2 files changed, 27 insertions, 11 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 9a87667827a..f0c02eec4e2 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -4077,13 +4077,6 @@ static OverflowResult mapOverflowResult(ConstantRange::OverflowResult OR) {
   llvm_unreachable("Unknown OverflowResult");
 }
 
-static ConstantRange constantRangeFromKnownBits(const KnownBits &Known) {
-  if (Known.isUnknown())
-    return ConstantRange(Known.getBitWidth(), /* full */ true);
-
-  return ConstantRange(Known.One, ~Known.Zero + 1);
-}
-
 OverflowResult llvm::computeOverflowForUnsignedAdd(
     const Value *LHS, const Value *RHS, const DataLayout &DL,
     AssumptionCache *AC, const Instruction *CxtI, const DominatorTree *DT,
@@ -4092,8 +4085,10 @@ OverflowResult llvm::computeOverflowForUnsignedAdd(
                                         nullptr, UseInstrInfo);
   KnownBits RHSKnown = computeKnownBits(RHS, DL, /*Depth=*/0, AC, CxtI, DT,
                                         nullptr, UseInstrInfo);
-  ConstantRange LHSRange = constantRangeFromKnownBits(LHSKnown);
-  ConstantRange RHSRange = constantRangeFromKnownBits(RHSKnown);
+  ConstantRange LHSRange =
+      ConstantRange::fromKnownBits(LHSKnown, /*signed*/ false);
+  ConstantRange RHSRange =
+      ConstantRange::fromKnownBits(RHSKnown, /*signed*/ false);
   return mapOverflowResult(LHSRange.unsignedAddMayOverflow(RHSRange));
 }
 
@@ -4208,8 +4203,10 @@ OverflowResult llvm::computeOverflowForUnsignedSub(const Value *LHS,
                                                    const DominatorTree *DT) {
   KnownBits LHSKnown = computeKnownBits(LHS, DL, /*Depth=*/0, AC, CxtI, DT);
   KnownBits RHSKnown = computeKnownBits(RHS, DL, /*Depth=*/0, AC, CxtI, DT);
-  ConstantRange LHSRange = constantRangeFromKnownBits(LHSKnown);
-  ConstantRange RHSRange = constantRangeFromKnownBits(RHSKnown);
+  ConstantRange LHSRange =
+      ConstantRange::fromKnownBits(LHSKnown, /*signed*/ false);
+  ConstantRange RHSRange =
+      ConstantRange::fromKnownBits(RHSKnown, /*signed*/ false);
   return mapOverflowResult(LHSRange.unsignedSubMayOverflow(RHSRange));
 }
 
diff --git a/llvm/lib/IR/ConstantRange.cpp b/llvm/lib/IR/ConstantRange.cpp
index 23737bb4650..d41914ba4a9 100644
--- a/llvm/lib/IR/ConstantRange.cpp
+++ b/llvm/lib/IR/ConstantRange.cpp
@@ -31,6 +31,7 @@
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/KnownBits.h"
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
 #include <cassert>
@@ -53,6 +54,24 @@ ConstantRange::ConstantRange(APInt L, APInt U)
          "Lower == Upper, but they aren't min or max value!");
 }
 
+ConstantRange ConstantRange::fromKnownBits(const KnownBits &Known,
+                                           bool IsSigned) {
+  if (Known.isUnknown())
+    return ConstantRange(Known.getBitWidth(), /* full */ true);
+
+  // For unsigned ranges, or signed ranges with known sign bit, create a simple
+  // range between the smallest and largest possible value.
+  if (!IsSigned || Known.isNegative() || Known.isNonNegative())
+    return ConstantRange(Known.One, ~Known.Zero + 1);
+
+  // If we don't know the sign bit, pick the lower bound as a negative number
+  // and the upper bound as a non-negative one.
+  APInt Lower = Known.One, Upper = ~Known.Zero;
+  Lower.setSignBit();
+  Upper.clearSignBit();
+  return ConstantRange(Lower, Upper + 1);
+}
+
 ConstantRange ConstantRange::makeAllowedICmpRegion(CmpInst::Predicate Pred,
                                                    const ConstantRange &CR) {
   if (CR.isEmptySet())