1 files changed, 41 insertions, 2 deletions

diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 11b33326fb7..1a0a106ee1f 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -254,7 +254,10 @@ static Value *GetLinearExpression(Value *V, APInt &Scale, APInt &Offset,
     Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension,
                                         DL, Depth+1, AT, DT);
     Scale = Scale.zext(OldWidth);
-    Offset = Offset.zext(OldWidth);
+
+    // We have to sign-extend even if Extension == EK_ZeroExt as we can't
+    // decompose a sign extension (i.e. zext(x - 1) != zext(x) - zext(-1)).
+    Offset = Offset.sext(OldWidth);
 
     return Result;
   }
@@ -1055,8 +1058,38 @@ BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
   // Grab the least significant bit set in any of the scales.
   if (!GEP1VariableIndices.empty()) {
     uint64_t Modulo = 0;
-    for (unsigned i = 0, e = GEP1VariableIndices.size(); i != e; ++i)
+    bool AllPositive = true;
+    for (unsigned i = 0, e = GEP1VariableIndices.size();
+         i != e && AllPositive; ++i) {
+      const Value *V = GEP1VariableIndices[i].V;
       Modulo |= (uint64_t)GEP1VariableIndices[i].Scale;
+
+      bool SignKnownZero, SignKnownOne;
+      ComputeSignBit(
+        const_cast<Value *>(V),
+        SignKnownZero, SignKnownOne,
+        DL, 0, AT, nullptr, DT);
+
+      // Zero-extension widens the variable, and so forces the sign
+      // bit to zero.
+      bool IsZExt = GEP1VariableIndices[i].Extension == EK_ZeroExt;
+      SignKnownZero |= IsZExt;
+      SignKnownOne &= !IsZExt;
+
+      // If the variable begins with a zero then we know it's
+      // positive, regardless of whether the value is signed or
+      // unsigned.
+      int64_t Scale = GEP1VariableIndices[i].Scale;
+      AllPositive &= 
+        (SignKnownZero && Scale >= 0) ||
+        (SignKnownOne && Scale < 0);
+
+      // If the Value is currently positive but could change in a cycle,
+      // then we can't guarantee it'll always br positive.
+      if (AllPositive && !isValueEqualInPotentialCycles(V, V))
+        AllPositive = false;
+    }
+
     Modulo = Modulo ^ (Modulo & (Modulo - 1));
 
     // We can compute the difference between the two addresses
@@ -1066,6 +1099,12 @@ BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
     if (V1Size != UnknownSize && V2Size != UnknownSize &&
         ModOffset >= V2Size && V1Size <= Modulo - ModOffset)
       return NoAlias;
+
+    // If we know all the variables are positive, then GEP1 >= GEP1BasePtr.
+    // If GEP1BasePtr > V2 (GEP1BaseOffset > 0) then we know the pointers
+    // don't alias if V2Size can fit in the gap between V2 and GEP1BasePtr.
+    if (AllPositive && GEP1BaseOffset > 0 && V2Size <= (uint64_t) GEP1BaseOffset)
+      return NoAlias;
   }
 
   // Statically, we can see that the base objects are the same, but the