[clang] Make guard(nocf) attribute available only for Windows

Control Flow Guard is only supported on Windows target, therefore there
is no point to make it an accepted attribute for other targets.

Reviewed By: rnk, aaron.ballman

Differential Revision: https://reviews.llvm.org/D132661

Author: alvinhochun

clang/docs/ReleaseNotes.rst

@@ -146,7 +146,8 @@ Attribute Changes in Clang
   ``[[clang::guard(nocf)]]``, which is equivalent to ``__declspec(guard(nocf))``
   when using the MSVC environment. This is to support enabling Windows Control
   Flow Guard checks with the ability to disable them for specific functions when
-  using the MinGW environment.
+  using the MinGW environment. This attribute is only available for Windows
+  targets.
 
 Windows Support
 ---------------

clang/include/clang/Basic/Attr.td

@@ -399,6 +399,9 @@ def TargetRISCV : TargetArch<["riscv32", "riscv64"]>;
 def TargetX86 : TargetArch<["x86"]>;
 def TargetAnyX86 : TargetArch<["x86", "x86_64"]>;
 def TargetWebAssembly : TargetArch<["wasm32", "wasm64"]>;
+def TargetWindows : TargetSpec {
+  let OSes = ["Win32"];
+}
 def TargetHasDLLImportExport : TargetSpec {
   let CustomCode = [{ Target.getTriple().hasDLLImportExport() }];
 }
@@ -3494,7 +3497,7 @@ def MSAllocator : InheritableAttr {
   let Documentation = [MSAllocatorDocs];
 }
 
-def CFGuard : InheritableAttr {
+def CFGuard : InheritableAttr, TargetSpecificAttr<TargetWindows> {
   // Currently only the __declspec(guard(nocf)) modifier is supported. In future
   // we might also want to support __declspec(guard(suppress)).
   let Spellings = [Declspec<"guard">, Clang<"guard">];

clang/test/Sema/attr-guard_nocf.c

@@ -2,10 +2,14 @@
 // RUN: %clang_cc1 -triple %ms_abi_triple -fms-extensions -verify -std=c++11 -fsyntax-only -x c++ %s
 // RUN: %clang_cc1 -triple x86_64-w64-windows-gnu -verify -fsyntax-only %s
 // RUN: %clang_cc1 -triple x86_64-w64-windows-gnu -verify -std=c++11 -fsyntax-only -x c++ %s
+// RUN: %clang_cc1 -triple x86_64-pc-linux-gnu -verify -fsyntax-only %s
+// RUN: %clang_cc1 -triple x86_64-pc-linux-gnu -verify -std=c++11 -fsyntax-only -x c++ %s
 
 // The x86_64-w64-windows-gnu version tests mingw target, which relies on
 // __declspec(...) being defined as __attribute__((...)) by compiler built-in.
 
+#if defined(_WIN32)
+
 // Function definition.
 __declspec(guard(nocf)) void testGuardNoCF(void) { // no warning
 }
@@ -35,3 +39,9 @@ __declspec(guard(nocf, nocf)) void testGuardNoCFTooManyParams(void) { // expecte
 // 'guard' Attribute argument must be a supported identifier.
 __declspec(guard(cf)) void testGuardNoCFInvalidParam(void) { // expected-warning {{'guard' attribute argument not supported: 'cf'}}
 }
+
+#else
+
+__attribute((guard(nocf))) void testGNUStyleGuardNoCF(void) {} // expected-warning {{unknown attribute 'guard' ignored}}
+
+#endif