[PATCH] [RegisterScavenger] Fix handling of predicated instructions

[Matthias Braun]

I changed IfConversion in r192482 (and following fixes) to produce kill flags that are correct independently of predicates. I think updating the register scavenger for this is fine.

- Matthias

> On Jun 5, 2015, at 4:10 PM, Tobias Edler von Koch <tobias at codeaurora.org> wrote:
> 
> On Fri, 5 Jun 2015 15:55:16 -0700 Quentin Colombet
> <qcolombet at apple.com> wrote:
> 
>> 
>> Hold on, isn’t the change you are proposing supposed to allow that, i.e., reuse a register after it has a kill flag set?
>> 
>> I.e., this patch creates the very case I was saying is broken and that the verifier won’t complain about.
>> 
>> Consider:
>> 
>> BB1
>> a =
>> <predicate> = a<kill>
>> <predicate> return
>> 
>> BB2 <livein a>
>> = a
>> 
>> With your change, the scavenger would be allowed to do:
>> BB1
>> a =
>> <predicate> = a<kill>
>> <predicate> return
>> // Reuse a
>> BB2 <livein a>
>> = a <— a is wrong now.
>> 
> 
> Right, sorry, I see what you're saying now. Thanks for clarifying!
> 
> The problem is that currently it will do:
> 
> BB1
> a =
> <predicate> = a<kill>
> <predicate> return
> // Spill a  <-- not legal per the verifier, because a is killed
> // Reuse a
> // Reload a
> 
> BB2 <livein a>
> = a 
> 
> So then there are two solutions to fix the bug
> a) Figure out whether if conversion really doesn't set kill flags
>   correctly - any other passes that might be affected? If the flags
>   really are wrong, this should be fixed.
> b) Never scavenge a register that's killed by a predicated
>   instruction... I guess this would be OK as work-around.
> 
> Tobias
> 
> -- 
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a
> Linux Foundation Collaborative Project.
> 
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits