Don't try to rotate a loop more than once - we never do this anyway.

Summary:
I can't find a case where we can rotate a loop more than once, and it looks
like we never do this. To rotate a loop following conditions should be met:
1) its header should be exiting
2) its latch shouldn't be exiting

But after the first rotation the header becomes the new latch, so this
condition can never be true any longer.

Tested on with an assert on LNT testsuite and make check.

Reviewers: hfinkel, sanjoy

Subscribers: sebpop, sanjoy, llvm-commits, mzolotukhin

Differential Revision: http://reviews.llvm.org/D20181

llvm-svn: 272439

llvm/lib/Transforms/Scalar/LoopRotation.cpp

@@ -569,12 +569,10 @@ static bool simplifyLoopLatch(Loop *L, LoopInfo *LI, DominatorTree *DT) {
   return true;
 }
 
-/// Rotate \c L as many times as possible. Return true if the loop is rotated
-/// at least once.
-static bool iterativelyRotateLoop(Loop *L, unsigned MaxHeaderSize, LoopInfo *LI,
-                                  const TargetTransformInfo *TTI,
-                                  AssumptionCache *AC, DominatorTree *DT,
-                                  ScalarEvolution *SE) {
+/// Rotate \c L, and return true if any modification was made.
+static bool processLoop(Loop *L, unsigned MaxHeaderSize, LoopInfo *LI,
+                        const TargetTransformInfo *TTI, AssumptionCache *AC,
+                        DominatorTree *DT, ScalarEvolution *SE) {
   // Save the loop metadata.
   MDNode *LoopMD = L->getLoopID();
 
@@ -583,12 +581,10 @@ static bool iterativelyRotateLoop(Loop *L, unsigned MaxHeaderSize, LoopInfo *LI,
   // loop exit.
   bool SimplifiedLatch = simplifyLoopLatch(L, LI, DT);
 
-  // One loop can be rotated multiple times.
-  bool MadeChange = false;
-  while (rotateLoop(L, MaxHeaderSize, LI, TTI, AC, DT, SE, SimplifiedLatch)) {
-    MadeChange = true;
-    SimplifiedLatch = false;
-  }
+  bool MadeChange =
+      rotateLoop(L, MaxHeaderSize, LI, TTI, AC, DT, SE, SimplifiedLatch);
+  assert((!MadeChange || L->isLoopExiting(L->getLoopLatch())) &&
+         "Loop latch should be exiting after loop-rotate.");
 
   // Restore the loop metadata.
   // NB! We presume LoopRotation DOESN'T ADD its own metadata.
@@ -613,7 +609,7 @@ PreservedAnalyses LoopRotatePass::run(Loop &L, AnalysisManager<Loop> &AM) {
   auto *DT = FAM.getCachedResult<DominatorTreeAnalysis>(*F);
   auto *SE = FAM.getCachedResult<ScalarEvolutionAnalysis>(*F);
 
-  bool Changed = iterativelyRotateLoop(&L, MaxHeaderSize, LI, TTI, AC, DT, SE);
+  bool Changed = processLoop(&L, MaxHeaderSize, LI, TTI, AC, DT, SE);
   if (!Changed)
     return PreservedAnalyses::all();
   return getLoopPassPreservedAnalyses();
@@ -654,7 +650,7 @@ class LoopRotateLegacyPass : public LoopPass {
     auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
     auto *SE = SEWP ? &SEWP->getSE() : nullptr;
 
-    return iterativelyRotateLoop(L, MaxHeaderSize, LI, TTI, AC, DT, SE);
+    return processLoop(L, MaxHeaderSize, LI, TTI, AC, DT, SE);
   }
 };
 }