We regard a function as 'unused' from the codegen perspective, so our warnings diverge from

gcc's unused warnings which don't get emitted if the function is referenced even in an unevaluated context
(e.g. in templates, sizeof, etc.). Also, saying that a function is 'unused' because it won't get codegen'ed
is somewhat misleading.

- Don't emit 'unused' warnings for functions that are referenced in any part of the user's code.
- A warning that an internal function/variable won't get emitted is useful though, so introduce
  -Wunneeded-internal-declaration which will warn if a function/variable with internal linkage is not
  "needed" ('used' from the codegen perspective), e.g:

  static void foo() { }

  template <int>
  void bar() {
    foo();
  }

test.cpp:1:13: warning: function 'foo' is not needed and will not be emitted
static void foo() { }
            ^

Addresses rdar://8733476.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@129794 91177308-0d34-0410-b5e6-96231b3b80d8

11 files changed, 79 insertions, 13 deletions

diff --git a/include/clang/AST/DeclBase.h b/include/clang/AST/DeclBase.h
index 1656240bc4..0473d1ca1f 100644
--- a/include/clang/AST/DeclBase.h
+++ b/include/clang/AST/DeclBase.h
@@ -227,6 +227,12 @@ private:
   /// required.
   unsigned Used : 1;
 
+  /// \brief Whether this declaration was "referenced".
+  /// The difference with 'Used' is whether the reference appears in a
+  /// evaluated context or not, e.g. functions used in uninstantiated templates
+  /// are regarded as "referenced" but not "used".
+  unsigned Referenced : 1;
+
 protected:
   /// Access - Used by C++ decls for the access specifier.
   // NOTE: VC++ treats enums as signed, avoid using the AccessSpecifier enum
@@ -261,7 +267,7 @@ protected:
   Decl(Kind DK, DeclContext *DC, SourceLocation L)
     : NextDeclInContext(0), DeclCtx(DC),
       Loc(L), DeclKind(DK), InvalidDecl(0),
-      HasAttrs(false), Implicit(false), Used(false),
+      HasAttrs(false), Implicit(false), Used(false), Referenced(false),
       Access(AS_none), PCHLevel(0), ChangedAfterLoad(false),
       IdentifierNamespace(getIdentifierNamespaceForKind(DK)),
       HasCachedLinkage(0) 
@@ -271,7 +277,7 @@ protected:
 
   Decl(Kind DK, EmptyShell Empty)
     : NextDeclInContext(0), DeclKind(DK), InvalidDecl(0),
-      HasAttrs(false), Implicit(false), Used(false),
+      HasAttrs(false), Implicit(false), Used(false), Referenced(false),
       Access(AS_none), PCHLevel(0), ChangedAfterLoad(false),
       IdentifierNamespace(getIdentifierNamespaceForKind(DK)),
       HasCachedLinkage(0)
@@ -408,6 +414,11 @@ public:
 
   void setUsed(bool U = true) { Used = U; }
 
+  /// \brief Whether this declaration was referenced.
+  bool isReferenced() const;
+
+  void setReferenced(bool R = true) { Referenced = R; }
+
   /// \brief Determine the availability of the given declaration.
   ///
   /// This routine will determine the most restrictive availability of
diff --git a/include/clang/Basic/DiagnosticGroups.td b/include/clang/Basic/DiagnosticGroups.td
index c8465508c8..15f3dbd464 100644
--- a/include/clang/Basic/DiagnosticGroups.td
+++ b/include/clang/Basic/DiagnosticGroups.td
@@ -154,6 +154,8 @@ def UnusedLabel : DiagGroup<"unused-label">;
 def UnusedParameter : DiagGroup<"unused-parameter">;
 def UnusedValue    : DiagGroup<"unused-value">;
 def UnusedVariable : DiagGroup<"unused-variable">;
+def UnneededInternalDecl : DiagGroup<"unneeded-internal-declaration">;
+def UnneededMemberFunction : DiagGroup<"unneeded-member-function">;
 def UsedButMarkedUnused : DiagGroup<"used-but-marked-unused">;
 def ReadOnlySetterAttrs : DiagGroup<"readonly-setter-attrs">;
 def Reorder : DiagGroup<"reorder">;
@@ -198,11 +200,16 @@ def Conversion : DiagGroup<"conversion",
                             BoolConversions]>,
                  DiagCategory<"Value Conversion Issue">;
 
+def Unneeded : DiagGroup<"unneeded",
+                         [UnneededInternalDecl
+                       //,UnneededMemberFunction (clean-up llvm before enabling)
+                          ]>;
+
 def Unused : DiagGroup<"unused",
                        [UnusedArgument, UnusedFunction, UnusedLabel,
                         // UnusedParameter, (matches GCC's behavior)
                         // UnusedMemberFunction, (clean-up llvm before enabling)
-                        UnusedValue, UnusedVariable]>,
+                        UnusedValue, UnusedVariable, Unneeded]>,
                         DiagCategory<"Unused Entity Issue">;
 
