[AArch64] Alter arm_fp16.h to be target-based, not preprocessor based.

[dmgreen]

Summary

As the other recent patches, this alters the arm_fp16 intrinsics to be target based, not preprocessor based. Apparently arm_fp16.h is AArch64 only under clang, making this mostly trivial with the TargetGuard infrastructure.

[simon_tatham]

Nit: your summary line and commit message don't match :-) One of them says "changed from target to preprocessor" and the other vice versa.

[dmgreen]

Oh yeah, so I did. That should be fixed. Thanks.

[simon_tatham]

But otherwise this is a very easy-to-check change compared to the previous ones on the same lines :-) LGTM.

clang/include/clang/Basic/arm_fp16.td

 //===--- arm_fp16.td - ARM FP16 compiler interface ------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 //  This file defines the TableGen definitions from which the ARM FP16 header
 //  file will be generated.
 //
 //===----------------------------------------------------------------------===//
 
 include "arm_neon_incl.td"
 
 // ARMv8.2-A FP16 intrinsics.
-let ArchGuard = "defined(__ARM_FEATURE_FP16_SCALAR_ARITHMETIC) && defined(__aarch64__)" in {
+let ArchGuard = "defined(__aarch64__)", TargetGuard = "fullfp16" in {
 
   // Negate
   def VNEGSH          : SInst<"vneg", "11", "Sh">;
 
   // Reciprocal/Sqrt
   def SCALAR_FRECPSH  : IInst<"vrecps", "111", "Sh">;
   def FSQRTSH         : SInst<"vsqrt", "11", "Sh">;
   def SCALAR_FRSQRTSH : IInst<"vrsqrts", "111", "Sh">;

clang/test/Sema/aarch64-fp16-target.c

 // RUN: %clang_cc1 -triple aarch64-none-linux-gnu -target-feature +neon -fsyntax-only -verify -emit-llvm -o - %s
 // REQUIRES: aarch64-registered-target
 
 // Test that functions with the correct target attributes can use the correct FP16 intrinsics.
 
 #include <arm_fp16.h>
 
 __attribute__((target("fullfp16")))
 void test_fullfp16(float16_t f16) {
-  vabdh_f16(f16, f16); // expected-error {{call to undeclared function 'vabdh_f16'}}
+  vabdh_f16(f16, f16);
 }
 
 __attribute__((target("arch=armv8-a+fp16")))
 void test_fp16_arch(float16_t f16) {
-    vabdh_f16(f16, f16); // expected-error {{call to undeclared function 'vabdh_f16'}}
+    vabdh_f16(f16, f16);
 }
 
 __attribute__((target("+fp16")))
 void test_fp16(float16_t f16) {
-    vabdh_f16(f16, f16); // expected-error {{call to undeclared function 'vabdh_f16'}}
+    vabdh_f16(f16, f16);
 }
 
 void undefined(float16_t f16) {
-  vabdh_f16(f16, f16); // expected-error {{call to undeclared function 'vabdh_f16'}}
+  vabdh_f16(f16, f16); // expected-error {{'__builtin_neon_vabdh_f16' needs target feature fullfp16}}
 }