[Bug target/42235] New: redundant memory move from parameter space to spill space

[carrot at google dot com]

Compile the attached source code with options -Os -fpic -mthumb, gcc generates
following code for the constructor:

        push    {r4, r5, r6, r7, lr}
.LCFI2:
        .pad #12
        sub     sp, sp, #12
.LCFI3:
        str     r3, [sp]
        add     r3, sp, #32    //A,  B
        ldrb    r3, [r3]       //A,  B
        ldr     r5, .L14
        mov     r4, r0
        mov     r7, r2
        mov     r6, r1
        str     r3, [sp, #4]   //A
        bl      _ZN1AC2Ev
        ldr     r3, .L14+4
.LPIC2:
        add     r5, pc
        ldr     r3, [r5, r3]
        add     r3, r3, #8
        mov     r1, sp        //A
        str     r3, [r4]
        add     r2, r1, #4    //A, C
        mov     r3, r4
        add     r3, r3, #42
        ldrb    r1, [r2]      //A, C
        strb    r7, [r3]
        add     r3, r3, #1    //   C
        strb    r1, [r3]      //A, C
        cmp     r6, #0
        beq     .L9
        ldr     r2, [sp]
        mov     r3, #2
        cmp     r2, #0
        bne     .L12
        mov     r3, #1
.L12:
        strh    r3, [r4, #40]
        b       .L11
.L9:
        strh    r6, [r4, #40]
.L11:
        add     sp, sp, #12
        mov     r0, r4
        @ sp needed for prologue
        pop     {r4, r5, r6, r7}
        pop     {r1}
        bx      r1

1. The instructions marked A move a byte from parameter to the object body, but
it first moves that parameter to stack [sp, 4], then move the value from [sp,4]
to the target object. The move to [sp, 4] is redundant.

2. The Instructions marked B load a byte from the stack, then store the whole
word into another place on the stack. Since later instructions load a byte from
the new space, we can simply move the whole word at the first place, so
instructions marked as B can be reduced to one instruction "ldr r3, [sp, 32]"

3. Instructions marked C can be simplified to
     ldrb r1, [r1, 4]
     strb r1, [r3, 1]

4. Why not use "pop {r4, r5, r6, r7, pc}" as the epilogue?


-- 
           Summary: redundant memory move from parameter space to spill
                    space
           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=42235