[clang] Support member function poiners in Decl::getFunctionType() (#125077)

This seems consistent with the documentation, which claims it:

```
/// Looks through the Decl's underlying type to extract a FunctionType
/// when possible. Will return null if the type underlying the Decl does not
/// have a FunctionType.
const FunctionType *getFunctionType(bool BlocksToo = true) const;
```
Note: This patch rewords this doc comment to clarify it includes various
function pointer types.

Without this, attaching attributes (which use `HasFunctionProto`) to
member function pointers errors with:

```
error: '<attr>' only applies to non-K&R-style functions
```

...which does not really make sense, since member functions are not K&C
functions.

With this change the Arm SME TypeAttrs work correctly on member function
pointers.

Note, however, that not all attributes work correctly when applied to
function pointers or member function pointers. For example,
`alloc_align` crashes when applied to a function pointer (on truck):
https://godbolt.org/z/YvMhnhKfx (as it only expects a `FunctionDecl` not
a `ParmVarDecl`). The same crash applies to member function pointers
(for the same reason).

Author: MacDue

clang/include/clang/AST/DeclBase.h

@@ -1257,8 +1257,11 @@ class alignas(8) Decl {
   int64_t getID() const;
 
   /// Looks through the Decl's underlying type to extract a FunctionType
-  /// when possible. Will return null if the type underlying the Decl does not
-  /// have a FunctionType.
+  /// when possible. This includes direct FunctionDecls, along with various
+  /// function types and typedefs. This includes function pointers/references,
+  /// member function pointers, and optionally if \p BlocksToo is set
+  /// Objective-C block pointers. Returns nullptr if the type underlying the
+  /// Decl does not have a FunctionType.
   const FunctionType *getFunctionType(bool BlocksToo = true) const;
 
   // Looks through the Decl's underlying type to determine if it's a

clang/include/clang/Basic/Attr.td

@@ -198,7 +198,7 @@ def OpenCLKernelFunction
 // inclusive nature of subject testing).
 def HasFunctionProto : SubsetSubject<DeclBase,
                                      [{(S->getFunctionType(true) != nullptr &&
-                              isa<FunctionProtoType>(S->getFunctionType())) ||
+                                       isa<FunctionProtoType>(S->getFunctionType())) ||
                                        isa<ObjCMethodDecl>(S) ||
                                        isa<BlockDecl>(S)}],
                                      "non-K&R-style functions">;

clang/lib/AST/DeclBase.cpp

@@ -1203,6 +1203,8 @@ const FunctionType *Decl::getFunctionType(bool BlocksToo) const {
 
   if (Ty->isFunctionPointerType())
     Ty = Ty->castAs<PointerType>()->getPointeeType();
+  else if (Ty->isMemberFunctionPointerType())
+    Ty = Ty->castAs<MemberPointerType>()->getPointeeType();
   else if (Ty->isFunctionReferenceType())
     Ty = Ty->castAs<ReferenceType>()->getPointeeType();
   else if (BlocksToo && Ty->isBlockPointerType())

clang/test/AST/attr-print-emit.cpp

@@ -91,3 +91,8 @@ ANNOTATE_ATTR NONNULL_ATTR void fn_non_null_annotated_attr(int *) __attribute__(
 
 [[gnu::nonnull(1)]] [[gnu::always_inline]] void cxx11_attr(int*) ANNOTATE_ATTR;
 // CHECK: {{\[\[}}gnu::nonnull(1)]] {{\[\[}}gnu::always_inline]] void cxx11_attr(int *) __attribute__((annotate("Annotated")));
+
+struct Foo;
+
+// CHECK: void as_member_fn_ptr(int *(Foo::*member)(int) __attribute__((alloc_size(1))));
+void as_member_fn_ptr(int* (Foo::*member)(int)  __attribute__((alloc_size(1))));

clang/test/CodeGen/AArch64/sme-attributes-member-function-pointer.cpp

@@ -0,0 +1,37 @@
+// RUN: %clang_cc1 -triple aarch64 -target-feature +sme -target-feature +sme2 -x c++ -std=c++20  -emit-llvm -o - %s | FileCheck %s --check-prefixes=CHECK
+
+struct TestStruct;
+
+__arm_new("za", "zt0") void test(TestStruct& TS,
+  void (TestStruct::*streaming_member_ptr)() __arm_streaming,
+  void (TestStruct::*streaming_compat_member)() __arm_streaming_compatible,
+  void (TestStruct::*arm_in_member)() __arm_in("za", "zt0"),
+  void (TestStruct::*arm_inout_member)() __arm_inout("za", "zt0"),
+  void (TestStruct::*arm_preserves_member)() __arm_preserves("za", "zt0"),
+  void (TestStruct::*arm_agnostic_member)() __arm_agnostic("sme_za_state")) {
+
+  // CHECK: call void %{{.*}} [[STREAMING_MEMBER_CALL_ATTRS:#.+]]
+  (TS.*streaming_member_ptr)();
+
+  // CHECK: call void %{{.*}} [[STREAMING_COMPAT_MEMBER_CALL_ATTRS:#.+]]
+  (TS.*streaming_compat_member)();
+
+  // CHECK: call void %{{.*}} [[ARM_IN_MEMBER_CALL_ATTRS:#.+]]
+  (TS.*arm_in_member)();
+
+  // CHECK: call void %{{.*}} [[ARM_INOUT_MEMBER_CALL_ATTRS:#.+]]
+  (TS.*arm_inout_member)();
+
+  // CHECK: call void %{{.*}} [[ARM_PRESERVES_MEMBER_CALL_ATTRS:#.+]]
+  (TS.*arm_preserves_member)();
+
+  // CHECK: call void %{{.*}} [[ARM_AGNOSTIC_MEMBER_CALL_ATTRS:#.+]]
+  (TS.*arm_agnostic_member)();
+}
+
+// CHECK: attributes [[STREAMING_MEMBER_CALL_ATTRS]] = { "aarch64_pstate_sm_enabled" }
+// CHECK: attributes [[STREAMING_COMPAT_MEMBER_CALL_ATTRS]] = { "aarch64_pstate_sm_compatible" }
+// CHECK: attributes [[ARM_IN_MEMBER_CALL_ATTRS]] = { "aarch64_in_za" "aarch64_in_zt0" }
+// CHECK: attributes [[ARM_INOUT_MEMBER_CALL_ATTRS]] = { "aarch64_inout_za" "aarch64_inout_zt0" }
+// CHECK: attributes [[ARM_PRESERVES_MEMBER_CALL_ATTRS]] = { "aarch64_preserves_za" "aarch64_preserves_zt0" }
+// CHECK: attributes [[ARM_AGNOSTIC_MEMBER_CALL_ATTRS]] = { "aarch64_za_state_agnostic" }

clang/test/CodeGen/xfail-alloc-align-fn-pointers.cpp

@@ -0,0 +1,10 @@
+
+// RUN: %clang_cc1 %s
+
+// FIXME: These should not crash!
+// XFAIL: *
+
+void aa_fn_ptr(char* (*member)(char*)  __attribute__((alloc_align(1))));
+
+struct Test;
+void aa_member_fn_ptr(char* (Test::*member)(char*)  __attribute__((alloc_align(1))));