[PATCH] D76983: [InstCombine] Transform extractelement-trunc -> bitcast-extractelement

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added a comment.

In D76983#1954722 <https://reviews.llvm.org/D76983#1954722>, @jeroen.dobbelaere wrote:

> This patch triggers a regression on our side:
>
> <...>
>
> The tests expects to see:
>
>   define dso_local <4 x i16> @truncate_v_v(<4 x i32> %lhs) local_unnamed_addr #0 {
>   entry:
>     %0 = trunc <4 x i32> %lhs to <4 x i16>
>     ret <4 x i16> %0
>   }
>   
>
> which, in machine instructions, is mapped onto a vector trunc instruction.
>
> But now, we see:
>
>   define dso_local <4 x i16> @truncate_v_v(<4 x i32> %lhs) local_unnamed_addr #0 {
>   entry:
>     %0 = bitcast <4 x i32> %lhs to <8 x i16>
>     %vecinit9 = shufflevector <8 x i16> %0, <8 x i16> undef, <4 x i32> <i32 0, i32 2, i32 4, i32 6>
>     ret <4 x i16> %vecinit9
>   }
>   
>
> which is expanded into a large sequence of code going through the stack.


This looks like a simple missed transform to me, not a miscompile


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  https://reviews.llvm.org/D76983/new/

https://reviews.llvm.org/D76983