diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1336,9 +1336,7 @@
       // ---------------->|
       // |-->V1Size       |-------> V2Size
       // GEP1             V2
-      // We need to know that V2Size is not unknown, otherwise we might have
-      // stripped a gep with negative index ('gep <ptr>, -1, ...).
-      if (V1Size.hasValue() && V2Size.hasValue()) {
+      if (V1Size.hasValue()) {
         if ((-DecompGEP1.Offset).ult(V1Size.getValue()))
           return PartialAlias;
         return NoAlias;
diff --git a/llvm/test/Analysis/BasicAA/negoffset.ll b/llvm/test/Analysis/BasicAA/negoffset.ll
--- a/llvm/test/Analysis/BasicAA/negoffset.ll
+++ b/llvm/test/Analysis/BasicAA/negoffset.ll
@@ -117,3 +117,13 @@
   %2 = load i32, i32* %1
   ret i32 %2
 }
+
+; CHECK-LABEL: Function: one_size_unknown:
+; CHECK: NoModRef:  Ptr: i8* %p.minus1	<->  call void @llvm.memset.p0i8.i32(i8* %p, i8 0, i32 %size, i1 false)
+define void @one_size_unknown(i8* %p, i32 %size) {
+  %p.minus1 = getelementptr inbounds i8, i8* %p, i32 -1
+  call void @llvm.memset.p0i8.i32(i8* %p, i8 0, i32 %size, i1 false)
+  ret void
+}
+
+declare void @llvm.memset.p0i8.i32(i8*, i8, i32, i1)