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* [Bug middle-end/42961]  New: [4.5 regression] IRA register preferencing bug
@ 2010-02-04 22:05 hubicka at gcc dot gnu dot org
  2010-02-05 10:18 ` [Bug middle-end/42961] " rguenth at gcc dot gnu dot org
                   ` (6 more replies)
  0 siblings, 7 replies; 8+ messages in thread
From: hubicka at gcc dot gnu dot org @ 2010-02-04 22:05 UTC (permalink / raw)
  To: gcc-bugs

For following testcase:
int e,f;
float
main(float d,float c)
{
   float a;
   float e;
   if (e)
     a=d, e=c;
   else
     a=c, c=e;
   if (f)
     e=a;
   return e;
}
(it looks artifical, but all the moves are there just to make "a" to be only
source or destination of reg-reg move that is not that uncommon in real world
code)
we generate completely bogus:
main:
.LFB0:
        .cfi_startproc
        xorps   %xmm2, %xmm2
        movss   %xmm1, -4(%rsp)
        movl    -4(%rsp), %eax
        ucomiss %xmm2, %xmm2
        jp      .L7
        je      .L3
.L7:
        movss   %xmm0, -4(%rsp)
        movl    -4(%rsp), %eax
        movaps  %xmm1, %xmm2
.L3:
        movl    f(%rip), %edx
        testl   %edx, %edx
        je      .L4
        movl    %eax, -4(%rsp)
        movss   -4(%rsp), %xmm2
.L4:
        movaps  %xmm2, %xmm0
        ret

instead of
main:
.LFB2:
        movaps  %xmm0, %xmm3
        xorps   %xmm0, %xmm0
        movaps  %xmm1, %xmm2
        ucomiss %xmm0, %xmm0
        jp      .L2
        je      .L3
.L2:
        movaps  %xmm3, %xmm2
        movaps  %xmm1, %xmm0
.L3:
        movl    f(%rip), %eax
        testl   %eax, %eax
        je      .L4
        movaps  %xmm2, %xmm0
.L4:
        rep
        ret
.LFE2:

generated by GCC 4.3.2.

The problem is that IRA concludes wrong register preferencing:
    a3 (r62,l0) best SSE_REGS, cover SSE_REGS
    a2 (r61,l0) best SSE_FIRST_REG, cover SSE_REGS
    a1 (r59,l0) best GENERAL_REGS, cover GENERAL_REGS
    a0 (r58,l0) best SSE_FIRST_REG, cover SSE_REGS

this is because of movdf pattern having alternative for general regs.
This alternative is however useful primarily when the value is temporary for
mem-mem moves, and IRA should be able to work this out.


-- 
           Summary: [4.5 regression] IRA register preferencing bug
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: hubicka at gcc dot gnu dot org
  GCC host triplet: x86_64-linux
GCC target triplet: x86_64-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42961


^ permalink raw reply	[flat|nested] 8+ messages in thread
end of thread, other threads:[~2010-07-31  9:33 UTC | newest]

Thread overview: 8+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2010-02-04 22:05 [Bug middle-end/42961] New: [4.5 regression] IRA register preferencing bug hubicka at gcc dot gnu dot org
2010-02-05 10:18 ` [Bug middle-end/42961] " rguenth at gcc dot gnu dot org
2010-02-05 10:52 ` rguenth at gcc dot gnu dot org
2010-02-07 14:59 ` rguenth at gcc dot gnu dot org
2010-02-10  9:26 ` hubicka at ucw dot cz
2010-03-20 13:02 ` steven at gcc dot gnu dot org
2010-04-06 11:26 ` rguenth at gcc dot gnu dot org
2010-07-31  9:35 ` [Bug middle-end/42961] [4.5/4.6 " rguenth at gcc dot gnu dot org

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