[Bug target/41705] New: missed if conversion optimization

["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, [r0]
        cmp     r3, #0               // B
        bne     .L3
        ldr     r3, [r0, #4]
        b       .L2
.L3:
        mov     r3, #0               // A
.L2:
        ldr     r2, [r0, #8]
        @ sp needed for prologue
        ldr     r0, [r2]
        add     r0, r3, r0
        pop     {pc}

Instruction A can be moved before instruction B, which should be handled by
ifcvt.c:find_if_case_2. Notice the following code in find_if_header:

  if (dom_info_state (CDI_POST_DOMINATORS) >= DOM_NO_FAST_QUERY
      && (! HAVE_conditional_execution || reload_completed))
    {
      if (find_if_case_1 (test_bb, then_edge, else_edge))
        goto success;
      if (find_if_case_2 (test_bb, then_edge, else_edge))
        goto success;
    }

After reload_completed, the target of conditional assignment is happened to be
allocated to the same physical register as the condition variable. This prevent
it from moving to the front of compare and branch instructions.

Before reload_completed, HAVE_conditional_execution prevent find_if_case_2 to
be called. So we missed this optimization chance.

Target ARM has conditional execution capability, but thumb actually can't do
conditional execution. Do we have any method to let the compiler know this?


-- 
           Summary: missed if conversion optimization
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        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=41705