[DSE] Don't eagerly optimize MemorySSA uses

[aeubanks]

Summary

Compile time improvements:
https://llvm-compile-time-tracker.com/compare.php?from=a4a2b62495a63516a4f782acff1b19361906546b&to=a408521f71702a5c5fb65077adc23413d8631cfc&stat=instructions:u

[aeubanks]

update

[aeubanks]

depends on https://reviews.llvm.org/D152743

[nikic]

Can you please explain in more detail what the relation to D152743 is here?

[aeubanks]

With optimized uses, here in some cases we'd see more uses of a MemoryUse and this would imprecisely return Ref instead of NoModRef. I think it's because this bypasses the logic in D152743, but I didn't double check that

example where this happened

[fhahn]

Looking at the compiler-time tracker link, there are some codegen changes. Do you know where those are coming from and if we could avoid regressions?

[nikic]

In D152744#4416847, @fhahn wrote:

Looking at the compiler-time tracker link, there are some codegen changes. Do you know where those are coming from and if we could avoid regressions?

I think some codegen changes are expected, because DSE walks over uses, and this will mean that more "irrelevant" uses are visited that count towards the scan limit. The MemorySSAScanLimit is fairly generous at 150 accesses, so it's probably okay in practice, though we could also raise the limit if this causes performance regressions.

[nikic]

LGTM, assuming you did not see any perf regression from this.

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

 
 static bool eliminateDeadStores(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,
                                 DominatorTree &DT, PostDominatorTree &PDT,
                                 AssumptionCache &AC,
                                 const TargetLibraryInfo &TLI,
                                 const LoopInfo &LI) {
   bool MadeChange = false;
 
-  MSSA.ensureOptimizedUses();
   DSEState State(F, AA, MSSA, DT, PDT, AC, TLI, LI);
   // For each store:
   for (unsigned I = 0; I < State.MemDefs.size(); I++) {
     MemoryDef *KillingDef = State.MemDefs[I];
     if (State.SkipStores.count(KillingDef))
       continue;
     Instruction *KillingI = KillingDef->getMemoryInst();