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

6 files changed, 40 insertions, 7 deletions

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