f <foo+0x1f>
  1f:   48 83 c4 28             add    $0x28,%rsp
  23:   c3                      ret=20=20=20=20

I think the scalar variant is 5 uops up to the call while the vector variant
is 9 uops.  The scalar variant can also execute 4 of the uops in parallel
(well, I guess only up to 3 with 3 store ports).  I think the scalar
variant is better and so I'm inclined to adjust the testcase.=