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

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added a comment.

In D87149#2282224 <https://reviews.llvm.org/D87149#2282224>, @spatel wrote:

> 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!

Unreachable BB's were the obvious potential source for non-empty use-count, but i thought instcombine didn't combine instructions in unreachable blocks.
Maybe that is the bug?


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