[Bug rtl-optimization/16380] New: Use of uninitialised register after dbra conversion

["rsandifo at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

gcc-3.0 and above miscompile the following test case on mips64-elf
at -O2:

int main ()
{
  int i;
  for (i = 0; i < 10; i++)
    continue;
  if (i < 10)
    abort ();
  exit (0);
}

GCSE realises that the second i < 10 is redundant and reuses
the result of "i < 10" from the for(...) loop.  The loop
optimiser then converts the loop into decrement-and-branch
form and deletes the first comparison.  This leaves the
if (...) statement checking an uninitialised register:

        addiu   $sp,$sp,-40
        sd      $31,32($sp)
        li      $2,9
        addiu   $2,$2,-1
$L10:
        bgez    $2,$L10
        addiu   $2,$2,-1

        beq     $3,$0,$L6      # <---- $3 not initialised
        nop

        jal     abort
        nop

$L6:
        jal     exit
        move    $4,$0

-- 
           Summary: Use of uninitialised register after dbra conversion
           Product: gcc
           Version: 3.4.1
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P2
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rsandifo at gcc dot gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org
GCC target triplet: mips64-elf


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16380