1 files changed, 31 insertions, 16 deletions

diff --git a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
index 0bb3c8b..136dfae 100644
--- a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
+++ b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
@@ -2510,30 +2510,45 @@ Value *SCEVExpander::generateOverflowCheck(const SCEVAddRecExpr *AR,
   }
 
   // Compute:
-  //   Start + |Step| * Backedge < Start
-  //   Start - |Step| * Backedge > Start
+  //   1. Start + |Step| * Backedge < Start
+  //   2. Start - |Step| * Backedge > Start
+  //
+  // And select either 1. or 2. depending on whether step is positive or
+  // negative. If Step is known to be positive or negative, only create
+  // either 1. or 2.
   Value *Add = nullptr, *Sub = nullptr;
+  bool NeedPosCheck = !SE.isKnownNegative(Step);
+  bool NeedNegCheck = !SE.isKnownPositive(Step);
+
   if (PointerType *ARPtrTy = dyn_cast<PointerType>(ARTy)) {
     StartValue = InsertNoopCastOfTo(
         StartValue, Builder.getInt8PtrTy(ARPtrTy->getAddressSpace()));
     Value *NegMulV = Builder.CreateNeg(MulV);
-    Add = Builder.CreateGEP(Builder.getInt8Ty(), StartValue, MulV);
-    Sub = Builder.CreateGEP(Builder.getInt8Ty(), StartValue, NegMulV);
+    if (NeedPosCheck)
+      Add = Builder.CreateGEP(Builder.getInt8Ty(), StartValue, MulV);
+    if (NeedNegCheck)
+      Sub = Builder.CreateGEP(Builder.getInt8Ty(), StartValue, NegMulV);
   } else {
-    Add = Builder.CreateAdd(StartValue, MulV);
-    Sub = Builder.CreateSub(StartValue, MulV);
+    if (NeedPosCheck)
+      Add = Builder.CreateAdd(StartValue, MulV);
+    if (NeedNegCheck)
+      Sub = Builder.CreateSub(StartValue, MulV);
+  }
+
+  Value *EndCompareLT = nullptr;
+  Value *EndCompareGT = nullptr;
+  Value *EndCheck = nullptr;
+  if (NeedPosCheck)
+    EndCheck = EndCompareLT = Builder.CreateICmp(
+        Signed ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT, Add, StartValue);
+  if (NeedNegCheck)
+    EndCheck = EndCompareGT = Builder.CreateICmp(
+        Signed ? ICmpInst::ICMP_SGT : ICmpInst::ICMP_UGT, Sub, StartValue);
+  if (NeedPosCheck && NeedNegCheck) {
+    // Select the answer based on the sign of Step.
+    EndCheck = Builder.CreateSelect(StepCompare, EndCompareGT, EndCompareLT);
   }
 
-  Value *EndCompareGT = Builder.CreateICmp(
-      Signed ? ICmpInst::ICMP_SGT : ICmpInst::ICMP_UGT, Sub, StartValue);
-
-  Value *EndCompareLT = Builder.CreateICmp(
-      Signed ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT, Add, StartValue);
-
-  // Select the answer based on the sign of Step.
-  Value *EndCheck =
-      Builder.CreateSelect(StepCompare, EndCompareGT, EndCompareLT);
-
   // If the backedge taken count type is larger than the AR type,
   // check that we don't drop any bits by truncating it. If we are
   // dropping bits, then we have overflow (unless the step is zero).