Allow functions to be marked "implicit return zero" and so mark main().

Codegen by initializing the return value with its LLVM type's null value.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@77288 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 17 insertions, 1 deletions

diff --git a/lib/CodeGen/CGCall.cpp b/lib/CodeGen/CGCall.cpp
index 88a5eeb68e..d7019b605d 100644
--- a/lib/CodeGen/CGCall.cpp
+++ b/lib/CodeGen/CGCall.cpp
@@ -505,6 +505,19 @@ void CodeGenModule::ConstructAttributeList(const CGFunctionInfo &FI,
 void CodeGenFunction::EmitFunctionProlog(const CGFunctionInfo &FI,
                                          llvm::Function *Fn,
                                          const FunctionArgList &Args) {
+  // If this is an implicit-return-zero function, go ahead and
+  // initialize the return value.  TODO: it might be nice to have
+  // a more general mechanism for this that didn't require synthesized
+  // return statements.
+  if (const FunctionDecl* FD = dyn_cast<FunctionDecl>(CurFuncDecl)) {
+    if (FD->hasImplicitReturnZero()) {
+      QualType RetTy = FD->getResultType().getUnqualifiedType();
+      const llvm::Type* LLVMTy = CGM.getTypes().ConvertType(RetTy);
+      llvm::Constant* Zero = CGM.getLLVMContext().getNullValue(LLVMTy);
+      Builder.CreateStore(Zero, ReturnValue);
+    }
+  }
+
   // FIXME: We no longer need the types from FunctionArgList; lift up and
   // simplify.
 
diff --git a/lib/Sema/SemaDecl.cpp b/lib/Sema/SemaDecl.cpp
index 7ab7356aa0..9b6de12bb2 100644
--- a/lib/Sema/SemaDecl.cpp
+++ b/lib/Sema/SemaDecl.cpp
@@ -3501,8 +3501,11 @@ Sema::DeclPtrTy Sema::ActOnFinishFunctionBody(DeclPtrTy D, StmtArg BodyArg,
   Stmt *Body = BodyArg.takeAs<Stmt>();
   if (FunctionDecl *FD = dyn_cast_or_null<FunctionDecl>(dcl)) {
     FD->setBody(Body);
-    if (!FD->isMain())
+    if (FD->isMain())
       // C and C++ allow for main to automagically return 0.
+      // Implements C++ [basic.start.main]p5 and C99 5.1.2.2.3.
+      FD->setHasImplicitReturnZero(true);
+    else
       CheckFallThroughForFunctionDef(FD, Body);
     
     if (!FD->isInvalidDecl())