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Date: Thu, 19 Aug 2010 07:18:00 -0000
Subject: [Bug target/45335]  New: Use ldrd to load two consecutive words
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Compile the following code with options -march=armv7-a -mthumb -O2 -fpic

struct S
{
    void* p1;
    void* p2;
    void* p3;
    void* p4;
};

void foo1(struct S* fp, struct S* otherSaveArea)
{
    struct S* saveArea = fp - 1;
    printf("StackSaveArea for fp %p [%p/%p]:\n", fp, saveArea, otherSaveArea);
    printf("  prevFrame=%p savedPc=%p meth=%p curPc=%p fp[0]=0x%08x\n",
       saveArea->p1, saveArea->p2,saveArea->p3,saveArea->p4,*(unsigned
int*)fp);
}

GCC 4.6 generates

foo1:
        push    {r4, r5, r6, lr}
        mov     r4, r0
        ldr     r5, .L2
        sub     sp, sp, #8
        mov     r3, r1
        sub     r2, r0, #16
.LPIC0:
        add     r5, pc
        mov     r1, r0
        mov     r0, r5
        bl      printf(PLT)
        ldr     r6, [r4, #-4]     // A
        ldr     r5, [r4, #0]      // B
        ldr     r0, .L2+4
        ldr     r1, [r4, #-16]    // C
        ldr     r2, [r4, #-12]    // D
.LPIC1:
        add     r0, pc
        ldr     r3, [r4, #-8]
        str     r6, [sp, #0]      // E
        str     r5, [sp, #4]      // F
        bl      printf(PLT)
        add     sp, sp, #8
        pop     {r4, r5, r6, pc}

Notice instructions A and B loading two consecutive words, they can be written
as a single instruction
        ldrd    r6, r5, [r4, -4]

It results in shorter and potentially faster code. Similarly instructions C and
D can be replace by a ldrd. Instructions E and F shows a chance for strd
instruction.

We don't have a ldrd insn pattern to represent two separate loads. Maybe we can
define one and let combine to find the chances.

There is also the same opportunity for ARM instructions, but there are more
constraints on register usage. In arm mode, the first register must be even
numbered and the second register number must be exact successor of the first
one. These are similar to the constraints of ldm and stm, and our register
allocator can't handle them.


-- 
           Summary: Use ldrd to load two consecutive words
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: enhancement
          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=45335