[PATCH] D19821: [EarlyCSE] Use MemorySSA if available.

[Daniel Berlin via llvm-commits]

On Mon, Jun 20, 2016 at 12:56 PM, George Burgess IV <
george.burgess.iv at gmail.com> wrote:

> george.burgess.iv added a comment.
>
> > George, can you stare at these quickly and see if any of your caching
> changes/etc will help?
>
>
> That depends on what exactly is slowing the benchmarks down so much. If
> our usage pattern is query -> remove -> query -> remove, then our cache may
> become useless, since (worst case) we drop the entire thing on each
> removal. If we primarily query defs, then this pattern gives us the same
> effectively-n^2 behavior of MemDep.



With the caveat that this will never occur for loads unless you remove the
stores in the way of those specific loads ;)


> One of the big goals of the new cache/walker is to allow us to drop as
> little as possible.
>

The nice part about walkers is that we  can use different walkers for
different purposes.


> In terms of pure walker/cache speed, the current walker is happy to do a
> lot of potentially useless work walking phis we can't optimize; the one I'm
> working on will do as little work as possible in that case. Also, the
> current walker potentially does a lot of domtree queries when caching
> results, whereas the one I'm working on does none (except in asserts).
> Glancing at some of the benchmarks, I'm not sure if any of that is what's
> slowing us down here, though.
>
> If you'd like, I'm happy to profile/poke around and give you a more
> definitive answer.
>
>
> ================
> Comment at: lib/Transforms/Scalar/EarlyCSE.cpp:500
> @@ -485,1 +499,3 @@
>
> +// Determine if the memory referenced by LaterInst is from the same heap
> version
> +// as EarlierInst.
> ----------------
> Nit: Please use ///
>
> ================
> Comment at: lib/Transforms/Scalar/EarlyCSE.cpp:571
> @@ +570,3 @@
> +      MSSAWalker->getClobberingMemoryAccess(EarlierInst);
> +  if (EarlierHeapGen == LaterHeapGen)
> +    return true;
> ----------------
> Nit: `return EarlierHeapGen == LaterHeapGen`?
>
>
> http://reviews.llvm.org/D19821
>
>
>
>
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