[PATCH] D57199: [MemorySSA] Extend removeMemoryAccess API to optimize MemoryPhis.

[George Burgess IV via Phabricator via llvm-commits]

george.burgess.iv added a comment.

Thanks for this!

Is there a reason we don't want to have this behavior by default? ISTR we're pretty good about keeping Phis well-optimized, but could be wrong.

Please also include a test-case.



================
Comment at: lib/Analysis/MemorySSAUpdater.cpp:1108
+  for (MemoryPhi *MP : PhisToOptimize)
+    if (onlySingleValue(MP))
+      removeMemoryAccess(MP, true);
----------------
It looks like this may result in a use-after-free. Consider a CFG:

```
|---A
|  / \
| B   C
|/ \ /|
D<--E | 
\     /
 \   /
  \ / 
   F
``` 

Control-flow flows down, except for the edge E -> D. (and there's an A->D edge that only looks somewhat awkward. :) )

All BBs are clear, though B has a Def in it. This Def would cause us to generate a Phi in E, D, and F.

So we remove that Def, remembering that the Phis in {D, E} are removal candidates. We check that E only has `liveOnEntry` first, it does, so we recurse on it. E's removal makes the Phi in D trivial, so we recurse on it, delete it, etc. We then land back at the top level of our stack and check `onlySingleValue(D)`.

If you buy all of this, ISTM we could either fix it by hoisting the `onlySingleValue` check into the `while (!MA->use_empty()) {` loop (if the Phi only points to one Def, we're not going to find it otherwise...). I'm willing to bet that the number of cases where the same Phi points to the same value N times is pretty small.

Alternatively, we could `takeVector()`, completely filter out the Phis that aren't `onlySingleValue`, from that, and *then* recurse on those Phis.


Repository:
  rL LLVM

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