[PATCH] D55866: [DAGCombiner] allow narrowing of add followed by truncate

Wed Dec 19 07:00:13 PST 2018

andreadb added a comment.

In D55866#1336263 <https://reviews.llvm.org/D55866#1336263>, @spatel wrote:

> In D55866#1336120 <https://reviews.llvm.org/D55866#1336120>, @andreadb wrote:
>
> > From an x86 point of view, this change looks good.
> >
> > At some point, we should address the poor codegen from `test/CodeGen/X86/pr32345.ll`. Maybe you could raise a bug for it.
>
>
> I don't think it's worth the effort if we look at the background for this test; the intent was to check for a crash with *unoptimized* code:
>  https://bugs.llvm.org/show_bug.cgi?id=32345
>  rL298923 <https://reviews.llvm.org/rL298923>
>  ...so the test includes CSE values that the DAG never expects to encounter. I think the optimized RUN was just added for completeness/sanity. The whole thing is folded to 'unreachable' in IR.

Fair enough. I didn't check if it was folded by the optimizers at IR level.
Given that the whole sequence is optimized out in IR, then I agree with you: it is not not worthy to look into it.

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D55866/new/

https://reviews.llvm.org/D55866