Excerpts from Richard Biener's message of März 3, 2024 11:41 am:
>
>
>> Am 03.03.2024 um 02:51 schrieb Iain Buclaw <ibuc...@gdcproject.org>:
>>
>> Hi,
>>
>> This patch fixes a wrong code issue in the D front-end where lowered
>> struct comparisons would reinterpret fields with a different (usually
>> bigger) alignment than the original. Use `build_aligned_type' to
>> preserve the alignment when casting away for such comparisons.
>>
>> Bootstrapped and regression tested on x86_64-linux-gnu/-m32, committed
>> to mainline, and backported to releases/gcc-13, releases/gcc-12, and
>> releases/gcc-11.
>
> LGTM. You might want to experiment with not doing the premature optimization
> but instead use __builtin_memcmp_eq (assuming that’s a general good fit).
> The middle-end should better turn that into target optimal code.
>
Indeed, just looking at the history, it was introduced well over ten
years ago so I can't comment on the original context for it (it doesn't
directly fix any old issues).
Small comparison between this optimization and memcmp_eq
---
_5 = newitem ();
# DEBUG bn => _5
_1 = MEM[(ucent *)_5 + 8B];
_2 = _1 != 0;
_6 = (int) _2;
return _6;
---
call _D8pr1141717newitemFNbNiZPSQz2S2
.LVL29:
vmovdqu 8(%rax), %xmm0
xorl %eax, %eax
.LVL30:
vptest %xmm0, %xmm0
setne %al
addq $8, %rsp
ret
---
_6 = newitem ();
# DEBUG bn => _6
D.2335.length = 0;
D.2335.ptr = 0B;
_1 = &_6->label.label;
_2 = __builtin_memcmp_eq (_1, &D.2335, 16);
_3 = _2 != 0;
_9 = (int) _3;
return _9;
---
call _D8pr1141717newitemFNbNiZPSQz2S2
.LVL29:
movq $0, (%rsp)
movq $0, 8(%rsp)
vmovdqu 8(%rax), %xmm0
xorl %eax, %eax
.LVL30:
vpxor (%rsp), %xmm0, %xmm0
vptest %xmm0, %xmm0
setne %al
addq $24, %rsp
ret
