[Bug c++/49358] New: optimization regression in 4.6.1 from 4.5.4

[Bug c++/49358] New: optimization regression in 4.6.1 from 4.5.4

[v.haisman at sh dot cvut.cz]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49358

           Summary: optimization regression in 4.6.1 from 4.5.4
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: v.haisman@sh.cvut.cz


The code:

--------8<----------
double f (double y)
{
  return y*y*y*y;
}
--------8<----------

This compiles using version 4.5.4 20110602 (prerelease) (FreeBSD Ports
Collection) to this:

--------8<----------
_Z1fd:
.LFB87:
    .cfi_startproc
    movapd    %xmm0, %xmm1    # y, y
    mulsd    %xmm1, %xmm0    # y, tmp63
    mulsd    %xmm1, %xmm0    # y, tmp63
    mulsd    %xmm1, %xmm0    # y, tmp63
    ret
--------8<----------

But gcc version 4.6.1 20110408 (prerelease) (GCC) produces what seems to be
less optimal version:

--------8<----------
_Z1fd:
.LFB85:
    .cfi_startproc
    movapd    %xmm0, %xmm1    # y, tmp64
    mulsd    %xmm0, %xmm1    # y, tmp64
    mulsd    %xmm0, %xmm1    # y, tmp64
    mulsd    %xmm0, %xmm1    # y, tmp64
    movapd    %xmm1, %xmm0    # tmp64,
    ret
--------8<----------

GCC 4.5.4 seems to be smarter in register allocation avoiding the final movapd.

In both cases I have used -O3 -fverbose-asm -save-temps parameters to g++.

[Bug rtl-optimization/49358] optimization regression in 4.6.1 from 4.5.4

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49358

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization, ra
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011.06.10 09:33:33
                 CC|                            |law at gcc dot gnu.org,
                   |                            |vmakarov at gcc dot gnu.org
          Component|c++                         |rtl-optimization
      Known to work|                            |4.4.5
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-06-10 09:33:33 UTC ---
Confirmed.

[Bug rtl-optimization/49358] optimization regression in 4.6.1 from 4.5.4

[v.haisman at sh dot cvut.cz]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49358

--- Comment #2 from v.haisman at sh dot cvut.cz 2011-12-30 11:31:54 UTC ---
The problem is still there with GCC 4.7.0 20111203 snapshot.

[Bug rtl-optimization/49358] optimization regression in 4.7.0, 4.6.1 from 4.5.4

[vhaisman at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49358

--- Comment #3 from Václav Zeman <vhaisman at gmail dot com> ---
And still a problem with gcc version 4.8.1 (Ubuntu/Linaro 4.8.1-10ubuntu9).
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Subject: [Bug fortran/60543] [4.7/4.8/4.9 Regression] Function with side effect removed by the optimizer.
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id`543

--- Comment #11 from Sarantis Pantazis <sarantis.pantazis at gmail dot com> ---
Yes, I did; I used "make clean" first (also "rm *.o *.f90 *.s" in the main
folder to clean up the files generated by -save-temps).