The patch fixes PR27392.

Summary:
 It is incorrect to compare TripCount (which is BECount + 1)
  with extraiters (or Count) to check if we should enter unrolled
  loop or not, because TripCount can potentially overflow
  (when BECount is max unsigned integer).
 While comparing BECount with (Count - 1) is overflow safe and
  therefore correct.

Reviewer: hfinkel

Differential Revision: http://reviews.llvm.org/D19256

From: Evgeny Stupachenko <evstupac@gmail.com>
llvm-svn: 267662

1 files changed, 10 insertions, 9 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp b/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
index da66da0e85f..861a50cf354 100644
--- a/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp
@@ -246,7 +246,7 @@ static void ConnectEpilog(Loop *L, Value *ModVal, BasicBlock *NewExit,
 
   Instruction *InsertPt = NewExit->getTerminator();
   IRBuilder<> B(InsertPt);
-  Value *BrLoopExit = B.CreateIsNotNull(ModVal);
+  Value *BrLoopExit = B.CreateIsNotNull(ModVal, "lcmp.mod");
   assert(Exit && "Loop must have a single exit block only");
   // Split the exit to maintain loop canonicalization guarantees
   SmallVector<BasicBlock*, 4> Preds(predecessors(Exit));
@@ -416,7 +416,7 @@ static void CloneLoopBlocks(Loop *L, Value *NewIter,
 ///
 /// ***Epilog case***
 ///        extraiters = tripcount % loopfactor
-///        if (extraiters == tripcount) jump LoopExit:
+///        if (tripcount < loopfactor) jump LoopExit:
 ///        unroll_iters = tripcount - extraiters
 /// Loop:  LoopBody; (executes unroll_iter times);
 ///        unroll_iter -= 1
@@ -575,14 +575,15 @@ bool llvm::UnrollRuntimeLoopRemainder(Loop *L, unsigned Count,
                           ConstantInt::get(BECount->getType(), Count),
                           "xtraiter");
   }
-  Value *CmpOperand =
-      UseEpilogRemainder ? TripCount :
-                           ConstantInt::get(TripCount->getType(), 0);
-  Value *BranchVal = B.CreateICmpNE(ModVal, CmpOperand, "lcmp.mod");
-  BasicBlock *FirstLoop = UseEpilogRemainder ? NewPreHeader : PrologPreHeader;
-  BasicBlock *SecondLoop = UseEpilogRemainder ? NewExit : PrologExit;
+  Value *BranchVal =
+      UseEpilogRemainder ? B.CreateICmpULT(BECount,
+                                           ConstantInt::get(BECount->getType(),
+                                                            Count - 1)) :
+                           B.CreateIsNotNull(ModVal, "lcmp.mod");
+  BasicBlock *RemainderLoop = UseEpilogRemainder ? NewExit : PrologPreHeader;
+  BasicBlock *UnrollingLoop = UseEpilogRemainder ? NewPreHeader : PrologExit;
   // Branch to either remainder (extra iterations) loop or unrolling loop.
-  B.CreateCondBr(BranchVal, FirstLoop, SecondLoop);
+  B.CreateCondBr(BranchVal, RemainderLoop, UnrollingLoop);
   PreHeaderBR->eraseFromParent();
   Function *F = Header->getParent();
   // Get an ordered list of blocks in the loop to help with the ordering of the