[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++.