[PATCH] D25847: [ValueTracking] Don't assume we can speculate shifts

[Davide Italiano via llvm-commits]

davide added a comment.

In https://reviews.llvm.org/D25847#575978, @mkuper wrote:

> In https://reviews.llvm.org/D25847#575972, @sanjoy wrote:
>
> > This looks wrong -- shifts should always be safe to speculate.  I wasn't following PR30708 closely, but can you describe why you think shift speculation is the root cause for PR30708?
>
>
> Because of this comment on PR30708:
>
> > This instruction:
> > 
> >   %shr.i.us21 = lshr i32 1, 53
> > 
> > has UB as we're trying to shift right > 32
>
> But, as Eli pointed out (thanks!), this isn't true.


Oh, sorry, this is entirely my fault. I misread `result undefined` == `undefined behaviour`, that's where the confusion started. Sorry again!


https://reviews.llvm.org/D25847