Add support for function attribute 'disable_tail_calls'.

The ``disable_tail_calls`` attribute instructs the backend to not
perform tail call optimization inside the marked function.

For example, 

int callee(int);

int foo(int a) __attribute__((disable_tail_calls)) {
  return callee(a); // This call is not tail-call optimized.
}

Note that this attribute is different from 'not_tail_called', which
prevents tail-call optimization to the marked function.

rdar://problem/8973573

Differential Revision: http://reviews.llvm.org/D12547

llvm-svn: 252986

2 files changed, 28 insertions, 2 deletions

diff --git a/clang/lib/CodeGen/CGCall.cpp b/clang/lib/CodeGen/CGCall.cpp
index 430184d4398..5dea876451f 100644
--- a/clang/lib/CodeGen/CGCall.cpp
+++ b/clang/lib/CodeGen/CGCall.cpp
@@ -1481,8 +1481,12 @@ void CodeGenModule::ConstructAttributeList(const CGFunctionInfo &FI,
       FuncAttrs.addAttribute("no-frame-pointer-elim-non-leaf");
     }
 
+    bool DisableTailCalls =
+        CodeGenOpts.DisableTailCalls ||
+        (TargetDecl && TargetDecl->hasAttr<DisableTailCallsAttr>());
     FuncAttrs.addAttribute("disable-tail-calls",
-                           llvm::toStringRef(CodeGenOpts.DisableTailCalls));
+                           llvm::toStringRef(DisableTailCalls));
+
     FuncAttrs.addAttribute("less-precise-fpmad",
                            llvm::toStringRef(CodeGenOpts.LessPreciseFPMAD));
     FuncAttrs.addAttribute("no-infs-fp-math",
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index a9473d2e119..abdab9a0692 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -1583,6 +1583,15 @@ static void handleCommonAttr(Sema &S, Decl *D, const AttributeList &Attr) {
     D->addAttr(CA);
 }
 
+static void handleNakedAttr(Sema &S, Decl *D, const AttributeList &Attr) {
+  if (checkAttrMutualExclusion<DisableTailCallsAttr>(S, D, Attr.getRange(),
+                                                     Attr.getName()))
+    return;
+
+  D->addAttr(::new (S.Context) NakedAttr(Attr.getRange(), S.Context,
+                                         Attr.getAttributeSpellingListIndex()));
+}
+
 static void handleNoReturnAttr(Sema &S, Decl *D, const AttributeList &attr) {
   if (hasDeclarator(D)) return;
 
@@ -1713,6 +1722,16 @@ static void handleNotTailCalledAttr(Sema &S, Decl *D,
       Attr.getRange(), S.Context, Attr.getAttributeSpellingListIndex()));
 }
 
+static void handleDisableTailCallsAttr(Sema &S, Decl *D,
+                                       const AttributeList &Attr) {
+  if (checkAttrMutualExclusion<NakedAttr>(S, D, Attr.getRange(),
+                                          Attr.getName()))
+    return;
+
+  D->addAttr(::new (S.Context) DisableTailCallsAttr(
+      Attr.getRange(), S.Context, Attr.getAttributeSpellingListIndex()));
+}
+
 static void handleUsedAttr(Sema &S, Decl *D, const AttributeList &Attr) {
   if (const VarDecl *VD = dyn_cast<VarDecl>(D)) {
     if (VD->hasLocalStorage()) {
@@ -4933,7 +4952,7 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
     handleHotAttr(S, D, Attr);
     break;
   case AttributeList::AT_Naked:
-    handleSimpleAttribute<NakedAttr>(S, D, Attr);
+    handleNakedAttr(S, D, Attr);
     break;
   case AttributeList::AT_NoReturn:
     handleNoReturnAttr(S, D, Attr);
@@ -5056,6 +5075,9 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
   case AttributeList::AT_NotTailCalled:
     handleNotTailCalledAttr(S, D, Attr);
     break;
+  case AttributeList::AT_DisableTailCalls:
+    handleDisableTailCallsAttr(S, D, Attr);
+    break;
   case AttributeList::AT_Used:
     handleUsedAttr(S, D, Attr);
     break;