[SCEVExpander] Skip creating <u 0 check, which is always false.

Unsigned compares of the form <u 0 are always false. Do not create such
a redundant check in generateOverflowCheck.

The patch introduces a new lambda to create the check, so we can
exit early conveniently and skip creating some instructions feeding the
check.

I am planning to sink a few additional instructions as follow-ups, but I
would prefer to do this separately, to keep the changes and diff
smaller.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D116811

1 files changed, 38 insertions, 30 deletions

diff --git a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
index 136dfae..b749847 100644
--- a/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
+++ b/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp
@@ -2516,38 +2516,46 @@ Value *SCEVExpander::generateOverflowCheck(const SCEVAddRecExpr *AR,
   // 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);
-    if (NeedPosCheck)
-      Add = Builder.CreateGEP(Builder.getInt8Ty(), StartValue, MulV);
-    if (NeedNegCheck)
-      Sub = Builder.CreateGEP(Builder.getInt8Ty(), StartValue, NegMulV);
-  } else {
+  auto ComputeEndCheck = [&]() -> Value * {
+    // Checking <u 0 is always false.
+    if (!Signed && Start->isZero() && SE.isKnownPositive(Step))
+      return ConstantInt::getFalse(Loc->getContext());
+
+    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);
+      if (NeedPosCheck)
+        Add = Builder.CreateGEP(Builder.getInt8Ty(), StartValue, MulV);
+      if (NeedNegCheck)
+        Sub = Builder.CreateGEP(Builder.getInt8Ty(), StartValue, NegMulV);
+    } else {
+      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)
-      Add = Builder.CreateAdd(StartValue, MulV);
+      EndCheck = EndCompareLT = Builder.CreateICmp(
+          Signed ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT, Add, StartValue);
     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);
-  }
+      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);
+    }
+    return EndCheck;
+  };
+  Value *EndCheck = ComputeEndCheck();
 
   // 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