[Bug tree-optimization/41778] New: missed dead store elimination

["carrot at google dot com" <gcc-bugzilla at gcc dot gnu dot org>]

Compile the attached source code with options -Os -march=armv5te -mthumb, gcc
generates:

        push    {lr}
        ldr     r3, [r1, #4]      // redundant
        ldrb    r3, [r3]          // redundant
        @ sp needed for prologue
        pop     {pc}

There are two redundant instructions.
Compile it with options -O2 -march=armv5te -mthumb, gcc generates following
expected results.

foo:
        @ sp needed for prologue
        bx      lr

The optimization done in -O2 is from this patch
http://gcc.gnu.org/viewcvs?view=revision&revision=145172. But this piece of
code was in pre pass, it is disabled when -Os is specified, so the unoptimized
code was passed to rtl passes. In rtl passes, the dead store is caught and
removed with some related code, but not all of them were removed, so we can
still see the two redundant instruction.

We should also add this optimization to dead store elimination pass to benefit
-Os cases.


-- 
           Summary: missed dead store elimination
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: carrot at google dot com
 GCC build triplet: i686-linux
  GCC host triplet: i686-linux
GCC target triplet: arm-eabi


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41778