Re: [PATCH v2 02/10] Finish move of clock_* functions to libc.

[Zack Weinberg <zackw at panix dot com>]

On Tue, Sep 3, 2019 at 9:44 AM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
> On 03/09/2019 10:31, Florian Weimer wrote:
> > * Adhemerval Zanella:
> >> On 03/09/2019 04:29, Florian Weimer wrote:
> >>> * Zack Weinberg:
> >>>> diff --git a/rt/Versions b/rt/Versions
> >>>> index 91e3fd2a20..84d1345420 100644
> >>>> --- a/rt/Versions
> >>>> +++ b/rt/Versions
> >>>> @@ -1,15 +1,3 @@
> >>>> -libc {
> >>>> -  GLIBC_2.17 {
> >>>> -    # c*
> >>>> -    clock_getres; clock_gettime; clock_settime; clock_getcpuclockid;
> >>>> -    clock_nanosleep;
> >>>> -  }
> >>>> -  GLIBC_PRIVATE {
> >>>> -    __clock_getres; __clock_gettime; __clock_settime; __clock_getcpuclockid;
> >>>> -    __clock_nanosleep;
> >>>> -  }
> >>>> -}
> >>>
> >>> Sorry, you cannot remove the GLIBC_2.17 symbol version in this way,
> >>> otherwise old binaries will fail to load.  You need to leave behind a
> >>> dummy function definition.  See __libpthread_version_placeholder for how
> >>> I handled this in the libpthread/vfork case.
> >>
> >> Are you sure about it?
> >
> > Yes. 8-)  But I missed that this block was just moved to time/Versions.
> > Either place will work and generate the same symbol versions.
>
> Right, because that was my understanding (the symbol versions will still
> be generated correctly on libc).

Yes, I moved the GLIBC_2.17 versions from rt/Versions to time/Versions
to keep them together with their .c files.

Adhemerval, Florian, I suspect I won't have very much time to work on
glibc for the next two weeks.  If either or both of you would like to
take over the branch for these patches (zack/y2038-preliminaries) and
keep working on it, please feel free.

Incidentally, regarding the cost of converting from a struct timespec
to a struct timeval on systems where 'long' is 64 bits, I'm getting
what looks like decent code generation out of this definition of
TIMESPEC_TO_TIMEVAL:

#define TIMESPEC_TO_TIMEVAL(tv, ts) do {        \
    uint32_t __nsec = (ts)->tv_nsec;            \
    (tv)->tv_sec = (ts)->tv_sec;                \
    (tv)->tv_usec = __nsec / 1000;              \
} while (0)

e.g.

    movq    8(%rsi), %rax
    movq    (%rsi), %rdx
    movl    %eax, %eax
    movq    %rdx, (%rdi)
    imulq    $274877907, %rax, %rax
    shrq    $38, %rax
    movl    %eax, %eax
    movq    %rax, 8(%rdi)
    ret

(Not sure why the 'movl %eax, %eax', though - clearing the upper 32
bits maybe?  I might throw this at a superoptimizer and see what it
comes up with.)

zw