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Re: [patch] Default to --enable-libstdcxx-time=auto


On 23 May 2013 11:28, Jakub Jelinek wrote:
> On Wed, May 22, 2013 at 02:35:40PM +0200, Jakub Jelinek wrote:
>> non-steady clock instead.  Or, have you also considered just using
>> for this routine
>> #if _GLIBCXX_HAS_SYS_SYSCALL_H
>> #include <sys/syscall.h>
>> #endif
>>
>> #if defined (SYS_clock_gettime) && defined (CLOCK_MONOTONIC)
>> syscall (SYS_clock_gettime, CLOCK_MONOTONIC, &tp);
>> #endif
>> if clock_gettime isn't available, at least on Linux?
>> The implementation seems to ignore ENOSYS from clock_gettime, so ignoring it
>> even here wouldn't make it worse.
>
> I mean something like completely untested following patch, then it would
> be pretty much enabled for all non-prehistoric Linux builds (there is a risk
> of it returning garbage on 2.4.x and earlier kernels, if you compile it on
> something that defines __NR_clock_gettime in their headers, but the exact
> same risk is if you do the same with --enable-libstdcxx-time=rt
> (clock_gettime wrapper in glibc will return -1/ENOSYS in that case, so will
> the syscall, but chrono.cc seems to ignore that return value)).
> 2.6+ kernels (2004-ish and later or so) should support CLOCK_MONOTONIC just

This looks great to me, thanks.

> fine.  Of course, there is a possibility of fallback, at least for the
> clock_gettime/syscall CLOCK_RUNTIME or gettimeofday, if they fail, fall
> through into the time case, and for CLOCK_MONOTONIC perhaps just lie and
> return time as well, shouldn't really affect almost anybody.

We should consider doing that yes, but it's less urgent.

> Still, the ABI question is there, would we want to apply to 4.8.1 (can we
> get agreement on that RSN, this is pretty much the only blocker for 4.8.1
> rc2 right now) and, would we export that symbol as @@GLIBCXX_3.4.18 (with
> all trunk @@GLIBCXX_3.4.18 symbols moved to 3.4.19) and add @GLIBCXX_3.4.17
> alias for backwards compatibility with those that configured with
> --enable-libstdcxx-time=rt ?

I like that plan.


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