Re: [RFA/C] Reimplementation of std::call_once.

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

On Wed, 9 Aug 2023 at 13:51, Iain Sandoe <iain@sandoe.co.uk> wrote:
>
> Hi Jonathan,
>
> > On 9 Aug 2023, at 13:38, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
> >
> > On Wed, 9 Aug 2023 at 13:22, Iain Sandoe <iain@sandoe.co.uk> wrote:
> >>
> >> Hello.
> >>
> >> With the current [posix-y] implementation of std::call_once, we are seeing significant issues for Darwin. The issues vary in strength from “hangs or segvs sometimes depending on optimisation and the day of the week” (newer OS, faster h/w) to “segvs or std::terminates every time” older OS slower h/w.
> >>
> >> Actually, I think that in the presence of exceptions it is not working on Linux either.
> >>
> >> As a use-case: this problem prevents us from building working LLVM/clang tools on Darwin using GCC as the bootstrap compiler.
> >>
> >> the test code I am using is: https://godbolt.org/z/Mxcnv9YEx (derived from a cppreference example, IIRC).
> >>
>
> >> This is a prototype, so I’m looking for advice/comment on:
> >>
> >> (a) if this can be generalised to be part of libstdc++
> >> (b) what that would take ...
> >> (c) ..or if I should figure out how to make this a darwin-specific impl.
> >
> >
> > It's an ABI break. I already tried to replace std::call_once once, see
> > r11-4691-g93e79ed391b9c6 and r11-7688-g6ee24638ed0ad5 and PR 99341.
>
> blast.
>
> >> ——  here is the prototype implementation as a non-patch .. (patch attached).
> >>
>
> >> =====
> >> the implementation in mutex.cc can sit togethe with the old version so that the symbols for that remain available (or, for versioned libraries, the old code can be deleted).
> >
> > If you have old and new code sharing a std::once_flag object you're dead.
>
> indeed - I was only considering the case where existing binaries needed to run with the new lib, not the case where code compiled with two different impls was combined.
>
> > Something like the abi_tag transition for std::string in GCC 5 would be needed.
>
> That sounds quite complicated and likely to produce similar pain?

Not nearly as complicated (since we don't use std::call_once
throughout the entire library) but it would still cause pain for the
ecosystem.

>
> ====
>
> Presumably an alternative is that I just have to accept that Darwin needs to use a versioned library (which is a direction I am close to heading in because of co-existence of mulitple c++ runtimes anyway).

We do need to fix PR 99341 somehow, I just don't know how.