[PATCH] D87149: [InstCombine] erase instructions leading up to unreachable

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

In D87149#2281145 <https://reviews.llvm.org/D87149#2281145>, @uabelho wrote:

> Hi!
>
> We've run into a case where instcombine crashes with this patch:
>
>   opt -o /dev/null bbi-47401.ll -instcombine
>
> on
>
>   @c = external global i16*, align 1
>   
>   define void @main() {
>   entry:
>     br label %for.cond
>   
>   for.cond:                                         ; preds = %g.exit, %entry
>     %conv1 = phi double [ %conv, %g.exit ], [ undef, %entry ]
>     br i1 undef, label %for.end, label %for.body
>   
>   for.body:                                         ; preds = %for.cond
>     %0 = load i16*, i16** @c, align 1
>     %1 = load i16, i16* %0, align 1
>     %conv = sitofp i16 %1 to double
>     unreachable
>   
>   g.exit:                                           ; No predecessors!
>     br label %for.cond
>   
>   for.end:                                          ; preds = %for.cond
>     %conv1.lcssa = phi double [ %conv1, %for.cond ]
>     store double %conv1.lcssa, double* undef, align 1
>     ret void
>   }
>
> results in
>
>   opt: ../lib/Transforms/InstCombine/InstCombineInternal.h:448: virtual llvm::Instruction *llvm::InstCombinerImpl::eraseInstFromFunction(llvm::Instruction &): Assertion `I.use_empty() && "Cannot erase instruction that is used!"' failed.
>
> I suppose it's the dead block %g.exit that messes up things.

We had a use_empty() guard in an earlier draft of this patch and decided that could not happen. :)
https://reviews.llvm.org/D87149#2259207
Thanks for the test case!


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