[Bug rtl-optimization/37262] New: Two branches of the same condition being emitted

[Bug rtl-optimization/37262] New: Two branches of the same condition being emitted

[pinskia at gcc dot gnu dot org]

Take:
unsigned ReverseBits (unsigned index, unsigned NumBits)
{
  unsigned i, rev;

  for (i = rev = 0; i < NumBits; i++)
  {
    rev = (rev << 1) | (index & 1);
    index >>= 1;
  }
  return rev;
}
---- CUT ---
Currently we get:
        mtctr 9
        beq- 7,.L8
        beq- 7,.L8
        .p2align 3,,7

Which is obviously broken as we should have only one beq as they use the same
CR and go to the same block and there is no way to get to the second one
without going through the first.

4.1.1 -fno-ivopts produces even worse code:
        cmplwi 7,4,1
        blt- 7,.L8
        cmpwi 7,4,0
        beq- 7,.L8

But we know that this a logicial compare so r4 < 1 is the same as r4 ==0 so 4.3
produces better code but still needs slight improvement with respect of getting
rid of the extra branch (though we have regression between 4.1 and 4.3 which I
will file seperately as it is unrelated to this bug).


-- 
           Summary: Two branches of the same condition being emitted
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: pinskia at gcc dot gnu dot org
GCC target triplet: powerpc64-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37262

[Bug rtl-optimization/37262] Two branches of the same condition being emitted

[pinskia at gcc dot gnu dot org]

-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37262

[Bug rtl-optimization/37262] Two branches of the same condition being emitted

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2008-08-30 01:41 -------
I have seen this in other cases even for the non doloop case, though I don't
know if it is because of -O1 or because it is not removing it.
Testcase:
int _bfd_xcoff_canonicalize_dynamic_reloc (unsigned long long l_symndx)
{
  if (l_symndx < 3)
    {
      switch (l_symndx)
      {
        case 0:
        case 1:
         break;
        case 2:
         return _bfd_abort ();
    }
  }
}
--- CUT ---
Compile at -O1 on powerpc-linux and you will see the double branches:
        bne 0,.L7
        bne 0,.L8


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37262

[Bug rtl-optimization/37262] Two branches of the same condition being emitted

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2010-03-02 18:29 -------
Still happens as of today on the trunk.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37262