[clang][ARM64EC] Add support for hybrid_patchable attribute.

clang/docs/ReleaseNotes.rst

@@ -614,6 +614,9 @@ Attribute Changes in Clang
   The attributes declare constraints about a function's behavior pertaining to blocking and
   heap memory allocation.
 
+- The ``hybrid_patchable`` attribute is now supported on ARM64EC targets. It can be used to specify
+  that a function requires an additional x86-64 thunk, which may be patched at runtime.
+
 Improvements to Clang's diagnostics
 -----------------------------------
 - Clang now emits an error instead of a warning for ``-Wundefined-internal``

clang/include/clang/Basic/Attr.td

@@ -477,6 +477,9 @@ def TargetELF : TargetSpec {
 def TargetELFOrMachO : TargetSpec {
   let ObjectFormats = ["ELF", "MachO"];
 }
+def TargetWindowsArm64EC : TargetSpec {
+  let CustomCode = [{ Target.getTriple().isWindowsArm64EC() }];
+}
 
 def TargetSupportsInitPriority : TargetSpec {
   let CustomCode = [{ !Target.getTriple().isOSzOS() }];
@@ -4027,6 +4030,12 @@ def SelectAny : InheritableAttr {
   let SimpleHandler = 1;
 }
 
+def HybridPatchable : InheritableAttr, TargetSpecificAttr<TargetWindowsArm64EC> {
+  let Spellings = [Declspec<"hybrid_patchable">, Clang<"hybrid_patchable">];
+  let Subjects = SubjectList<[Function]>;
+  let Documentation = [HybridPatchableDocs];
+}
+
 def Thread : Attr {
   let Spellings = [Declspec<"thread">];
   let LangOpts = [MicrosoftExt];

clang/include/clang/Basic/AttrDocs.td

@@ -5984,6 +5984,16 @@ For more information see
 or `msvc documentation <https://docs.microsoft.com/pl-pl/cpp/cpp/selectany>`_.
 }]; }
 
+def HybridPatchableDocs : Documentation {
+  let Category = DocCatFunction;
+  let Content = [{
+The ``hybrid_patchable`` attribute declares an ARM64EC function with an additional
+x86-64 thunk, which may be patched at runtime.
+
+For more information see
+`ARM64EC ABI documentation <https://learn.microsoft.com/en-us/windows/arm/arm64ec-abi>`_.
+}]; }
+
 def WebAssemblyExportNameDocs : Documentation {
   let Category = DocCatFunction;
   let Content = [{

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3671,6 +3671,9 @@ def err_attribute_weak_static : Error<
   "weak declaration cannot have internal linkage">;
 def err_attribute_selectany_non_extern_data : Error<
   "'selectany' can only be applied to data items with external linkage">;
+def warn_attribute_hybrid_patchable_non_extern : Warning<
+  "'hybrid_patchable' is ignored on functions without external linkage">,
+  InGroup<IgnoredAttributes>;
 def err_declspec_thread_on_thread_variable : Error<
   "'__declspec(thread)' applied to variable that already has a "
   "thread-local storage specifier">;

clang/lib/CodeGen/CodeGenFunction.cpp

@@ -989,6 +989,9 @@ void CodeGenFunction::StartFunction(GlobalDecl GD, QualType RetTy,
   if (D && D->hasAttr<NoProfileFunctionAttr>())
     Fn->addFnAttr(llvm::Attribute::NoProfile);
 
+  if (D && D->hasAttr<HybridPatchableAttr>())
+    Fn->addFnAttr(llvm::Attribute::HybridPatchable);
+
   if (D) {
     // Function attributes take precedence over command line flags.
     if (auto *A = D->getAttr<FunctionReturnThunksAttr>()) {

clang/lib/Sema/SemaDecl.cpp

@@ -6890,6 +6890,11 @@ static void checkAttributesAfterMerging(Sema &S, NamedDecl &ND) {
     }
   }
 
+  if (HybridPatchableAttr *Attr = ND.getAttr<HybridPatchableAttr>()) {
+    if (!ND.isExternallyVisible())
+      S.Diag(Attr->getLocation(),
+             diag::warn_attribute_hybrid_patchable_non_extern);
+  }
   if (const InheritableAttr *Attr = getDLLAttr(&ND)) {
     auto *VD = dyn_cast<VarDecl>(&ND);
     bool IsAnonymousNS = false;

clang/lib/Sema/SemaDeclAttr.cpp

@@ -7039,6 +7039,9 @@ ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D, const ParsedAttr &AL,
   case ParsedAttr::AT_MSConstexpr:
     handleMSConstexprAttr(S, D, AL);
     break;
+  case ParsedAttr::AT_HybridPatchable:
+    handleSimpleAttribute<HybridPatchableAttr>(S, D, AL);
+    break;
 
   // HLSL attributes:
   case ParsedAttr::AT_HLSLNumThreads:

clang/test/CodeGen/arm64ec-hybrid-patchable.c

@@ -0,0 +1,34 @@
+// REQUIRES: aarch64-registered-target
+// RUN: %clang_cc1 -triple arm64ec-pc-windows -fms-extensions -emit-llvm -o - %s -verify | FileCheck %s
+
+// CHECK: ;    Function Attrs: hybrid_patchable noinline nounwind optnone
+// CHECK-NEXT: define dso_local i32 @func() #0 {
+int __attribute__((hybrid_patchable)) func(void) {  return 1; }
+
+// CHECK: ;    Function Attrs: hybrid_patchable noinline nounwind optnone
+// CHECK-NEXT: define dso_local i32 @func2() #0 {
+int __declspec(hybrid_patchable) func2(void) {  return 2; }
+
+// CHECK: ;    Function Attrs: hybrid_patchable noinline nounwind optnone
+// CHECK-NEXT: define dso_local i32 @func3() #0 {
+int __declspec(hybrid_patchable) func3(void);
+int func3(void) {  return 3; }
+
+// CHECK: ;    Function Attrs: hybrid_patchable noinline nounwind optnone
+// CHECK-NEXT: define dso_local i32 @func4() #0 {
+[[clang::hybrid_patchable]] int func4(void);
+int func4(void) {  return 3; }
+
+// CHECK: ; Function Attrs: hybrid_patchable noinline nounwind optnone
+// CHECK-NEXT: define internal void @static_func() #0 {
+// expected-warning@+1 {{'hybrid_patchable' is ignored on functions without external linkage}}
+static void __declspec(hybrid_patchable) static_func(void) {}
+
+// CHECK: ;    Function Attrs: hybrid_patchable noinline nounwind optnone
+// CHECK-NEXT: define linkonce_odr dso_local i32 @func5() #0 comdat {
+int inline __declspec(hybrid_patchable) func5(void) {  return 4; }
+
+void caller(void) {
+  static_func();
+  func5();
+}

clang/test/Misc/pragma-attribute-supported-attributes-list.test

@@ -83,6 +83,7 @@
 // CHECK-NEXT: HIPManaged (SubjectMatchRule_variable)
 // CHECK-NEXT: HLSLResourceClass (SubjectMatchRule_record_not_is_union)
 // CHECK-NEXT: Hot (SubjectMatchRule_function)
+// CHECK-NEXT: HybridPatchable (SubjectMatchRule_function)
 // CHECK-NEXT: IBAction (SubjectMatchRule_objc_method_is_instance)
 // CHECK-NEXT: IFunc (SubjectMatchRule_function)
 // CHECK-NEXT: InitPriority (SubjectMatchRule_variable)