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Re: Question about the highly optimized aspects of libc implementation
- From: Will Hawkins <whh8b at virginia dot edu>
- To: Florian Weimer <fweimer at redhat dot com>
- Cc: libc-help at sourceware dot org
- Date: Sun, 3 Dec 2017 20:15:44 -0500
- Subject: Re: Question about the highly optimized aspects of libc implementation
- Authentication-results: sourceware.org; auth=none
- References: <CAE+MWFsT5vu4yW0zDKgyinZky=2s-i2wzK7N-fnKuQP0kGQ_Bw@mail.gmail.com> <firstname.lastname@example.org>
Thank you for your response and forgive me for not responding more
quickly. I just had a few follow-ups that I've sprinkled below!
On Thu, Nov 30, 2017 at 3:36 AM, Florian Weimer <email@example.com> wrote:
> On 11/30/2017 07:47 AM, Will Hawkins wrote:
>> I've been digging through the glibc implementation and looking for
>> examples of where compiler directives or hand-written assembly have
>> been used to improve performance at the "expense" of standards or
> The fcntl implementation calls va_arg on a variadic argument which might not
> actually exist. The syscall function does something similar (but it is
> actually implemented in machine code, so it's less of a problem).
Are you referring to
__libc_fcntl (int fd, int cmd, ...)
va_start (ap, cmd);
arg = va_arg (ap, void *);
> The NSS internals in general and getaddrinfo in particular call functions
> through a mis-matching function pointer (with an additional argument added,
> or with a void * argument where the function is defined with a concrete
> function pointer).
Are you referring to, for example,
if (fct != NULL)
if (req->ai_family == AF_INET6
|| req->ai_family == AF_UNSPEC)
gethosts (AF_INET6, struct in6_addr);
no_inet6_data = no_data;
inet6_status = status;
from sysdeps/posix/getaddrinfo.c where gethosts uses DL_CALL_FCT to
invoke fct as if it were a function that returned void* and with an
additional void* parameter as a result of the call through
> Calling functions such as getpwuid_r with a pointer which has not been
> allocated on the heap (or reusing an existing allocation for a second call)
> is probably not quite valid C due to aliasing violations.
> A lot of the code which manipulates struct sockaddr/sockaddr_in/sockaddr_in6
> objects does not make the additional copies which are needed to avoid
> aliasing violations.
> Do you need more? I can probably go on for quite some time.
What you've given me is great! However, if there are other interesting
ones, I'd love to hear them! I love seeing the /expert/ uses of the C
language for learning. You are all amazing craftpersons -- it's great
to watch you work.
Are there any places where, I know this sounds crazy, but functions
are invoked with push/jump (or straight jumps) because, for instance
they are tail calls or somehow the return address is known statically?
Again, thank you so much for taking the time to respond! I really appreciate it!