[PATCH] Add support for nest attribute to AArch64 backend

[Stephen Cross]

Hi t.p.northover,

The `nest` attribute is currently supported on the x86 (32-bit) and x86-64 backends, but not on ARM (32-bit) or AArch64. This patch adds support for `nest` to the AArch64 backend.

Register x18 is used by GCC for this purpose (see https://github.com/gcc-mirror/gcc/blob/7c62dfbbcd3699efcbbadc9fb3aa14f23a123add/gcc/testsuite/gcc.dg/cwsc1.c ) and hence is used here. As discussed on the GCC mailing list (see http://www.mail-archive.com/gcc@gcc.gnu.org/msg76966.html ) the register choice is an ABI issue and so choosing the same register as GCC means `__builtin_call_with_static_chain` is compatible.

A slight complexity to this issue is that x18 is reserved as the 'platform register' on iOS and Mac (see https://developer.apple.com/library/ios/documentation/Xcode/Conceptual/iPhoneOSABIReference/Articles/ARM64FunctionCallingConventions.html ) and no register has been selected by GCC yet, so `nest` remains unsupported for those platforms.

Relevant LLVM mailing list discussion: http://comments.gmane.org/gmane.comp.compilers.llvm.devel/86370

http://reviews.llvm.org/D10585

Files:
  lib/Target/AArch64/AArch64CallingConvention.td
  test/CodeGen/AArch64/nest-register.ll

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
+}

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