Implemented [[clang::musttail]] attribute for guaranteed tail calls.

This is a Clang-only change and depends on the existing "musttail"
support already implemented in LLVM.

The [[clang::musttail]] attribute goes on a return statement, not
a function definition. There are several constraints that the user
must follow when using [[clang::musttail]], and these constraints
are verified by Sema.

Tail calls are supported on regular function calls, calls through a
function pointer, member function calls, and even pointer to member.

Future work would be to throw a warning if a users tries to pass
a pointer or reference to a local variable through a musttail call.

Reviewed By: rsmith

Differential Revision: https://reviews.llvm.org/D99517

1 files changed, 11 insertions, 3 deletions

diff --git a/clang/lib/CodeGen/CodeGenFunction.h b/clang/lib/CodeGen/CodeGenFunction.h
index 0b3da9e..7723382 100644
--- a/clang/lib/CodeGen/CodeGenFunction.h
+++ b/clang/lib/CodeGen/CodeGenFunction.h
@@ -520,6 +520,10 @@ public:
   /// True if the current statement has nomerge attribute.
   bool InNoMergeAttributedStmt = false;
 
+  // The CallExpr within the current statement that the musttail attribute
+  // applies to.  nullptr if there is no 'musttail' on the current statement.
+  const CallExpr *MustTailCall = nullptr;
+
   /// True if the current function should be marked mustprogress.
   bool FnIsMustProgress = false;
 
@@ -568,6 +572,8 @@ public:
   llvm::Instruction *CurrentFuncletPad = nullptr;
 
   class CallLifetimeEnd final : public EHScopeStack::Cleanup {
+    bool isRedundantBeforeReturn() override { return true; }
+
     llvm::Value *Addr;
     llvm::Value *Size;
 
@@ -3913,12 +3919,14 @@ public:
   /// LLVM arguments and the types they were derived from.
   RValue EmitCall(const CGFunctionInfo &CallInfo, const CGCallee &Callee,
                   ReturnValueSlot ReturnValue, const CallArgList &Args,
-                  llvm::CallBase **callOrInvoke, SourceLocation Loc);
+                  llvm::CallBase **callOrInvoke, bool IsMustTail,
+                  SourceLocation Loc);
   RValue EmitCall(const CGFunctionInfo &CallInfo, const CGCallee &Callee,
                   ReturnValueSlot ReturnValue, const CallArgList &Args,
-                  llvm::CallBase **callOrInvoke = nullptr) {
+                  llvm::CallBase **callOrInvoke = nullptr,
+                  bool IsMustTail = false) {
     return EmitCall(CallInfo, Callee, ReturnValue, Args, callOrInvoke,
-                    SourceLocation());
+                    IsMustTail, SourceLocation());
   }
   RValue EmitCall(QualType FnType, const CGCallee &Callee, const CallExpr *E,
                   ReturnValueSlot ReturnValue, llvm::Value *Chain = nullptr);