[PATCH] D130648: [SCEV] Avoid repeated proveNoUnsignedWrapViaInduction calls.

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.
fhahn added reviewers: reames, nikic, mkazantsev.
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At the moment, proveNoUnsignedWrapViaInduction may be called for the
same AddRec a large number of times via getZeroExtendExpr. This can have
a severe compile-time impact for very loop-heavy code. One one
particular workload, LSR takes ~51s without this patch, almost
exlusively in proveNoUnsignedWrapViaInduction. With this patch, the time
in LSR drops to ~0.4s.

If proveNoUnsignedWrapViaInduction failed to prove NUW the first time,
it is unlikely to succeed on subsequent tries and the cost doesn't seem
to be justified.

Besides drastically improving compile-time in some excessive cases, this
also has a slightly positive compile-time impact on CTMark:

NewPM-O3: -0.07%
NewPM-ReleaseThinLTO: -0.08%
NewPM-ReleaseLTO-g: -0.06

https://llvm-compile-time-tracker.com/compare.php?from=b435da027d7774c24cdb8c88d09f6b771e07fb14&to=f2729e33e8284b502f6c35a43345272252f35d12&stat=instructions


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D130648

Files:
  llvm/include/llvm/Analysis/ScalarEvolution.h
  llvm/lib/Analysis/ScalarEvolution.cpp


Index: llvm/lib/Analysis/ScalarEvolution.cpp
===================================================================
--- llvm/lib/Analysis/ScalarEvolution.cpp
+++ llvm/lib/Analysis/ScalarEvolution.cpp
@@ -5007,6 +5007,10 @@
   if (!AR->isAffine())
     return Result;
 
+  // This function can be expensive, only try to prove NUW once per AddRec.
+  if (!UnsignedWrapViaInductionTried.insert(AR).second)
+    return Result;
+
   const SCEV *Step = AR->getStepRecurrence(*this);
   unsigned BitWidth = getTypeSizeInBits(AR->getType());
   const Loop *L = AR->getLoop();
@@ -13580,6 +13584,9 @@
   HasRecMap.erase(S);
   MinTrailingZerosCache.erase(S);
 
+  if (auto *AR = dyn_cast<SCEVAddRecExpr>(S))
+    UnsignedWrapViaInductionTried.erase(AR);
+
   auto ExprIt = ExprValueMap.find(S);
   if (ExprIt != ExprValueMap.end()) {
     for (Value *V : ExprIt->second) {
Index: llvm/include/llvm/Analysis/ScalarEvolution.h
===================================================================
--- llvm/include/llvm/Analysis/ScalarEvolution.h
+++ llvm/include/llvm/Analysis/ScalarEvolution.h
@@ -2115,6 +2115,10 @@
            std::pair<const SCEV *, SmallVector<const SCEVPredicate *, 3>>>
       PredicatedSCEVRewrites;
 
+  /// Set of AddRecs for which proving NUW via an induction has already been
+  /// tried.
+  SmallPtrSet<const SCEVAddRecExpr *, 16> UnsignedWrapViaInductionTried;
+
   /// The head of a linked list of all SCEVUnknown values that have been
   /// allocated. This is used by releaseMemory to locate them all and call
   /// their destructors.


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