[llvm-dev] [RFC] IR-level Region Annotations

[Mehdi Amini via llvm-dev]

> On Jan 17, 2017, at 7:01 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> 
> On 01/17/2017 06:59 PM, Mehdi Amini wrote:
>> 
>>> On Jan 17, 2017, at 4:36 PM, Hal Finkel via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>> 
>>> 
>>> On 01/17/2017 05:36 PM, Wael Yehia via llvm-dev wrote:
>>>> Hi. Regarding the token approach, I've read some documentation (review D11861, EH in llvm, and Reid and David's presentation) but couldn't answer the following question.
>>>> Does the intrinsic or the instruction returning a token type object act as a code motion barrier? In other words, does it prevent other operations from being reordered with it?
>>>> If the answer is no, then does it mean the intrinsic has to be marked with the convergent property to achieve the code motion barrier effect?
>>> 
>>> To clarify, convergent does not form a code-motion barrier. It prevents transformations from adding additional control dependencies. I suspect that the token type does the same (because tokens can't appear in PHIs).
>> 
>> What if the only user of the token is in a branch? It seems to me that you’d be allowed to sink the token producer there, but not if it is marked convergent.
> 
> Can that happen if you start out with a "well formed" region?

LangRef seems to allow it, there might not be any users of token that would do that though?
I haven’t given much thought about it, I just raw-analyzed your suspected equivalence on this aspect :)

— 
Mehdi



>> 
>> 
>> 
>>> 
>>> Actual code motion barriers are generally achieved by adding unknown memory dependencies.
>>> 
>>>  -Hal
>>> 
>>>> If yes, can someone explain how is that achieved in llvm?
>>>> Greatly appreciated. Thanks.
>>>> 
>>>> Wael
>>>> 
>>>> 
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>>> 
>>> -- 
>>> Hal Finkel
>>> Lead, Compiler Technology and Programming Languages
>>> Leadership Computing Facility
>>> Argonne National Laboratory
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>> 
> 
> -- 
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory

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