[LLVMdev] Stale SCEV pointer to deleted AssumptionCache

[Steve King]

Hello LLVM,
I have a small-ish custom pass that runs after LoopStrengthReduce.
After a recent upstream merge, this custom pass caused an impossible
assert(), the root cause of which is that the SCEV pass has a stale
pointer to a deleted AssumptionCache.  Specifically:

bool ScalarEvolution::runOnFunction(Function &F) {
...
  AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
 ...

AC is long lived raw pointer to memory owned by a unique_ptr.  Sounds bad.

The sequence of events is quite hairy, but what seems to be happening
is that PMDataManager::removeDeadPasses() blows away the
AssumptionCache data, while the SCEV object and its dangling pointer
live on.  As far as I can tell, all the passes have correctly
registered their dependencies.

My previous upstream merge was June 29, and everything worked fine then.

I'm a little stumped as to what the real problem might be.  Pointers
for chasing this own would be much appreciated.

Thanks,
-steve