[PATCH] D130694: [SCEV] Avoid repeated proveNoSignedWrapViaInduction calls.

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.
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At the moment, proveNoSignedWrapViaInduction may be called for the
same AddRec a large number of times via getSignExtendExpr. This can have
a severe compile-time impact for very loop-heavy code.

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

This is the signed version of 8daa338297d533d <https://reviews.llvm.org/rG8daa338297d533db4d1ae8d3770613eb25c29688> / D130648 <https://reviews.llvm.org/D130648>.

This can drastically improve compile-time in some excessive cases and
also has a slightly positive compile-time impact on CTMark:

NewPM-O3: -0.06%
NewPM-ReleaseThinLTO: -0.04%
NewPM-ReleaseLTO-g: -0.04%

https://llvm-compile-time-tracker.com/compare.php?from=8daa338297d533db4d1ae8d3770613eb25c29688&to=aed126a196e7a5a9803543d9b4d6bdb233d0009c&stat=instructions


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D130694

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
@@ -4958,6 +4958,10 @@
   if (!AR->isAffine())
     return Result;
 
+  // This function can be expensive, only try to prove NSW once per AddRec.
+  if (!SignedWrapViaInductionTried.insert(AR).second)
+    return Result;
+
   const SCEV *Step = AR->getStepRecurrence(*this);
   const Loop *L = AR->getLoop();
 
@@ -13584,8 +13588,10 @@
   HasRecMap.erase(S);
   MinTrailingZerosCache.erase(S);
 
-  if (auto *AR = dyn_cast<SCEVAddRecExpr>(S))
+  if (auto *AR = dyn_cast<SCEVAddRecExpr>(S)) {
     UnsignedWrapViaInductionTried.erase(AR);
+    SignedWrapViaInductionTried.erase(AR);
+  }
 
   auto ExprIt = ExprValueMap.find(S);
   if (ExprIt != ExprValueMap.end()) {
Index: llvm/include/llvm/Analysis/ScalarEvolution.h
===================================================================
--- llvm/include/llvm/Analysis/ScalarEvolution.h
+++ llvm/include/llvm/Analysis/ScalarEvolution.h
@@ -2119,6 +2119,10 @@
   /// tried.
   SmallPtrSet<const SCEVAddRecExpr *, 16> UnsignedWrapViaInductionTried;
 
+  /// Set of AddRecs for which proving NSW via an induction has already been
+  /// tried.
+  SmallPtrSet<const SCEVAddRecExpr *, 16> SignedWrapViaInductionTried;
+
   /// 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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