http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57176
Bug #: 57176
Summary: copy elision with function arguments passed by value
Classification: Unclassified
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: enhancement
Priority: P3
Component: c++
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: gli...@gcc.gnu.org
Hello,
it is a well know issue that copy elision is allowed but never implemented in
cases like:
A f(A x){
return x;
}
and the reason is that the caller and the callee have to communicate for it to
happen. In C, the function would be:
void f_impl(A* ret_p, A* x_p);
with the caller being responsible for allocating space for ret and x and making
the copy into x.
It seems that to make it work, we would need to notice that f is eligible for
this optimization, mark it somehow (indicating which argument can be used as
return value) and clone it:
void f_impl_nrvo(A* ret_x_p);
Callers who would see this mark would instead call the clone with one less
argument.
There are clearly many parts of the front-end I don't understand enough to do
this, but does it look like a correct and workable approach?
