On Sun, Jan 1, 2023 at 11:47 PM Iain Sandoe wrote: > > > > On 2 Jan 2023, at 00:53, Thomas Rodgers wrote: > > > > __platform_wait_t should be whatever the platform supports lock free > natively. The use of a 64 bit int there in the fall through was copied from > Olivier's original implementation for libc++, which uses __ulock_wait/wake > on Darwin which takes a unit64_t, because I had intended to add support > Darwin, but haven't done so yet. > > In general, libc++ supports fewer versions of Darwin than GCC does (I > don’t know right now which versions/archs we support have the > __ulock_wait/wake). Of course, I would very much like to see an efficient > solution for Darwin - so please let me know/share patches with me when you > get to it - I can test on older supported versions. > I *think* it's supported for 10.12 onward, for anything older the mutex/condvar implementation would have to be used. > > However, the issue here is not really Darwin-specific - it will effect any > target that does not have either a futex or a 64b lock-free atomic. > > I agree. I was just relating the back-story of why it was 64b in the not-Linux case. > thanks > Iain > > > > > On Fri, Dec 30, 2022 at 2:51 AM Iain Sandoe wrote: > > > > > > > On 29 Dec 2022, at 17:02, Jonathan Wakely > wrote: > > > > > > > > > > > > On Thu, 29 Dec 2022, 16:22 Iain Sandoe, wrote: > > > > > > > > > > On 29 Dec 2022, at 15:44, Jonathan Wakely > wrote: > > > > > > > > > > > > > > > > On Thu, 29 Dec 2022, 15:30 Iain Sandoe, wrote: > > > > Hi Folks, > > > > > > > > > On 29 Dec 2022, at 12:09, Jonathan Wakely > wrote: > > > > > > > > > On Thu, 29 Dec 2022, 11:29 Iain Sandoe, wrote: > > > > > > > > >> The recent addition of the tz handling has pulled in a dependency > on > > > > >> > > > > >> This currently specifies __platform_wait_t as a 64bit quatity on > platforms without _GLIBCXX_HAVE_LINUX_FUTEX. > > > > >> > > > > >> PowerPC does not have a 64b atomic without library support - so > that this causes a bootstrap > > > > >> fail on powerpc-darwin (and I guess any other 32b powerpc > non-futex target). > > > > >> > > > > >> Rather than contrive to build and add libatomic (which is not at > present available at the point > > > > >> that libstdc++ is built), I wonder if there is any specific > reason that __platform_wait_t needs > > > > >> to be 64 bits on these platforms? (Especially since the futex > case uses an int.) > > > > >> > > > > > I think we do want the generic case's _M_wait atomic variable to > be lock free, otherwise we use two locks for every operation, the one in > libatomic and the waiter mutex. That's more important than it being any > specific width. > > > > > > > > Definitely, that’s probably a recipe for some subtle race condition > .. nested locks etc. > > > > > > > > I didn't see any nested cases from a quick look, but it would still > be better to avoid two locks. > > > > > > > > > > > > >> Advice on the right way to fix this welcome — as a work-around to > allow bootstrap to complete > > > > >> I applied the patch below - but that seems unlikely to be the > right thing generically . > > > > >> > > > > > Rather than __lp64__ I think we should check the > ATOMIC_LONG_LOCK_FREE macro and use long if it's lock free and int > otherwise. But Tom needs to confirm that. That would be approximately the > same as your patch in practice. > > > > > > > > OK.. that makes sense here’s a proposed patch (pending subsequent > input from Tom). > > > > > > > > I am using “lock free always” as the criterion, “sometimes” does not > seem useful here. > > > > > > > > Agreed. > > > > > > > > > > > > Although we normally build libstdc++ with the just-built GCC... > > > > .. AFAIK the __SIZEOF_* are available from any version of GCC or > clang that would > > > > be capable of building the sources. > > > > > > > > Yep, but do we need the size checks at all? > > > > > > > > I was thinking we could just use 'unsigned long' or 'unsigned int' > directly, instead of a uintN_t typedef. Using the typedef just seems to > complicate things. > > > > > > That’s fine by me - I was just copying what was there :) > > > > > > In this patch I made it so that a target without a ‘suitable' > lock-free size would fail to > > > compile the source, which seems better than a link fail later — I > could make it more > > > specific (e.g. # fail clause) or we could test for smaller lock-free > entities… > > > > > > I think we can just eschew atomics altogether in that case, and just > use the mutex for all accesses. I can do that after the break when I'm back > online. > > > > Great, thanks! > > cheers > > Iain > > > > I’m using this locally in the meantime: > > > > # if ATOMIC_LONG_LOCK_FREE == 2 > > using __platform_wait_t = long; > > # elif ATOMIC_INT_LOCK_FREE == 2 > > using __platform_wait_t = int; > > # elif ATOMIC_SHORT_LOCK_FREE == 2 > > using __platform_wait_t = short; > > # elif ATOMIC_CHAR_LOCK_FREE == 2 > > using __platform_wait_t = char; > > # else > > # fail No suitable lock-free type found. > > # endif > > > >