Re: C aggregate passing (Rust kernel policy)

Laurent Pinchart
2025-02-21T20:24:23Z

On Fri, Feb 21, 2025 at 10:23:33PM +0200, Laurent Pinchart wrote:
> On Fri, Feb 21, 2025 at 09:06:14PM +0100, Jan Engelhardt wrote:
> > On Friday 2025-02-21 19:34, David Laight wrote:
> > >>
> > >> Returning aggregates in C++ is often implemented with a secret extra
> > >> pointer argument passed to the function. The C backend does not
> > >> perform that kind of transformation automatically. I surmise ABI reasons.
> > >
> > > Have you really looked at the generated code?
> > > For anything non-trivial if gets truly horrid.
> > >
> > > To pass a class by value the compiler has to call the C++ copy-operator to
> > > generate a deep copy prior to the call, and then call the destructor after
> > > the function returns - compare against passing a pointer to an existing
> > > item (and not letting it be written to).
> >
> > And that is why people generally don't pass aggregates by value,
> > irrespective of the programming language.
>
> It's actually sometimes more efficient to pass aggregates by value.
> Considering std::string for instance,
>
> std::string global;
>
> void setSomething(std::string s)
> {
> global = std::move(s);
> }
>
> void foo(int x)
> {
> std::string s = std::to_string(x);
>
> setSomething(std::move(s));
> }
>
> Passing by value is the most efficient option. The backing storage for
> the string is allocated once in foo(). If you instead did
>
> std::string global;
>
> void setSomething(const std::string &s)
> {
> global = s;
> }
>
> void foo(int x)
> {
> std::string s = std::to_string(x);
>
> setSomething(s);
> }
>
> then the data would have to be copied when assigned global.
>
> The std::string object itself needs to be copied in the first case of
> course, but that doesn't require heap allocation. The best solution
> depends on the type of aggregates you need to pass. It's one of the
> reasons string handling is messy in C++, due to the need to interoperate
> with zero-terminated strings, the optimal API convention depends on the
> expected usage pattern in both callers and callees. std::string_view is
> no silver bullet :-(
>
> > > Returning a class member is probably worse and leads to nasty bugs.
> > > In general the called code will have to do a deep copy from the item
> > > being returned
> >
> > People have thought of that already and you can just
> > `return std::move(a.b);`.
>
> Doesn't that prevent NRVO (named return value optimization) in C++ ?
> Starting in C++17, compilers are required to perform copy ellision.

Ah my bad, I missed the 'a.'. NRVO isn't possible.

> > > Then you get code like:
> > > const char *foo = data.func().c_str();
> > > very easily written looks fine, but foo points to garbage.
> >
> > Because foo is non-owning, and the only owner has gone out of scope.
> > You have to be wary of that.
> >
> > > You can return a reference - that doesn't go out of scope.
> >
> > That depends on the refererred item.
> > string &f() { string z; return z; }
> > is going to explode (despite returning a reference).
> >
> > > (Apart from the fact that c++ makes it hard to ensure all the non-class
> > > members are initialised.)
> >
> > struct stat x{};
> > struct stat x = {};
> >
> > all of x's members (which are scalar and thus non-class) are
> > initialized. The second line even works in C.

--
Regards,

Laurent Pinchart