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

Mehdi Amini via llvm-dev llvm-dev at lists.llvm.org
Tue Jan 17 16:59:30 PST 2017

> On Jan 17, 2017, at 4:36 PM, Hal Finkel via llvm-dev <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.


> 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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