Implement semantic analysis and an AST representation for the named

return value optimization. Sema marks return statements with their
NRVO candidates (which may or may not end up using the NRVO), then, at
the end of a function body, computes and marks those variables that
can be allocated into the return slot.

I've checked this locally with some debugging statements (not
committed), but there won't be any tests until CodeGen comes along.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@103865 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 35 insertions, 0 deletions

diff --git a/lib/Sema/SemaDecl.cpp b/lib/Sema/SemaDecl.cpp
index 5bb0b524d1..fb3100353b 100644
--- a/lib/Sema/SemaDecl.cpp
+++ b/lib/Sema/SemaDecl.cpp
@@ -4548,6 +4548,38 @@ Sema::DeclPtrTy Sema::ActOnStartOfFunctionDef(Scope *FnBodyScope, DeclPtrTy D) {
   return DeclPtrTy::make(FD);
 }
 
+/// \brief Given the set of return statements within a function body,
+/// compute the variables that are subject to the named return value 
+/// optimization.
+///
+/// Each of the variables that is subject to the named return value 
+/// optimization will be marked as NRVO variables in the AST, and any
+/// return statement that has a marked NRVO variable as its NRVO candidate can
+/// use the named return value optimization.
+///
+/// This function applies a very simplistic algorithm for NRVO: if every return
+/// statement in the function has the same NRVO candidate, that candidate is
+/// the NRVO variable.
+///
+/// FIXME: Employ a smarter algorithm that accounts for multiple return 
+/// statements and the lifetimes of the NRVO candidates. We should be able to
+/// find a maximal set of NRVO variables.
+static void ComputeNRVO(Stmt *Body, ReturnStmt **Returns, unsigned NumReturns) {
+  const VarDecl *NRVOCandidate = 0;
+  for (unsigned I = 0; I != NumReturns; ++I) {
+    if (!Returns[I]->getNRVOCandidate())
+      return;
+    
+    if (!NRVOCandidate)
+      NRVOCandidate = Returns[I]->getNRVOCandidate();
+    else if (NRVOCandidate != Returns[I]->getNRVOCandidate())
+      return;
+  }
+  
+  if (NRVOCandidate)
+    const_cast<VarDecl*>(NRVOCandidate)->setNRVOVariable(true);
+}
+
 Sema::DeclPtrTy Sema::ActOnFinishFunctionBody(DeclPtrTy D, StmtArg BodyArg) {
   return ActOnFinishFunctionBody(D, move(BodyArg), false);
 }
@@ -4581,6 +4613,9 @@ Sema::DeclPtrTy Sema::ActOnFinishFunctionBody(DeclPtrTy D, StmtArg BodyArg,
       // If this is a constructor, we need a vtable.
       if (CXXConstructorDecl *Constructor = dyn_cast<CXXConstructorDecl>(FD))
         MarkVTableUsed(FD->getLocation(), Constructor->getParent());
+      
+      ComputeNRVO(Body, FunctionScopes.back()->Returns.data(),
+                  FunctionScopes.back()->Returns.size());
     }
     
     assert(FD == getCurFunctionDecl() && "Function parsing confused");