Index: lib/Target/AArch64/AArch64CallingConvention.td
===================================================================
--- lib/Target/AArch64/AArch64CallingConvention.td
+++ lib/Target/AArch64/AArch64CallingConvention.td
@@ -40,6 +40,11 @@
   // slot is 64-bit.
   CCIfByVal<CCPassByVal<8, 8>>,
 
+  // The 'nest' parameter, if any, is passed in X18.
+  // Darwin uses X18 as the platform register and hence 'nest' isn't currently
+  // supported there.
+  CCIfNest<CCAssignToReg<[X18]>>,
+
   CCIfConsecutiveRegs<CCCustom<"CC_AArch64_Custom_Block">>,
 
   // Handle i1, i8, i16, i32, i64, f32, f64 and v2f64 by passing in registers,
Index: test/CodeGen/AArch64/nest-register.ll
===================================================================
--- test/CodeGen/AArch64/nest-register.ll
+++ test/CodeGen/AArch64/nest-register.ll
@@ -0,0 +1,23 @@
+; RUN: llc -verify-machineinstrs < %s -mtriple=aarch64-none-linux-gnu | FileCheck %s
+
+; Tests that the 'nest' parameter attribute causes the relevant parameter to be
+; passed in the right register.
+
+define i8* @nest_receiver(i8* nest %arg) nounwind {
+; CHECK-LABEL: nest_receiver:
+; CHECK-NEXT: // BB#0:
+; CHECK-NEXT: mov x0, x18
+; CHECK-NEXT: ret
+
+  ret i8* %arg
+}
+
+define i8* @nest_caller(i8* %arg) nounwind {
+; CHECK-LABEL: nest_caller:
+; CHECK: mov x18, x0
+; CHECK-NEXT: bl nest_receiver
+; CHECK: ret
+
+  %result = call i8* @nest_receiver(i8* nest %arg)
+  ret i8* %result
+}