 // Format settings.
diff --git a/include/clang/Basic/DiagnosticSemaKinds.td b/include/clang/Basic/DiagnosticSemaKinds.td
index 3ca45d4cf3..ce592a3c95 100644
--- a/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/include/clang/Basic/DiagnosticSemaKinds.td
@@ -116,6 +116,12 @@ def warn_unused_member_function : Warning<"unused member function %0">,
   InGroup<UnusedMemberFunction>, DefaultIgnore;
 def warn_used_but_marked_unused: Warning<"%0 was marked unused but was used">,
   InGroup<UsedButMarkedUnused>, DefaultIgnore;
+def warn_unneeded_internal_decl : Warning<
+  "%select{function|variable}0 %1 is not needed and will not be emitted">,
+  InGroup<UnneededInternalDecl>, DefaultIgnore;
+def warn_unneeded_member_function : Warning<
+  "member function %0 is not needed and will not be emitted">,
+  InGroup<UnneededMemberFunction>, DefaultIgnore;
 
 def warn_parameter_size: Warning<
   "%0 is a large (%1 bytes) pass-by-value argument; "
diff --git a/lib/AST/DeclBase.cpp b/lib/AST/DeclBase.cpp
index 673b718ba0..41a0361425 100644
--- a/lib/AST/DeclBase.cpp
+++ b/lib/AST/DeclBase.cpp
@@ -242,6 +242,18 @@ bool Decl::isUsed(bool CheckUsedAttr) const {
   return false; 
 }
 
+bool Decl::isReferenced() const { 
+  if (Referenced)
+    return true;
+
+  // Check redeclarations.
+  for (redecl_iterator I = redecls_begin(), E = redecls_end(); I != E; ++I)
+    if (I->Referenced)
+      return true;
+
+  return false; 
+}
+
 /// \brief Determine the availability of the given declaration based on
 /// the target platform.
 ///
diff --git a/lib/Sema/Sema.cpp b/lib/Sema/Sema.cpp
index a83722d824..403cf6246c 100644
--- a/lib/Sema/Sema.cpp
+++ b/lib/Sema/Sema.cpp
@@ -473,16 +473,30 @@ void Sema::ActOnEndOfTranslationUnit() {
           DiagD = FD;
         if (DiagD->isDeleted())
           continue; // Deleted functions are supposed to be unused.
-        Diag(DiagD->getLocation(),
-             isa<CXXMethodDecl>(DiagD) ? diag::warn_unused_member_function
-                                       : diag::warn_unused_function)
-              << DiagD->getDeclName();
+        if (DiagD->isReferenced()) {
+          if (isa<CXXMethodDecl>(DiagD))
+            Diag(DiagD->getLocation(), diag::warn_unneeded_member_function)
+                  << DiagD->getDeclName();
+          else
+            Diag(DiagD->getLocation(), diag::warn_unneeded_internal_decl)
+                  << /*function*/0 << DiagD->getDeclName();
+        } else {
+          Diag(DiagD->getLocation(),
+               isa<CXXMethodDecl>(DiagD) ? diag::warn_unused_member_function
+                                         : diag::warn_unused_function)
+                << DiagD->getDeclName();
+        }
       } else {
         const VarDecl *DiagD = cast<VarDecl>(*I)->getDefinition();
         if (!DiagD)
           DiagD = cast<VarDecl>(*I);
-        Diag(DiagD->getLocation(), diag::warn_unused_variable)
-              << DiagD->getDeclName();
+        if (DiagD->isReferenced()) {
+          Diag(DiagD->getLocation(), diag::warn_unneeded_internal_decl)
+                << /*variable*/1 << DiagD->getDeclName();
+        } else {
+          Diag(DiagD->getLocation(), diag::warn_unused_variable)
+                << DiagD->getDeclName();
+        }
       }
     }
 
diff --git a/lib/Sema/SemaExpr.cpp b/lib/Sema/SemaExpr.cpp
index 9415673a77..8eb0177046 100644
--- a/lib/Sema/SemaExpr.cpp
+++ b/lib/Sema/SemaExpr.cpp
@@ -9885,6 +9885,8 @@ Sema::PopExpressionEvaluationContext() {
 void Sema::MarkDeclarationReferenced(SourceLocation Loc, Decl *D) {
   assert(D && "No declaration?");
 
