[llvm-dev] RFC: Contained stateful AliasAnalysis

[Finkel, Hal J. via llvm-dev]

On 3/4/19 7:08 PM, Alina Sbirlea via llvm-dev wrote:
TL;DR: I'm looking to have AliasAnalysis passes have the ability keep a temporary cache when no transformations are performed.

I'm interested to first and foremost clarify what is the best way to even start such an infrastructure change, such that it is not abused (or even available) by other passes. We certainly don't want to keep arbitrary caches in all passes.
Would making this a feature used exclusively by MemorySSA make sense?
Only from other Analysis?

The usecases I have stem from BasicAA and TBAA being used to build MemorySSA.
MemorySSA is essentially a cache itself, and we're building it with a stateless tool.
We know that while building MemorySSA, there are no changes in the program, and yet, there is info that is computed again and again.

One example is this patch: D57627<https://reviews.llvm.org/D57627>. The patch adds a cache for PointerMayBeCaptured, which is cleared after every alias() call, in order to keep BasicAA stateless. In the example given in the patch, not clearing the isCapturedCache reduces compile times by another second. In some pathological cases I'm looking at, not-clearing this cache halves compile times (from 8s to 4s).
There are a few other examples such as TBAA's getLeastCommonType and BasicAA's DecomposeGEP.

Ideally we'd would have something along the lines of:
void keepCaches()
void clearCaches(),
with the first one called just before building MemorySSA, and the second one called afterwards.

Again, I'm looking into how to make this available but also contain the lifespan of these internal caches to only "as long as it takes for this other analysis pass to be built".


Why then? Isn't the information valid so long as the IR is not mutated?

 -Hal


Suggestions and feedback are most welcome!

Thanks,
Alina





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--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory
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