[PATCH] Partially fix PR20058: reduce compile time for loop unrolling with very high count

[Mark Heffernan]

http://reviews.llvm.org/D4285

Files:
  lib/Transforms/Utils/LoopUnroll.cpp

Index: lib/Transforms/Utils/LoopUnroll.cpp
===================================================================
--- lib/Transforms/Utils/LoopUnroll.cpp
+++ lib/Transforms/Utils/LoopUnroll.cpp
@@ -17,6 +17,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Transforms/Utils/UnrollLoop.h"
+#include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/InstructionSimplify.h"
 #include "llvm/Analysis/LoopIterator.h"
@@ -61,12 +62,19 @@
   }
 }
 
-/// FoldBlockIntoPredecessor - Folds a basic block into its predecessor if it
-/// only has one predecessor, and that predecessor only has one successor.
-/// The LoopInfo Analysis that is passed will be kept consistent.
-/// Returns the new combined block.
-static BasicBlock *FoldBlockIntoPredecessor(BasicBlock *BB, LoopInfo* LI,
-                                            LPPassManager *LPM) {
+/// FoldBlockIntoPredecessor - Folds a basic block into its
+/// predecessor if it only has one predecessor, and that predecessor
+/// only has one successor.  The LoopInfo Analysis that is passed will
+/// be kept consistent.  If folding is successful references to the
+/// containing loop must be removed from ScalarEvolution by calling
+/// ScalarEvolution::forgetLoop because SE may have references to the
+/// eliminated BB.  The argument ForgottenLoops contains a set of
+/// loops that have already been forgotten to prevent redundant,
+/// expensive calls to ScalarEvolution::forgetLoop.  Returns the new
+/// combined block.
+static BasicBlock *
+FoldBlockIntoPredecessor(BasicBlock *BB, LoopInfo* LI, LPPassManager *LPM,
+                         SmallPtrSetImpl<Loop *> &ForgottenLoops) {
   // Merge basic blocks into their predecessor if there is only one distinct
   // pred, and if there is only one distinct successor of the predecessor, and
   // if there are no PHI nodes.
@@ -103,8 +111,10 @@
   // ScalarEvolution holds references to loop exit blocks.
   if (LPM) {
     if (ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>()) {
-      if (Loop *L = LI->getLoopFor(BB))
-        SE->forgetLoop(L);
+      if (Loop *L = LI->getLoopFor(BB)) {
+        if (ForgottenLoops.insert(L))
+          SE->forgetLoop(L);
+      }
     }
   }
   LI->removeBlock(BB);
@@ -418,11 +428,13 @@
   }
 
   // Merge adjacent basic blocks, if possible.
+  SmallPtrSet<Loop *, 4> ForgottenLoops;
   for (unsigned i = 0, e = Latches.size(); i != e; ++i) {
     BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator());
     if (Term->isUnconditional()) {
       BasicBlock *Dest = Term->getSuccessor(0);
-      if (BasicBlock *Fold = FoldBlockIntoPredecessor(Dest, LI, LPM))
+      if (BasicBlock *Fold = FoldBlockIntoPredecessor(Dest, LI, LPM,
+                                                      ForgottenLoops))
         std::replace(Latches.begin(), Latches.end(), Dest, Fold);
     }
   }
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