[Bug rtl-optimization/43286] New: Missed related value optimization in cse.c

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

foo1:
        stmfd   sp!, {r4, r5, r6, lr}
        ldr     r4, .L2
        sub     r6, r0, #16     // A
        mov     r5, r0          // B
        mov     r3, r1
        mov     r1, r0
        ldr     r0, .L2+4
.LPIC0:
        add     r4, pc, r4
        sub     sp, sp, #8
        mov     r2, r6          //
        add     r0, r4, r0
        bl      printf(PLT)
        ldr     r0, [r6, #12]   //
        ldr     r1, [r5, #-16]  //
        ldmib   r6, {r2, r3}    @ phole ldm   //
        str     r0, [sp, #0]
        ldr     r0, [r5, #0]     //
        str     r0, [sp, #4]
        ldr     r0, .L2+8
        add     r0, r4, r0
        bl      printf(PLT)
        add     sp, sp, #8
        ldmfd   sp!, {r4, r5, r6, pc}

According to instructions AB, r5 = r6 + 16. All following instructions
involving r5 are memory loads. The usage of r5 can be replaced by r6 with an
adjusted offset. Then instruction B can be removed.

I assumed pass cse1 should handle this optimization. But after reading the code
of function insert_with_costs I realized it can't handle arbitrary related
expression, the only form that can be processed is constant related
expressions, such as (plus SYMBOL offset).


-- 
           Summary: Missed related value optimization in cse.c
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-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=43286