Don't trigger unused-parameter warnings on naked functions

This commit checks if a function is marked with the naked attribute
and, if it is, will silence the emission of any unused-parameter
warning.

Inside a naked function only the usage of basic ASM instructions is
expected. In this context the parameters can actually be used by
fetching them according to the underlying ABI. Since parameters might
be used through ASM instructions, the linter and the compiler will have
a hard time understanding if one of those is unused or not, therefore
no unused-parameter warning should ever be triggered whenever a
function is marked naked.

clang-tools-extra/clang-tidy/misc/UnusedParametersCheck.cpp

@@ -31,10 +31,11 @@ bool isOverrideMethod(const FunctionDecl *Function) {
 } // namespace
 
 void UnusedParametersCheck::registerMatchers(MatchFinder *Finder) {
-  Finder->addMatcher(
-      functionDecl(isDefinition(), hasBody(stmt()), hasAnyParameter(decl()))
-          .bind("function"),
-      this);
+  Finder->addMatcher(functionDecl(isDefinition(), hasBody(stmt()),
+                                  hasAnyParameter(decl()),
+                                  unless(hasAttr(attr::Kind::Naked)))
+                         .bind("function"),
+                     this);
 }
 
 template <typename T>

clang-tools-extra/test/clang-tidy/checkers/misc-unused-parameters-strict.cpp

@@ -22,4 +22,8 @@ public:
 // CHECK-MESSAGES: :[[@LINE-1]]:9: warning: parameter 'j' is unused
 // CHECK-FIXES: {{^}}  F(int  /*j*/) : i() {}{{$}}
 };
+
+// Do not warn on naked functions.
+[[gnu::naked]] int nakedFunction(int a, float b, const char *c) { ; }
+__attribute__((naked)) void nakedFunction(int a, int b) { ; }
 }

clang-tools-extra/test/clang-tidy/checkers/misc-unused-parameters.c

@@ -15,3 +15,5 @@ static void b(int i) {;}
 // ===============
 void h(i, c, d) int i; char *c, *d; {} // Don't mess with K&R style
 
+// Do not warn on naked functions.
+__attribute__((naked)) void nakedFunction(int a, int b) { ; }

clang-tools-extra/test/clang-tidy/checkers/misc-unused-parameters.cpp

@@ -286,3 +286,7 @@ void test() {
   f([](int I) { return; });
 }
 } // namespace lambda
+
+// Do not warn on naked functions.
+[[gnu::naked]] int nakedFunction(int a, float b, const char *c) { ; }
+__attribute__((naked)) void nakedFunction(int a, int b) { ; }

clang/lib/Sema/SemaDecl.cpp

@@ -14694,8 +14694,10 @@ Decl *Sema::ActOnFinishFunctionBody(Decl *dcl, Stmt *Body,
         Diag(FD->getLocation(), diag::ext_pure_function_definition);
 
       if (!FD->isInvalidDecl()) {
-        // Don't diagnose unused parameters of defaulted or deleted functions.
-        if (!FD->isDeleted() && !FD->isDefaulted() && !FD->hasSkippedBody())
+        // Don't diagnose unused parameters of defaulted, deleted or naked
+        // functions.
+        if (!FD->isDeleted() && !FD->isDefaulted() && !FD->hasSkippedBody() &&
+            !FD->hasAttr<NakedAttr>())
           DiagnoseUnusedParameters(FD->parameters());
         DiagnoseSizeOfParametersAndReturnValue(FD->parameters(),
                                                FD->getReturnType(), FD);

clang/test/Sema/warn-unused-parameters.c

@@ -17,14 +17,17 @@ void f1(void) {
 // warnings for the above cases.
 static void achor() {};
 
+// Do not warn on naked functions.
+__attribute__((naked)) static void nakedFunction(int a, int b) { }
+
 // CHECK: 5:12: warning: unused parameter 'y'
 // CHECK: 12:15: warning: unused parameter 'y'
-// CHECK-unused: 1 warning generated
+// CHECK-unused: 2 warnings generated
 
 // RUN: %clang_cc1 -fblocks -fsyntax-only -Weverything %s 2>&1 | FileCheck -check-prefix=CHECK-everything %s
 // RUN: not %clang_cc1 -fblocks -fsyntax-only -Weverything -Werror %s 2>&1 | FileCheck -check-prefix=CHECK-everything-error %s
 // RUN: %clang_cc1 -fblocks -fsyntax-only -Weverything -Wno-unused %s 2>&1 | FileCheck -check-prefix=CHECK-everything-no-unused %s
-// CHECK-everything: 6 warnings generated
+// CHECK-everything: 7 warnings generated
 // CHECK-everything-error: 5 errors generated
 // CHECK-everything-no-unused: 5 warnings generated
 

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

@@ -32,3 +32,7 @@ static int test_pack(T... t, T... s)
   auto l = [&t...]() { return sizeof...(s); };
   return l();
 }
+
+// Do not warn on naked functions.
+[[gnu::naked]] int nakedFunction(int a, float b, const char* c) { ; }
+__attribute__((naked)) void nakedFunction(int a, int b) { ; }