Index: lib/Analysis/Loads.cpp
===================================================================
--- lib/Analysis/Loads.cpp
+++ lib/Analysis/Loads.cpp
@@ -100,9 +100,12 @@
     return isDereferenceableAndAlignedPointer(
         RelocateInst->getDerivedPtr(), Align, Size, DL, CtxI, DT, Visited);
 
-  if (const AddrSpaceCastInst *ASC = dyn_cast<AddrSpaceCastInst>(V))
-    return isDereferenceableAndAlignedPointer(ASC->getOperand(0), Align, Size,
-                                              DL, CtxI, DT, Visited);
+  if (const AddrSpaceCastInst *ASC = dyn_cast<AddrSpaceCastInst>(V)) {
+    const Value *Ptr = ASC->getPointerOperand();
+    if (Size.getBitWidth() == DL.getPointerTypeSizeInBits(Ptr->getType()))
+      return isDereferenceableAndAlignedPointer(ASC->getOperand(0), Align,
+                                                Size, DL, CtxI, DT, Visited);
+  }
 
   if (auto CS = ImmutableCallSite(V))
     if (const Value *RV = CS.getReturnedArgOperand())
Index: lib/IR/Value.cpp
===================================================================
--- lib/IR/Value.cpp
+++ lib/IR/Value.cpp
@@ -516,6 +516,13 @@
       V = GEP->getPointerOperand();
     } else if (Operator::getOpcode(V) == Instruction::BitCast) {
       V = cast<Operator>(V)->getOperand(0);
+    } else if (AddrSpaceCastInst *ASCI = dyn_cast<AddrSpaceCastInst>(V)) {
+      Value *SourcePtr = ASCI->getPointerOperand();
+      // Don't look through addrspace cast which changes pointer size
+      if (Offset.getBitWidth() !=
+            DL.getPointerTypeSizeInBits(SourcePtr->getType()))
+        return V;
+      V = SourcePtr;
     } else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
       V = GA->getAliasee();
     } else {
Index: test/Analysis/ValueTracking/dereferenceable-addrspace-cast.ll
===================================================================
--- /dev/null
+++ test/Analysis/ValueTracking/dereferenceable-addrspace-cast.ll
@@ -0,0 +1,15 @@
+; RUN: opt -default-data-layout="e-m:e-p:16:16-p1:32:16-i32:16-i64:16-n8:16" -print-memderefs -analyze -S <%s | FileCheck %s --check-prefix=CHECK-DL
+; RUN: opt -print-memderefs -analyze -S <%s | FileCheck %s --check-prefix=CHECK-NO-DL
+
+; CHECK-DL-LABEL: 'test'
+; CHECK-DL: The following are dereferenceable:
+; CHECK-DL-NOT: %cast{{.*}}(aligned)
+; CHECK-NO-DL-LABEL: 'test'
+; CHECK-NO-DL: The following are dereferenceable:
+; CHECK-NO-DL: %cast{{.*}}(aligned)
+define void @test(i32 addrspace(1)* dereferenceable(8) %dparam) {
+entry:
+    %cast = addrspacecast i32 addrspace(1)* %dparam to i32*
+    %load3 = load i32, i32* %cast
+    ret void
+}
Index: test/Transforms/InstSimplify/addrspace-cast.ll
===================================================================
--- /dev/null
+++ test/Transforms/InstSimplify/addrspace-cast.ll
@@ -0,0 +1,21 @@
+; RUN: opt -S -instsimplify -default-data-layout="p:32:32-p1:32:32" < %s | FileCheck %s --check-prefix=CHECK-32
+; RUN: opt -S -instsimplify -default-data-layout="p:32:32-p1:64:64" < %s | FileCheck %s --check-prefix=CHECK-64
+
+; Check that we don't look through an addrspacecast if it changes pointer size
+; and that we do look if it doesn't
+
+define i32 @test4() {
+; CHECK-32-LABEL: @test4(
+; CHECK-32-NEXT: ret i32 4
+; CHECK-64-LABEL: @test4(
+; CHECK-64: ret i32 %add_to_int
+  %alloca = alloca i32, align 4                                     ; alloca + 0
+  %gep = getelementptr inbounds i32, i32* %alloca, i32 1            ; alloca + 4
+  %bc = bitcast i32* %gep to [4 x i8]*                              ; alloca + 4
+  %ac = addrspacecast [4 x i8]* %bc to [4 x i8] addrspace(1)*       ; alloca + 4
+  %pti = ptrtoint i32* %alloca to i32                               ; alloca
+  %sub = sub i32 0, %pti                                            ; -alloca
+  %add = getelementptr [4 x i8], [4 x i8] addrspace(1)* %ac, i32 0, i32 %sub     ; alloca + 4 - alloca == 4
+  %add_to_int = ptrtoint i8 addrspace(1)* %add to i32               ; 4
+  ret i32 %add_to_int                                               ; 4
+}
Index: test/Transforms/LoopUnswitch/2015-09-18-Addrspace.ll
===================================================================
--- test/Transforms/LoopUnswitch/2015-09-18-Addrspace.ll
+++ test/Transforms/LoopUnswitch/2015-09-18-Addrspace.ll
@@ -6,10 +6,10 @@
 
 target datalayout = "e-m:e-p:16:16-p1:32:16-i32:16-i64:16-n8:16"
 
-define void @foo() {
+define void @foo(i16* %p) {
 ; CHECK-LABEL: @foo
 entry:
-  %arrayidx.i1 = getelementptr inbounds i16, i16* undef, i16 undef
+  %arrayidx.i1 = getelementptr inbounds i16, i16* %p, i16 4
   %arrayidx.i = addrspacecast i16* %arrayidx.i1 to i16 addrspace(1)*
   br i1 undef, label %for.body.i, label %bar.exit