[PATCH] [RegisterScavenger] Fix handling of predicated instructions

[Quentin Colombet]

Ah great!

Thanks Matthias!

Tobias, please add a testcase to your patch and you can push your changes.

Cheers,
Q.
> On Jun 5, 2015, at 5:08 PM, Matthias Braun <mbraun at apple.com> wrote:
> 
> 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.
>> 
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>