[llvm] [InstCombine] Simplify and/or of icmp eq with op replacement (PR #70335)

[via llvm-commits]

mikaelholmen wrote:

> @mikaelholmen Thank you! Here is a reduced version:
> 
> ```llvm
> declare void @barrier()
> 
> define i1 @test1(i32 %x) {
>   %cmp1 = icmp ne i32 %x, 0
>   call void @barrier()
>   %div = udiv i32 2147483647, %x
>   %cmp2 = icmp ugt i32 %x, %div
>   %or = or i1 %cmp1, %cmp2
>   ret i1 %or
> }
> ```
> 
> What happens is that we try to evaluate `%cmp2` with `%x==0`, in which case the division is UB and folds to poison. We then fold `or` with a poison operand, resulting in an overall `poison` result.

But... this @test1 example is indeed UB in case %x is 0, right? Since the div is always carried out, even if  %x is 0, and then the result is used to calculate the return value %or (via icmp and or)? Or what am I missing here?

In the original ic.ll example the udiv was avoided if the denominator was 0 and UB was avoided.

https://github.com/llvm/llvm-project/pull/70335