diff --git a/llvm/include/llvm/IR/IRBuilder.h b/llvm/include/llvm/IR/IRBuilder.h
--- a/llvm/include/llvm/IR/IRBuilder.h
+++ b/llvm/include/llvm/IR/IRBuilder.h
@@ -540,6 +540,11 @@
     return Type::getVoidTy(Context);
   }
 
+  /// Fetch the type representing a pointer.
+  PointerType *getPtrTy(unsigned AddrSpace = 0) {
+    return PointerType::get(Context, AddrSpace);
+  }
+
   /// Fetch the type representing a pointer to an 8-bit integer value.
   PointerType *getInt8PtrTy(unsigned AddrSpace = 0) {
     return Type::getInt8PtrTy(Context, AddrSpace);
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
@@ -1391,15 +1391,15 @@
 
   // If both arms of the Select node NoAlias or MustAlias V2, then returns
   // NoAlias / MustAlias. Otherwise, returns MayAlias.
-  AliasResult Alias = getBestAAResults().alias(
-      MemoryLocation(V2, V2Size),
-      MemoryLocation(SI->getTrueValue(), SISize), AAQI);
+  AliasResult Alias =
+      getBestAAResults().alias(MemoryLocation(SI->getTrueValue(), SISize),
+                               MemoryLocation(V2, V2Size), AAQI);
   if (Alias == AliasResult::MayAlias)
     return AliasResult::MayAlias;
 
-  AliasResult ThisAlias = getBestAAResults().alias(
-      MemoryLocation(V2, V2Size),
-      MemoryLocation(SI->getFalseValue(), SISize), AAQI);
+  AliasResult ThisAlias =
+      getBestAAResults().alias(MemoryLocation(SI->getFalseValue(), SISize),
+                               MemoryLocation(V2, V2Size), AAQI);
   return MergeAliasResults(ThisAlias, Alias);
 }
 
@@ -1521,8 +1521,7 @@
   AAQueryInfo *UseAAQI = BlockInserted ? &NewAAQI : &AAQI;
 
   AliasResult Alias = getBestAAResults().alias(
-      MemoryLocation(V2, V2Size),
-      MemoryLocation(V1Srcs[0], PNSize), *UseAAQI);
+      MemoryLocation(V1Srcs[0], PNSize), MemoryLocation(V2, V2Size), *UseAAQI);
 
   // Early exit if the check of the first PHI source against V2 is MayAlias.
   // Other results are not possible.
@@ -1539,7 +1538,7 @@
     Value *V = V1Srcs[i];
 
     AliasResult ThisAlias = getBestAAResults().alias(
-        MemoryLocation(V2, V2Size), MemoryLocation(V, PNSize), *UseAAQI);
+        MemoryLocation(V, PNSize), MemoryLocation(V2, V2Size), *UseAAQI);
     Alias = MergeAliasResults(ThisAlias, Alias);
     if (Alias == AliasResult::MayAlias)
       break;
diff --git a/llvm/unittests/Analysis/BasicAliasAnalysisTest.cpp b/llvm/unittests/Analysis/BasicAliasAnalysisTest.cpp
--- a/llvm/unittests/Analysis/BasicAliasAnalysisTest.cpp
+++ b/llvm/unittests/Analysis/BasicAliasAnalysisTest.cpp
@@ -63,16 +63,17 @@
 
 public:
   BasicAATest()
-      : M("BasicAATest", C), B(C), DL(DLString), TLI(TLII), F(nullptr) {}
+      : M("BasicAATest", C), B(C), DL(DLString), TLI(TLII), F(nullptr) {
+    C.setOpaquePointers(true);
+  }
 };
 
 // Check that a function arg can't trivially alias a global when we're accessing
 // >sizeof(global) bytes through that arg, unless the access size is just an
 // upper-bound.
 TEST_F(BasicAATest, AliasInstWithObjectOfImpreciseSize) {
-  F = Function::Create(
-      FunctionType::get(B.getVoidTy(), {B.getInt32Ty()->getPointerTo()}, false),
-      GlobalValue::ExternalLinkage, "F", &M);
+  F = Function::Create(FunctionType::get(B.getVoidTy(), {B.getPtrTy()}, false),
+                       GlobalValue::ExternalLinkage, "F", &M);
 
