Index: llvm/include/llvm/Analysis/BasicAliasAnalysis.h
===================================================================
--- llvm/include/llvm/Analysis/BasicAliasAnalysis.h
+++ llvm/include/llvm/Analysis/BasicAliasAnalysis.h
@@ -118,6 +118,9 @@
 
     APInt Scale;
 
+    // Context instruction to use when querying information about this index.
+    const Instruction *CxtI;
+
     bool operator==(const VariableGEPIndex &Other) const {
       return V == Other.V && ZExtBits == Other.ZExtBits &&
              SExtBits == Other.SExtBits && Scale == Other.Scale;
Index: llvm/lib/Analysis/BasicAliasAnalysis.cpp
===================================================================
--- llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -422,6 +422,7 @@
   // Limit recursion depth to limit compile time in crazy cases.
   unsigned MaxLookup = MaxLookupSearchDepth;
   SearchTimes++;
+  const Instruction *CxtI = dyn_cast<Instruction>(V);
 
   unsigned MaxPointerSize = getMaxPointerSize(DL);
   DecomposedGEP Decomposed;
@@ -584,7 +585,7 @@
       Scale = adjustToPointerSize(Scale, PointerSize);
 
       if (!!Scale) {
-        VariableGEPIndex Entry = {Index, ZExtBits, SExtBits, Scale};
+        VariableGEPIndex Entry = {Index, ZExtBits, SExtBits, Scale, CxtI};
         Decomposed.VarIndices.push_back(Entry);
       }
     }
@@ -1237,9 +1238,9 @@
         // the Value this cycle may not hold in the next cycle. We'll just
         // give up if we can't determine conditions that hold for every cycle:
         const Value *V = DecompGEP1.VarIndices[i].V;
+        const Instruction *CxtI = DecompGEP1.VarIndices[i].CxtI;
 
-        KnownBits Known =
-            computeKnownBits(V, DL, 0, &AC, dyn_cast<Instruction>(GEP1), DT);
+        KnownBits Known = computeKnownBits(V, DL, 0, &AC, CxtI, DT);
         bool SignKnownZero = Known.isNonNegative();
         bool SignKnownOne = Known.isNegative();
 
@@ -1767,7 +1768,7 @@
 
     // If we didn't consume this entry, add it to the end of the Dest list.
     if (!!Scale) {
-      VariableGEPIndex Entry = {V, ZExtBits, SExtBits, -Scale};
+      VariableGEPIndex Entry = {V, ZExtBits, SExtBits, -Scale, Src[i].CxtI};
       Dest.push_back(Entry);
     }
   }
Index: llvm/test/Analysis/BasicAA/assume-index-positive.ll
===================================================================
--- llvm/test/Analysis/BasicAA/assume-index-positive.ll
+++ llvm/test/Analysis/BasicAA/assume-index-positive.ll
@@ -113,4 +113,20 @@
   ret void
 }
 
+define void @symmetry([0 x i8]* %ptr, i32 %a, i32 %b, i32 %c) {
+; CHECK-LABEL: Function: symmetry
+; CHECK: NoAlias: i8* %gep1, i8* %gep2
+;
+  %b.cmp = icmp slt i32 %b, 0
+  call void @llvm.assume(i1 %b.cmp)
+  %gep1 = getelementptr [0 x i8], [0 x i8]* %ptr, i32 %a, i32 %b
+  call void @barrier()
+  %c.cmp = icmp sgt i32 %c, -1
+  call void @llvm.assume(i1 %c.cmp)
+  %c.off = add nuw nsw i32 %c, 1
+  %gep2 = getelementptr [0 x i8], [0 x i8]* %ptr, i32 %a, i32 %c.off
+  ret void
+}
+
 declare void @llvm.assume(i1 %cond)
+declare void @barrier()