[llvm] 8daa338 - [SCEV] Avoid repeated proveNoUnsignedWrapViaInduction calls.

[Florian Hahn via llvm-commits]

Author: Florian Hahn
Date: 2022-07-28T10:02:19+01:00
New Revision: 8daa338297d533db4d1ae8d3770613eb25c29688

URL: https://github.com/llvm/llvm-project/commit/8daa338297d533db4d1ae8d3770613eb25c29688
DIFF: https://github.com/llvm/llvm-project/commit/8daa338297d533db4d1ae8d3770613eb25c29688.diff

LOG: [SCEV] Avoid repeated proveNoUnsignedWrapViaInduction calls.

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

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D130648

Added: 
    

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

Removed: 
    


################################################################################
diff  --git a/llvm/include/llvm/Analysis/ScalarEvolution.h b/llvm/include/llvm/Analysis/ScalarEvolution.h
index fd00c744840b9..d49775cfd6f0e 100644
--- a/llvm/include/llvm/Analysis/ScalarEvolution.h
+++ b/llvm/include/llvm/Analysis/ScalarEvolution.h
@@ -2115,6 +2115,10 @@ class ScalarEvolution {
            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.

diff  --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 2958a5054afc7..7e1306895a85e 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -5007,6 +5007,10 @@ ScalarEvolution::proveNoUnsignedWrapViaInduction(const SCEVAddRecExpr *AR) {
   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 @@ void ScalarEvolution::forgetMemoizedResultsImpl(const SCEV *S) {
   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) {