[clang][sema] Fix -Wunused-function on target_version'd file-scope Fn's (#81167)

We should only warn if the default version is the one that is unused.

Fixes: https://github.com/llvm/llvm-project/issues/80227

clang/include/clang/AST/Decl.h

@@ -2615,6 +2615,10 @@ public:
   /// the target functionality.
   bool isTargetMultiVersion() const;
 
+  /// True if this function is the default version of a multiversioned dispatch
+  /// function as a part of the target functionality.
+  bool isTargetMultiVersionDefault() const;
+
   /// True if this function is a multiversioned dispatch function as a part of
   /// the target-clones functionality.
   bool isTargetClonesMultiVersion() const;

clang/lib/AST/Decl.cpp

@@ -3537,6 +3537,11 @@ bool FunctionDecl::isTargetMultiVersion() const {
          (hasAttr<TargetAttr>() || hasAttr<TargetVersionAttr>());
 }
 
+bool FunctionDecl::isTargetMultiVersionDefault() const {
+  return isMultiVersion() && hasAttr<TargetVersionAttr>() &&
+         getAttr<TargetVersionAttr>()->isDefaultVersion();
+}
+
 bool FunctionDecl::isTargetClonesMultiVersion() const {
   return isMultiVersion() && hasAttr<TargetClonesAttr>();
 }

clang/lib/Sema/Sema.cpp

@@ -1393,7 +1393,8 @@ void Sema::ActOnEndOfTranslationUnit() {
               Diag(DiagD->getLocation(), diag::warn_unneeded_internal_decl)
                   << /*function=*/0 << DiagD << DiagRange;
           }
-        } else {
+        } else if (!FD->isTargetMultiVersion() ||
+                   FD->isTargetMultiVersionDefault()) {
           if (FD->getDescribedFunctionTemplate())
             Diag(DiagD->getLocation(), diag::warn_unused_template)
                 << /*function=*/0 << DiagD << DiagRange;

clang/test/SemaCXX/warn-unused-filescoped.cpp

@@ -236,4 +236,20 @@ constexpr int constexpr4() { return 2; }
 #endif
 }
 
+__attribute__((target_version("fp16")))
+static int not_used_fmv(void) { return 1; }
+__attribute__((target_version("fp16fml")))
+static int not_used_fmv(void) { return 2; }
+__attribute__((target_version("default")))
+static int not_used_fmv(void) { return 0; } // expected-warning {{unused function 'not_used_fmv'}}
+
+
+__attribute__((target_version("fp16")))
+static int definitely_used_fmv(void) { return 1; }
+__attribute__((target_version("fp16fml")))
+static int definitely_used_fmv(void) { return 2; }
+__attribute__((target_version("default")))
+static int definitely_used_fmv(void) { return 0; }
+int definite_user(void) { return definitely_used_fmv(); }
+
 #endif