[PATCH] D108091: [AggressiveInstCombine] Add shift left instruction to `TruncInstCombine` DAG

[Anton Afanasyev via Phabricator via llvm-commits]

anton-afanasyev added a comment.

In D108091#2950930 <https://reviews.llvm.org/D108091#2950930>, @bjope wrote:

> Seen some failures and I'm suspecting this commit.
>
> For input like this:
>
>   define i32 @foo(i32 %call62, i16 %call63) {
>     %conv64142 = zext i32 %call62 to i64
>     %conv65 = sext i16 %call63 to i64
>     %sh_prom66 = and i64 %conv65, 4294967295
>     %shl67 = shl i64 %conv64142, %sh_prom66
>     %conv68 = trunc i64 %shl67 to i32
>     ret i32 %conv68
>   }
>
> we now get the following after aggressive instcombine
>
>   define i32 @foo(i32 %call62, i16 %call63) {
>     %conv65 = sext i16 %call63 to i32
>     %sh_prom66 = and i32 %conv65, -1
>     %shl67 = shl i32 %call62, %sh_prom66
>     ret i32 %shl67
>   }
>
> which is more poisonous according to https://alive2.llvm.org/ce/z/88tNrs
>
> In my original test case %call63 is 32, so we shift leftby 32, which is ok when shifting the i64 but not after having rewritten into a 32-bit shift.

Thanks @bjope, it was overflow of `unsigned = uint64_t` assignment, fixed it here: https://reviews.llvm.org/rG803270c0c691


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https://reviews.llvm.org/D108091