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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/56069] New: [4.6/4.7/4.8 Regression] RA pessimization
Date: Mon, 21 Jan 2013 17:37:00 -0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: rtl-optimization
X-Bugzilla-Keywords: missed-optimization, ra
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X-Bugzilla-Priority: P3
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56069

             Bug #: 56069
           Summary: [4.6/4.7/4.8 Regression] RA pessimization
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Keywords: missed-optimization, ra
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: jakub@gcc.gnu.org
                CC: rsandifo@gcc.gnu.org, vmakarov@gcc.gnu.org
            Target: x86_64-linux


Since http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=139993
unsigned long foo (unsigned long mem)
{
  return (mem >> 3) | (1UL << 44);
}

unsigned long bar (unsigned long mem)
{
  return (mem >> 3) + (1UL << 44);
}

on x86_64-linux at -O2 (and -Os) the generated code for foo is one insn longer.
We used to emit:
        shrq    $3, %rdi
        movabsq $17592186044416, %rax
        orq     %rdi, %rax
        ret
but now emit:
        movq    %rdi, %rax
        movabsq $17592186044416, %rdx
        shrq    $3, %rax
        orq     %rdx, %rax
        ret