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

[Iain Sandoe <iain@sandoe.co.uk>]

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?

====

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).

thanks
Iain