Re: Guaranteed copy elision

[Jonathan Wakely <jwakely.gcc@gmail.com>]

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On Fri, 18 Nov 2022, 22:00 Segher Boessenkool, <segher@kernel.crashing.org>
wrote:

> On Fri, Nov 18, 2022 at 10:35:20PM +0100, Martin Oberzalek via Gcc-help
> wrote:
> > Am Freitag, dem 18.11.2022 um 16:05 +0100 schrieb Stefan Ring via Gcc-
> > help:
> > > On Tue, Nov 15, 2022 at 6:48 AM Yubin Ruan via Gcc-help
> > > <gcc-help@gcc.gnu.org> wrote:
> > > > Is there any language facility to help us guarantee that at compile
> > > > time
> > > > (such as some kind of static_assert() ) so that we can be confident
> > > > writing
> > > > those one-liner ?
>
> > With c++ language features this may is a solution:
>
> Nope.  Try with -O0 for example.
>
> There is no way to guarantee copy elision.  It isn't even clear what
> *exactly* you want guaranteed, what "copy elision" means *exactly*, what
> "guaranteeing copy elision" means, etc.
>

The standard defines what it means here:
https://eel.is/c++draft/class.copy.elision

Since C++17 it is guaranteed in certain situations:
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0135r0.html

There is a proposal to guarantee it in more cases, including the one the OP
asked about:
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2025r2.html
That has not been approved for the standard, but it explains the status quo
quite well.


> If you write clear and simple (which means: not artificially made
> complex) and correct source code, you can trust the compiler will
> generate good machine code for you (if you use -O2 or such).  If not,
> bug reports are welcome!
>
> A C compiler is not a "portable assembler", you have no direct control
> over generated code.  This is a good thing: the compiler is much better
> at writing fast machine code than users are.
>
>
> Segher
>