   BasicBlock *Entry(BasicBlock::Create(C, "", F));
   B.SetInsertPoint(Entry);
@@ -131,3 +132,71 @@
                 AAQI),
             AliasResult::MayAlias);
 }
+
+TEST_F(BasicAATest, PartialAliasOffsetPhi) {
+  F = Function::Create(
+      FunctionType::get(B.getVoidTy(), {B.getPtrTy(), B.getInt1Ty()}, false),
+      GlobalValue::ExternalLinkage, "F", &M);
+
+  Value *Ptr = F->arg_begin();
+  Value *I = F->arg_begin() + 1;
+
+  BasicBlock *Entry(BasicBlock::Create(C, "", F));
+  BasicBlock *B1(BasicBlock::Create(C, "", F));
+  BasicBlock *B2(BasicBlock::Create(C, "", F));
+  BasicBlock *End(BasicBlock::Create(C, "", F));
+
+  B.SetInsertPoint(Entry);
+  B.CreateCondBr(I, B1, B2);
+
+  B.SetInsertPoint(B1);
+  auto *Ptr1 =
+      cast<GetElementPtrInst>(B.CreateGEP(B.getInt8Ty(), Ptr, B.getInt32(1)));
+  B.CreateBr(End);
+
+  B.SetInsertPoint(B2);
+  auto *Ptr2 =
+      cast<GetElementPtrInst>(B.CreateGEP(B.getInt8Ty(), Ptr, B.getInt32(1)));
+  B.CreateBr(End);
+
+  B.SetInsertPoint(End);
+  auto *Phi = B.CreatePHI(B.getPtrTy(), 2);
+  Phi->addIncoming(Ptr1, B1);
+  Phi->addIncoming(Ptr2, B2);
+  B.CreateRetVoid();
+
+  auto &AllAnalyses = setupAnalyses();
+  BasicAAResult &BasicAA = AllAnalyses.BAA;
+  AAQueryInfo &AAQI = AllAnalyses.AAQI;
+  AliasResult AR =
+      BasicAA.alias(MemoryLocation(Ptr, LocationSize::precise(2)),
+                    MemoryLocation(Phi, LocationSize::precise(1)), AAQI);
+  ASSERT_EQ(AR.getOffset(), 1);
+}
+
+TEST_F(BasicAATest, PartialAliasOffsetSelect) {
+  F = Function::Create(
+      FunctionType::get(B.getVoidTy(), {B.getPtrTy(), B.getInt1Ty()}, false),
+      GlobalValue::ExternalLinkage, "F", &M);
+
+  Value *Ptr = F->arg_begin();
+  Value *I = F->arg_begin() + 1;
+
+  BasicBlock *Entry(BasicBlock::Create(C, "", F));
+  B.SetInsertPoint(Entry);
+
+  auto *Ptr1 =
+      cast<GetElementPtrInst>(B.CreateGEP(B.getInt8Ty(), Ptr, B.getInt32(1)));
+  auto *Ptr2 =
+      cast<GetElementPtrInst>(B.CreateGEP(B.getInt8Ty(), Ptr, B.getInt32(1)));
+  auto *Select = B.CreateSelect(I, Ptr1, Ptr2);
+  B.CreateRetVoid();
+
+  auto &AllAnalyses = setupAnalyses();
+  BasicAAResult &BasicAA = AllAnalyses.BAA;
+  AAQueryInfo &AAQI = AllAnalyses.AAQI;
+  AliasResult AR =
+      BasicAA.alias(MemoryLocation(Ptr, LocationSize::precise(2)),
+                    MemoryLocation(Select, LocationSize::precise(1)), AAQI);
+  ASSERT_EQ(AR.getOffset(), 1);
+}