[llvm-bugs] [Bug 31232] New: [Thumb2] Rebase ld/st to avoid negative offsets

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=31232

            Bug ID: 31232
           Summary: [Thumb2] Rebase ld/st to avoid negative offsets
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: weimingz at codeaurora.org
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

given test.c:

typedef struct
{
 unsigned char* Src;
 unsigned char* Dst;
 unsigned int Len;
} ContextType;

void test(ContextType* pContext)
{
   unsigned char* Src = (pContext->Src) - 26;
   unsigned int Offset = (pContext->Len) - 26;
   unsigned char* pDst = pContext->Dst;
   unsigned int dest_p1 = *((unsigned int*)(&Src[4]));
   unsigned short dest_p2 = *((unsigned short*)(&Src[8]));
   unsigned short src_p1 = *((unsigned short*)(&Src[10]));
   unsigned int src_p2 = *((unsigned int*)(&Src[12]));
   unsigned short EType = *((unsigned short*)(&Src[24]));
   *((unsigned int*)(&pDst[4+Offset])) = dest_p1;
   *((unsigned short*)(&pDst[8+Offset])) = dest_p2;
   *((unsigned short*)(&pDst[10+Offset])) = src_p1;
   *((unsigned int*)(&pDst[12+Offset])) = src_p2;
   *((unsigned short*)(&pDst[24+Offset])) = EType;
}

clang -Os -mcpu=cortex-m3 test.c -o llvm.s generates:

@ BB#0:                                 @ %entry
    .save    {r4, r6, r7, lr}
    push    {r4, r6, r7, lr}
    .setfp    r7, sp, #8
    add    r7, sp, #8
    ldrd    r1, lr, [r0]
    ldr    r0, [r0, #8]
    ldrh    r12, [r1, #-2]
    ldrh    r3, [r1, #-16]
    ldrh    r2, [r1, #-18]
    ldr    r4, [r1, #-22]
    ldr    r1, [r1, #-14]
    add    r0, lr
    str    r4, [r0, #-22]
    strh    r2, [r0, #-18]
    strh    r3, [r0, #-16]
    str    r1, [r0, #-14]
    strh    r12, [r0, #-2]
    pop    {r4, r6, r7, pc}

Since the offset are all negative, the ld/st are wide instructions. The
expected code are like:
        subs r1, #26
        subs r0, #26

            ldrh    r12, [r1, #-2]
    ldrh    r3, [r1, #10]
    ldrh    r2, [r1, #8]
    ldr    r4, [r1, #4]
    ldr    r1, [r1, #12]

    str    r4, [r0, #4]
    strh    r2, [r0, #8]
    strh    r3, [r0, #10]
    str    r1, [r0, #12]

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