diff --git a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
--- a/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FastISel.cpp
@@ -410,8 +410,8 @@
   else if (isa<ConstantPointerNull>(V))
     // Translate this as an integer zero so that it can be
     // local-CSE'd with actual integer zeros.
-    Reg = getRegForValue(
-        Constant::getNullValue(DL.getIntPtrType(V->getContext())));
+    Reg =
+        getRegForValue(Constant::getNullValue(DL.getIntPtrType(V->getType())));
   else if (const auto *CF = dyn_cast<ConstantFP>(V)) {
     if (CF->isNullValue())
       Reg = fastMaterializeFloatZero(CF);
diff --git a/llvm/lib/Target/X86/X86.h b/llvm/lib/Target/X86/X86.h
--- a/llvm/lib/Target/X86/X86.h
+++ b/llvm/lib/Target/X86/X86.h
@@ -152,4 +152,15 @@
 void initializeX86SpeculativeLoadHardeningPassPass(PassRegistry &);
 } // End llvm namespace
 
+namespace X86AS {
+enum : unsigned {
+  GS = 256,
+  FS = 257,
+  SS = 258,
+  PTR32_SPTR = 270,
+  PTR32_UPTR = 271,
+  PTR64 = 272
+};
+}
+
 #endif
diff --git a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
--- a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
+++ b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
@@ -2224,12 +2224,11 @@
   AM.Scale = cast<ConstantSDNode>(Mgs->getScale())->getZExtValue();
 
   unsigned AddrSpace = cast<MemSDNode>(Parent)->getPointerInfo().getAddrSpace();
-  // AddrSpace 256 -> GS, 257 -> FS, 258 -> SS.
-  if (AddrSpace == 256)
+  if (AddrSpace == X86AS::GS)
     AM.Segment = CurDAG->getRegister(X86::GS, MVT::i16);
-  if (AddrSpace == 257)
+  if (AddrSpace == X86AS::FS)
     AM.Segment = CurDAG->getRegister(X86::FS, MVT::i16);
-  if (AddrSpace == 258)
+  if (AddrSpace == X86AS::SS)
     AM.Segment = CurDAG->getRegister(X86::SS, MVT::i16);
 
   SDLoc DL(N);
diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -284,6 +284,10 @@
     setOperationAction(ISD::FP_TO_UINT, MVT::i64, Custom);
   }
 
+  // Handle address space casts between mixed sized pointers.
+  setOperationAction(ISD::ADDRSPACECAST, MVT::i32, Custom);
+  setOperationAction(ISD::ADDRSPACECAST, MVT::i64, Custom);
+
   // TODO: when we have SSE, these could be more efficient, by using movd/movq.
   if (!X86ScalarSSEf64) {
     setOperationAction(ISD::BITCAST        , MVT::f32  , Expand);
@@ -2383,6 +2387,10 @@
                                             unsigned DestAS) const {
   assert(SrcAS != DestAS && "Expected different address spaces!");
 
+  const TargetMachine &TM = getTargetMachine();
+  if (TM.getPointerSize(SrcAS) != TM.getPointerSize(DestAS))
+    return false;
+
   return SrcAS < 256 && DestAS < 256;
 }
 
@@ -27620,6 +27628,30 @@
   return DAG.getMergeValues({Extract, NewGather.getValue(2)}, dl);
 }
 