+  D->setReferenced();
+
   if (D->isUsed(false))
     return;
 
diff --git a/lib/Sema/SemaTemplateInstantiateDecl.cpp b/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 3fe5fd5c95..0ff7ff4d40 100644
--- a/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -298,8 +298,10 @@ Decl *TemplateDeclInstantiator::VisitVarDecl(VarDecl *D) {
 
   Var->setAccess(D->getAccess());
   
-  if (!D->isStaticDataMember())
+  if (!D->isStaticDataMember()) {
     Var->setUsed(D->isUsed(false));
+    Var->setReferenced(D->isReferenced());
+  }
   
   // FIXME: In theory, we could have a previous declaration for variables that
   // are not static data members.
diff --git a/lib/Serialization/ASTReaderDecl.cpp b/lib/Serialization/ASTReaderDecl.cpp
index 5b4a64eeb5..889ef7324c 100644
--- a/lib/Serialization/ASTReaderDecl.cpp
+++ b/lib/Serialization/ASTReaderDecl.cpp
@@ -214,6 +214,7 @@ void ASTDeclReader::VisitDecl(Decl *D) {
   }
   D->setImplicit(Record[Idx++]);
   D->setUsed(Record[Idx++]);
+  D->setReferenced(Record[Idx++]);
   D->setAccess((AccessSpecifier)Record[Idx++]);
   D->setPCHLevel(Record[Idx++] + (F.Type <= ASTReader::PCH));
 }
diff --git a/lib/Serialization/ASTWriterDecl.cpp b/lib/Serialization/ASTWriterDecl.cpp
index 1306ffda6d..b3b55f584a 100644
--- a/lib/Serialization/ASTWriterDecl.cpp
+++ b/lib/Serialization/ASTWriterDecl.cpp
@@ -141,6 +141,7 @@ void ASTDeclWriter::VisitDecl(Decl *D) {
     Writer.WriteAttributes(D->getAttrs(), Record);
   Record.push_back(D->isImplicit());
   Record.push_back(D->isUsed(false));
+  Record.push_back(D->isReferenced());
   Record.push_back(D->getAccess());
   Record.push_back(D->getPCHLevel());
 }
@@ -1133,6 +1134,7 @@ void ASTWriter::WriteDeclsBlockAbbrevs() {
   Abv->Add(BitCodeAbbrevOp(0));                       // HasAttrs
   Abv->Add(BitCodeAbbrevOp(0));                       // isImplicit
   Abv->Add(BitCodeAbbrevOp(0));                       // isUsed
+  Abv->Add(BitCodeAbbrevOp(0));                       // isReferenced
   Abv->Add(BitCodeAbbrevOp(AS_none));                 // C++ AccessSpecifier
   Abv->Add(BitCodeAbbrevOp(0));                       // PCH level
 
diff --git a/test/Sema/warn-unused-function.c b/test/Sema/warn-unused-function.c
index 5bbcf18a62..8f4cdcba88 100644
--- a/test/Sema/warn-unused-function.c
+++ b/test/Sema/warn-unused-function.c
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -Wunused-function -verify %s
+// RUN: %clang_cc1 -fsyntax-only -Wunused-function -Wunneeded-internal-declaration -verify %s
 // RUN: %clang_cc1 -fsyntax-only -verify -Wunused %s
 // RUN: %clang_cc1 -fsyntax-only -verify -Wall %s
 
@@ -6,7 +6,7 @@ void foo() {}
 static void f2() {} 
 static void f1() {f2();} // expected-warning{{unused}}
 
-static int f0() { return 17; } // expected-warning{{unused}}
+static int f0() { return 17; } // expected-warning{{not needed and will not be emitted}}
 int x = sizeof(f0());
 
 static void f3();
@@ -46,7 +46,7 @@ static void f12(void) { }  // expected-warning{{unused}}
 static void f12(void);
 
 // PR7923
-static void unused(void) { unused(); }  // expected-warning{{unused}}
+static void unused(void) { unused(); }  // expected-warning{{not needed and will not be emitted}}
 
 // rdar://8728293
 static void cleanupMalloc(char * const * const allocation) { }
diff --git a/test/SemaCXX/warn-unused-filescoped.cpp b/test/SemaCXX/warn-unused-filescoped.cpp
index c32acb0b48..dbff4b0e68 100644
--- a/test/SemaCXX/warn-unused-filescoped.cpp
+++ b/test/SemaCXX/warn-unused-filescoped.cpp
@@ -78,3 +78,12 @@ namespace test4 {
   void test(A a); // expected-warning {{unused function}}
   extern "C" void test4(A a);
 }
+
+namespace rdar8733476 {
+  static void foo() { } // expected-warning {{not needed and will not be emitted}}
+
+  template <int>
+  void bar() {
+    foo();
+  }
+}