[PATCH] D20907: [IfConversion] Bugfix: don't add Undef flag on use if reg is live.

[Matthias Braun via llvm-commits]

> On Jul 7, 2016, at 10:33 AM, Quentin Colombet via llvm-commits <llvm-commits at lists.llvm.org> wrote:
> 
> 
>> On Jul 6, 2016, at 3:30 PM, Matthias Braun <matze at braunis.de> wrote:
>> 
>> MatzeB added a comment.
>> 
>> In http://reviews.llvm.org/D20907#473455, @jonpa wrote:
>> 
>>> Matthias, from what I can see you have some experience with this part of IfConversion.cpp. Perhaps you could give some advice on or even fix this problem? In short, a bug in IfConversion was found and fixed, but when patch was applied, we ran into a problem relating to Redefs - see above.
>> 
>> 
>> To come back to this discussion: I added the UpdatePredRedefs() code to achieve the liveness effects stated in my previous post. I have no idea why I used the undef flag, it was one of my first commits to llvm so I assume I misunderstood the semantics back then. So removing this flag is the right call.
> 
> I believe Pete added that code, so you’re clear :P.
> 
>> 
>> To fix calls clobbering a live register; "pseudo" live in the sense that it is only live in our "ignore the predicates" worldview. I believe we have to add a implicit def and(!) and implicit use in the presence of clobbers or dead definitions.
> 
> The problem, as far as I understood, is that the liveness information is wrong when we call updatepredreg, which is why the add of implicit-use gets buggy.

If it is really just that, then my BranchFolder patch may have fixed that by accident. I'll give it a try.

- Matthias