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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/65847] SSE2 code for adding two structs is much worse at -O3 than at -O2
Date: Wed, 22 Apr 2015 14:03:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65847

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Target|                            |x86_64-*-*
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2015-04-22
                 CC|                            |rguenth at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed.  The issue is that the vectorizer thinks x and y reside in memory
and thus it vectorizes the code as

  <bb 2>:
  vect__2.5_11 = MEM[(double *)&x];
  vect__3.8_13 = MEM[(double *)&y];
  vect__4.9_14 = vect__2.5_11 + vect__3.8_13;
  MEM[(double *)&D.1840] = vect__4.9_14;
  return D.1840;

which looks good.  But now comes the ABI and passes x, y and the return
value in registers ...

But even the best vectorized sequence would have four stmts - two to
pack arguments into vector registers, one add and one upack for the
return value.

Thus it seems the vectorizer should be informed of this ABI detail
or simply as heuristic never consider function arguments "memory"
it can perform vector loads on (which probably means to disable
group analysis on them?).

On i?86 with SSE2 we get

        movupd  8(%esp), %xmm1
        movl    4(%esp), %eax
        movupd  24(%esp), %xmm0
        addpd   %xmm1, %xmm0
        movups  %xmm0, (%eax)

vs.

        movsd   16(%esp), %xmm0
        movl    4(%esp), %eax
        movsd   8(%esp), %xmm1
        addsd   32(%esp), %xmm0
        addsd   24(%esp), %xmm1
        movsd   %xmm0, 8(%eax)
        movsd   %xmm1, (%eax)

which eventually looks even profitable (with -mfpmath=sse).

So a simple heuristic might pessimize things too much.

Replicating calls.c code to compute how the arguments are passed sounds
odd though...

Eventually the target can pessimize the loads in the target cost model
though (at least it can perform a more reasonable "heuristic").