For the below case, pre-inc prep think it's a good candidate to use pre-inc for the bucket, but 64bit integer load/store update (pre-inc) instruction on Power requires the displacement field should be DS-form (4's multiple). Since it can't satisfy the constraint, we have to do some fix ups later. As below, the original load/stores could be well-form, it makes things worse.

unsigned long long result = 0; unsigned long long foo(char *p, unsigned long long n) { for (unsigned long long i = 0; i < n; i++) { unsigned long long x1 = *(unsigned long long *)(p - 50000 + i); unsigned long long x2 = *(unsigned long long *)(p - 61024 + i); unsigned long long x3 = *(unsigned long long *)(p - 62048 + i); unsigned long long x4 = *(unsigned long long *)(p - 64096 + i); result *= x1 * x2 * x3 * x4; } return result; }

The current instruction sequence:

.LBB0_2: addi 4, 5, -11023 addi 7, 5, 1 addi 8, 5, -12047 addi 5, 5, -14095 ld 9, 0(7) ld 4, 0(4) ld 8, 0(8) ld 5, 0(5) mulld 4, 4, 9 mulld 4, 4, 8 mulld 4, 4, 5 mr 5, 7 mulld 3, 4, 3 bdnz .LBB0_2

With the proposed fix:

.LBB0_2: ld 4, 14096(5) ld 7, 3072(5) ld 8, 2048(5) ld 9, 0(5) addi 5, 5, 1 mulld 4, 7, 4 mulld 4, 4, 8 mulld 4, 4, 9 mulld 3, 4, 3 bdnz .LBB0_2

For i64 loads/stores, the proposed fix is to check whether the displacement fits in a 16-bit signed field but isn't a multiple of 4, if so we don't make it as candidate any more.