[llvm-dev] AtomicExpandPass and branch weighting

[James Knight via llvm-dev]

Seems reasonable.

I'd note additionally that on some architectures, that the success block *must* be the fallthrough case (that is to say: you must not have any taken branches between the load-linked and store-conditional) in order to have an architectural guarantee that two such loops on different CPUs won't livelock against eachother.


On Dec 12, 2016, at 12:30 PM, Kyle Butt via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> I'm working on a change to the layout algorithm, and I noted that test/CodeGen/ARM/cmpxchg-weak.ll was affected.
> 
> Normally, that would be fine, but I noted that the layout changed the fallthrough from the success case to the failure case. I was surprised to see that the success case isn't annotated with a branch weight by AtomicExpandPass.cpp
> 
> Would it make sense to annotate the success case as more likely when we expand the intrinsic to help guarantee that the success case remains the fallthrough? Even a 2:1 or 3:2 weighting would correct the layout issue I noted.
> 
> Thanks,
> Kyle.
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