+static SDValue LowerADDRSPACECAST(SDValue Op, SelectionDAG &DAG) {
+  SDLoc dl(Op);
+  SDValue Src = Op.getOperand(0);
+  MVT DstVT = Op.getSimpleValueType();
+
+  AddrSpaceCastSDNode *N = cast<AddrSpaceCastSDNode>(Op.getNode());
+  unsigned SrcAS = N->getSrcAddressSpace();
+  unsigned DstAS = N->getDestAddressSpace();
+
+  assert(SrcAS != DstAS &&
+         "addrspacecast must be between different address spaces");
+
+  if (SrcAS == X86AS::PTR32_UPTR && DstVT == MVT::i64) {
+    Op = DAG.getNode(ISD::ZERO_EXTEND, dl, DstVT, Src);
+  } else if (DstVT == MVT::i64) {
+    Op = DAG.getNode(ISD::SIGN_EXTEND, dl, DstVT, Src);
+  } else if (DstVT == MVT::i32) {
+    Op = DAG.getNode(ISD::TRUNCATE, dl, DstVT, Src);
+  } else {
+    report_fatal_error("Bad address space in addrspacecast");
+  }
+  return Op;
+}
+
 SDValue X86TargetLowering::LowerGC_TRANSITION_START(SDValue Op,
                                                     SelectionDAG &DAG) const {
   // TODO: Eventually, the lowering of these nodes should be informed by or
@@ -27793,6 +27825,8 @@
   case ISD::GC_TRANSITION_START:
                                 return LowerGC_TRANSITION_START(Op, DAG);
   case ISD::GC_TRANSITION_END:  return LowerGC_TRANSITION_END(Op, DAG);
+  case ISD::ADDRSPACECAST:
+    return LowerADDRSPACECAST(Op, DAG);
   }
 }
 
@@ -28556,6 +28590,29 @@
     Results.push_back(Res.getValue(1));
     return;
   }
+  case ISD::ADDRSPACECAST: {
+    SDValue Src = N->getOperand(0);
+    EVT DstVT = N->getValueType(0);
+    AddrSpaceCastSDNode *CastN = cast<AddrSpaceCastSDNode>(N);
+    unsigned SrcAS = CastN->getSrcAddressSpace();
+    unsigned DstAS = CastN->getDestAddressSpace();
+
+    assert(SrcAS != DstAS &&
+           "addrspacecast must be between different address spaces");
+
+    SDValue Res;
+    if (SrcAS == X86AS::PTR32_UPTR && DstVT == MVT::i64)
+      Res = DAG.getNode(ISD::ZERO_EXTEND, dl, DstVT, Src);
+    else if (DstVT == MVT::i64)
+      Res = DAG.getNode(ISD::SIGN_EXTEND, dl, DstVT, Src);
+    else if (DstVT == MVT::i32)
+      Res = DAG.getNode(ISD::TRUNCATE, dl, DstVT, Src);
+    else
+      report_fatal_error("Unrecognized addrspacecast type legalization");
+
+    Results.push_back(Res);
+    return;
+  }
   }
 }
 
