[llvm-dev] Loop Load Elimination - RAR forward deps

[Adam Nemet via llvm-dev]

Hi Renato,

Thanks for working on this.

> On Nov 3, 2015, at 2:50 AM, Renato Golin <renato.golin at linaro.org> wrote:
> 
> Hi Adam,
> 
> I'm looking into your LLE pass in order to remove the RAR dependency
> from GVN, and I've hit a problem that might need some more thinking.
> 
> I've modified your patch to cope with both kinds of dependencies in
> the ForwardingCandidate class, but the way the pass gathers
> informations only brings RAW dependencies:
> 
>    const auto *Deps = LAI.getDepChecker().getDependences();
>    if (!Deps)
>      return Candidates;
> 
> If I add RAR dependency checking on LoopAccessInfo, that'll break
> everything else that depends on it, and if I scan the basic blocks for
> RAR dependency in LLE itself, it would be moving the logic away from
> its rightful place.
> 
> I'm thinking of renaming getDependencies() to something like
> getStoreToLoadDeps() and add a getLoadToLoadDeps(). Does that seem
> reasonable?

I don’t think so.  I think we want getDependences to return everything: true, output and input/RAR dependences and then have the client ignore the uninteresting types.

We probably need a type enum in struct Dependence for true, output and intput/RAR dependence (probably rename the current type to direction).

Would that work for you?  Let me know if you need help.  I should be on IRC all day. 

Just FYI, I am planning to push out the LLE set today unless testing uncovers something unexpected.

Adam

> 
> Or am I missing something obvious?
> 
> cheers,
> --renato