Index: lib/Target/X86/X86CallingConv.td
===================================================================
--- lib/Target/X86/X86CallingConv.td
+++ lib/Target/X86/X86CallingConv.td
@@ -590,9 +590,11 @@
 
 // Calling convention used on Win64
 def CC_X86_Win64_C : CallingConv<[
-  // FIXME: Handle byval stuff.
   // FIXME: Handle varargs.
 
+  // Byval aggregates are passed by pointer
+  CCIfByVal<CCPassIndirect<i64>>,
+
   // Promote i1/v1i1 arguments to i8.
   CCIfType<[i1, v1i1], CCPromoteToType<i8>>,
 
Index: lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- lib/Target/X86/X86ISelLowering.cpp
+++ lib/Target/X86/X86ISelLowering.cpp
@@ -3138,7 +3138,7 @@
     }
 
     // If value is passed via pointer - do a load.
-    if (VA.getLocInfo() == CCValAssign::Indirect)
+    if (VA.getLocInfo() == CCValAssign::Indirect && !Ins[I].Flags.isByVal())
       ArgValue =
           DAG.getLoad(VA.getValVT(), dl, Chain, ArgValue, MachinePointerInfo());
 
@@ -3621,13 +3621,29 @@
       Arg = DAG.getBitcast(RegVT, Arg);
       break;
     case CCValAssign::Indirect: {
-      // Store the argument.
-      SDValue SpillSlot = DAG.CreateStackTemporary(VA.getValVT());
-      int FI = cast<FrameIndexSDNode>(SpillSlot)->getIndex();
-      Chain = DAG.getStore(
-          Chain, dl, Arg, SpillSlot,
-          MachinePointerInfo::getFixedStack(DAG.getMachineFunction(), FI));
-      Arg = SpillSlot;
+      if (isByVal) {
+        // Memcpy the argument to a temporary stack slot to prevent
+        // the caller from seeing any modifications the callee may make
+        // as guaranteed by the `byval` attribute.
+        int FrameIdx = MF.getFrameInfo().CreateStackObject(
+            Flags.getByValSize(), std::max(16, (int)Flags.getByValAlign()),
+            false);
+        SDValue StackSlot =
+            DAG.getFrameIndex(FrameIdx, getPointerTy(DAG.getDataLayout()));
+        Chain =
+            CreateCopyOfByValArgument(Arg, StackSlot, Chain, Flags, DAG, dl);
+        // From now on treat this as a regular pointer
+        Arg = StackSlot;
+        isByVal = false;
+      } else {
+        // Store the argument.
+        SDValue SpillSlot = DAG.CreateStackTemporary(VA.getValVT());
+        int FI = cast<FrameIndexSDNode>(SpillSlot)->getIndex();
+        Chain = DAG.getStore(
+            Chain, dl, Arg, SpillSlot,
+            MachinePointerInfo::getFixedStack(DAG.getMachineFunction(), FI));
+        Arg = SpillSlot;
+      }
       break;
     }
     }
Index: test/CodeGen/X86/win64-byval.ll
===================================================================
--- /dev/null
+++ test/CodeGen/X86/win64-byval.ll
@@ -0,0 +1,33 @@
+; RUN: llc -mtriple x86_64-w64-mingw32 %s -o - | FileCheck %s
+
+declare void @foo({ float, double }* byval)
+@G = external constant { float, double }
+
+define void @bar()
+{
+; Make sure we're creating a temporary stack slot, rather than just passing
+; the pointer through unmodified.
+; CHECK-LABEL: @bar
+; CHECK: movq    G+8(%rip), %rax
+; CHECK: movq    %rax, 40(%rsp)
+; CHECK: movq    G(%rip), %rax
+; CHECK: movq    %rax, 32(%rsp)
+; CHECK: leaq    32(%rsp), %rcx
+    call void @foo({ float, double }* byval @G)
+    ret void
+}
+
+define void @baz({ float, double }* byval %arg)
+{
+; On Win64 the byval is effectively ignored on declarations, since we do
+; pass a real pointer in registers. However, by our semantics if we pass
+; the pointer on to another byval function, we do need to make a copy.
+; CHECK-LABEL: @baz
+; CHECK: movq	(%rcx), %rax
+; CHECK: movq	8(%rcx), %rcx
+; CHECK: movq	%rcx, 40(%rsp)
+; CHECK: movq	%rax, 32(%rsp)
+; CHECK: leaq	32(%rsp), %rcx
+    call void @foo({ float, double }* byval %arg)
+    ret void
+}