diff --git a/llvm/test/CodeGen/X86/mixed-ptr-sizes.ll b/llvm/test/CodeGen/X86/mixed-ptr-sizes.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/X86/mixed-ptr-sizes.ll
@@ -0,0 +1,128 @@
+; RUN: llc < %s | FileCheck --check-prefixes=CHECK %s
+; RUN: llc -O0 < %s | FileCheck --check-prefixes=CHECK %s
+
+; Source to regenerate:
+; struct Foo {
+;   int * __ptr32 p32;
+;   int * __ptr64 p64;
+;   __attribute__((address_space(9))) int *p_other;
+; };
+; void use_foo(Foo *f);
+; void test_sign_ext(Foo *f, int * __ptr32 __sptr i) {
+;   f->p64 = i;
+;   use_foo(f);
+; }
+; void test_zero_ext(Foo *f, int * __ptr32 __uptr i) {
+;   f->p64 = i;
+;   use_foo(f);
+; }
+; void test_trunc(Foo *f, int * __ptr64 i) {
+;   f->p32 = i;
+;   use_foo(f);
+; }
+; void test_noop1(Foo *f, int * __ptr32 i) {
+;   f->p32 = i;
+;   use_foo(f);
+; }
+; void test_noop2(Foo *f, int * __ptr64 i) {
+;   f->p64 = i;
+;   use_foo(f);
+; }
+; void test_null_arg(Foo *f, int * __ptr32 i) {
+;   test_noop1(f, 0);
+; }
+; void test_unrecognized(Foo *f, __attribute__((address_space(14))) int *i) {
+;   f->p32 = (int * __ptr32)i;
+;   use_foo(f);
+; }
+;
+; $ clang -cc1 -triple x86_64-windows-msvc -fms-extensions -O2 -S t.cpp
+
+target datalayout = "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-windows-msvc"
+
+%struct.Foo = type { i32 addrspace(270)*, i32*, i32 addrspace(9)* }
+declare dso_local void @use_foo(%struct.Foo*)
+
+define dso_local void @test_sign_ext(%struct.Foo* %f, i32 addrspace(270)* %i) {
+; CHECK-LABEL: test_sign_ext
+; CHECK:       movslq %edx, %rax
+entry:
+  %0 = addrspacecast i32 addrspace(270)* %i to i32*
+  %p64 = getelementptr inbounds %struct.Foo, %struct.Foo* %f, i64 0, i32 1
+  store i32* %0, i32** %p64, align 8
+  tail call void @use_foo(%struct.Foo* %f)
+  ret void
+}
+
+define dso_local void @test_zero_ext(%struct.Foo* %f, i32 addrspace(271)* %i) {
+; CHECK-LABEL: test_zero_ext
+; CHECK:       movl %edx, %eax
+entry:
+  %0 = addrspacecast i32 addrspace(271)* %i to i32*
+  %p64 = getelementptr inbounds %struct.Foo, %struct.Foo* %f, i64 0, i32 1
+  store i32* %0, i32** %p64, align 8
+  tail call void @use_foo(%struct.Foo* %f)
+  ret void
+}
+
+define dso_local void @test_trunc(%struct.Foo* %f, i32* %i) {
+; CHECK-LABEL: test_trunc
+; CHECK:       movl %edx, (%rcx)
+entry:
+  %0 = addrspacecast i32* %i to i32 addrspace(270)*
+  %p32 = getelementptr inbounds %struct.Foo, %struct.Foo* %f, i64 0, i32 0
+  store i32 addrspace(270)* %0, i32 addrspace(270)** %p32, align 8
+  tail call void @use_foo(%struct.Foo* %f)
+  ret void
+}
+
+define dso_local void @test_noop1(%struct.Foo* %f, i32 addrspace(270)* %i) {
+; CHECK-LABEL: test_noop1
+; CHECK:       movl %edx, (%rcx)
+entry:
+  %p32 = getelementptr inbounds %struct.Foo, %struct.Foo* %f, i64 0, i32 0
+  store i32 addrspace(270)* %i, i32 addrspace(270)** %p32, align 8
+  tail call void @use_foo(%struct.Foo* %f)
+  ret void
+}
+
+define dso_local void @test_noop2(%struct.Foo* %f, i32* %i) {
+; CHECK-LABEL: test_noop2
+; CHECK:       movq %rdx, 8(%rcx)
+entry:
+  %p64 = getelementptr inbounds %struct.Foo, %struct.Foo* %f, i64 0, i32 1
+  store i32* %i, i32** %p64, align 8
+  tail call void @use_foo(%struct.Foo* %f)
+  ret void
+}
+
+; Test that null can be passed as a 32-bit pointer.
+define dso_local void @test_null_arg(%struct.Foo* %f) {
+entry:
+  call void @test_noop1(%struct.Foo* %f, i32 addrspace(270)* null)
+  ret void
+}
+
+; Test casts between unrecognized address spaces.
+define void @test_unrecognized(%struct.Foo* %f, i32 addrspace(14)* %i) {
+; CHECK-LABEL: test_unrecognized
+; CHECK:       movl %edx, (%rcx)
+entry:
+  %0 = addrspacecast i32 addrspace(14)* %i to i32 addrspace(270)*
+  %p32 = getelementptr inbounds %struct.Foo, %struct.Foo* %f, i64 0, i32 0
+  store i32 addrspace(270)* %0, i32 addrspace(270)** %p32, align 8
+  tail call void @use_foo(%struct.Foo* %f)
+  ret void
+}
+
+define void @test_unrecognized2(%struct.Foo* %f, i32 addrspace(271)* %i) {
+; CHECK-LABEL: test_unrecognized2
+; CHECK:       movl %edx, %eax
+entry:
+  %0 = addrspacecast i32 addrspace(271)* %i to i32 addrspace(9)*
+  %p32 = getelementptr inbounds %struct.Foo, %struct.Foo* %f, i64 0, i32 2
+  store i32 addrspace(9)* %0, i32 addrspace(9)** %p32, align 8
+  tail call void @use_foo(%struct.Foo* %f)
+  ret void
+}