[PATCH] D112268: [LegalizeTypes][RISCV][PowerPC] Expand CTLZ/CTTZ/CTPOP instead of promoting if they'll be expanded later.

[Craig Topper via Phabricator via llvm-commits]

craig.topper added a comment.

In D112268#3137708 <https://reviews.llvm.org/D112268#3137708>, @thopre wrote:

> Hi @craig.topper ,
>
> This caused a regression for us for i16 cttz with is_undef_zero true because cttz is expanded to a sequence using ctpop which loses the fact that any output is ok for the 0 case.
>
>   t17: i16 = sub t9, Constant:i16<1>
>   t19: i16 = xor t9, Constant:i16<-1>
>   t20: i16 = and t19, t17
>   t21: i16 = ctpop t20
>
> So when this gets promoted, a mask is inserted to not have the result of ctpop change when `t9` is 0.
>
>   t15: i32 = extract_vector_elt t2, Constant:i32<0>
>   t25: i32 = xor t15, Constant:i32<-1>
>   t23: i32 = sub t15, Constant:i32<1>
>   t26: i32 = and t25, t23
>   t28: i32 = and t26, Constant:i32<65535>
>   t29: i32 = ctpop t28
>
> Prior to this patch, we'd get:
>
>   t19: i32 = sub t15, Constant:i32<1>
>   t21: i32 = xor t15, Constant:i32<-1>
>   t22: i32 = and t21, t19
>   t23: i32 = ctpop t22
>
> I thought about inserting some llvm.assume saying the value is non zero when expanding but those work at IR level. Any pointer on how to solve this? Best regards.

I take it i32 ctpop is legal for your target or the i16 ctpop would have gotten expanded too?


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https://reviews.llvm.org/D112268