[cfe-dev] Detecting (N)RVO

[Kim Gräsman via cfe-dev]

Hi all,

I'm experimenting with a patch to let users mark functions with an attribute

[[clang::requires_rvo]]
std::string f() {
    return std::string("Hello world");
}

and in the presence of this attribute, warn if (N)RVO didn't happen.
This would be useful to prevent small code changes from disabling RVO
in critical paths.

I have the attribute in place, and I've seen that I can check if it
exists with FunctionDecl::hasAttr. But finding where NRVO is actually
decided for a given function turns out to be more difficult...

SemaInit appears to do something for NRVO/copy elision more generally
when constructors are called. But it only marks the CXXConstructExpr
as elidable, as far as I can see, and I'm not sure if this is used to
effect RVO later (I found something in
CodeGenFunction::EmitCXXConstructExpr, but there seems to be more
playing into the decision of whether to construct at caller or not.

Also, I'm not sure if my terminology is right; I haven't found any
references to RVO-sans-the-N, is that refered to as copy elision? NRVO
only seems to address VarDecls and their lifetime.

Thanks for any ideas,
- Kim