[Thumb2] Rebase ld/st to avoid negative offsets

[llvmbot]

Bugzilla Link	31232
Version	trunk
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@smithp35

Extended Description

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]

[llvmbot]

when there are multiple ld/sts with the same base, pre-substract it will save code size