[PATCH] D35741: Add MemorySSA alternative to AliasSetTracker in LICM.

[Daniel Berlin via llvm-commits]

On Tue, Sep 26, 2017 at 6:18 PM, Sanjoy Das <sanjoy at playingwithpointers.com>
wrote:

> On Tue, Sep 26, 2017 at 5:57 PM, Daniel Berlin <dberlin at dberlin.org>
> wrote:
> > FWIW I suspect MemorySSAAliasSets will be too slow for use by anything
> for
> > real.
>
> I was under the impression is that it should be no worse than
> AliasSetTracker,


This is true, but AST is badly N^2 already, so realize i'm in the boat of
"AST is too slow for anything to use for real".


I also very much do not want to see *new* passes using MemorySSAAliasSets
to do anything.




> and better in some cases (large blocks with very few
> memory operations).


In the case of promoting loads alone, AST is not necessary at all.
In the case of promoting loads and stores, the way our LICM does it means
it's still going to be N^2.
You will save some walking time.

You also end up back in a world where clients are trying to do direct
aliasing queries, which sucks.


> Otherwise I don't see a reason to do this.
>

I'm torn.
It buys you something, but i also suspect at this level of complexity, we
could have just rewritten licm and build a memoryssi or a multi-phi
memoryssa or whatever it took to make handling stores fast.

Sigh.
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