From: Corinna Vinschen <corinna-cygwin@cygwin.com>
To: cygwin-patches@cygwin.com
Subject: Re: [PATCH] Cygwin: Make gcc-specific code in <sys/cpuset.h> compiler-agnostic
Date: Fri, 7 Jul 2023 12:45:58 +0200 [thread overview]
Message-ID: <ZKfs5ivJ94sVv77t@calimero.vinschen.de> (raw)
In-Reply-To: <465f8863-6559-e061-684a-a2a812e9c4c6@maxrnd.com>
On Jul 7 03:13, Mark Geisert wrote:
> Hi Corinna,
>
> Corinna Vinschen wrote:
> > On Jul 7 00:41, Mark Geisert wrote:
> > > The current version of <sys/cpuset.h> cannot be compiled by Clang due to
> > > the use of __builtin* functions. Their presence here was a dubious
> > > optimization anyway, so their usage has been converted to standard
> > > library functions. A popcnt (population count of 1 bits in a word)
> > > function is provided here because there isn't one in the standard library
> > > or elsewhere in the Cygwin DLL.
> >
> > And clang really doesn't provide it? That's unfortunate.
> >
> > Do you really think it's not worth to use it if it's available?
>
> I don't know for sure. I'd guess the popcnt op should be optimized if
> available; the others probably don't need it.
>
> > You could workaround it like this:
> >
> > > +/* Modern CPUs have popcnt* instructions but the need here is not worth
> > > + * worrying about builtins or inline assembler for different compilers. */
> > > +static inline int
> > > +__maskpopcnt (__cpu_mask mask)
> > > +{
> > #if (__GNUC__ >= 4)
> > return __builtin_popcountl (mask);
> > #else
> > > + int res = 0;
> > > + unsigned long ulmask = (unsigned long) mask;
> > > +
> > > + while (ulmask != 0)
> > > + {
> > > + if (ulmask & 1)
> > > + ++res;
> > > + ulmask >>= 1;
> > > + }
> > > + return res;
> > #endif
> > > +}
> > > +
>
> The first version of the patch (unsubmitted) worked something like that,
> though it was a chore figuring out how to tell the difference between gcc
> and clang. clang #defines __GNUC__ (?!) for example. I ended up using
Oh well...
> __GNUC_PREREQ__ with the hope clang version numbers stay lower than gcc
> version numbers. Has to be a better way than that.
>
> On the other hand, one compilation with clang or clang++, I forget which,
> and with some optimization flag, recognized the 'while' loop in that
> function and turned it into the Hackers Delight algorithm for popcnt in ~20
> instructions and no loop.
>
> TL;DR let me ponder this over the weekend.
> Thanks for listening,
No worries,
Corinna
next prev parent reply other threads:[~2023-07-07 10:46 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-07-07 7:41 Mark Geisert
2023-07-07 9:44 ` Corinna Vinschen
2023-07-07 10:13 ` Mark Geisert
2023-07-07 10:45 ` Corinna Vinschen [this message]
2023-07-07 15:46 ` Jon Turney
2023-07-07 15:49 ` Corinna Vinschen
2023-07-07 18:54 ` Brian Inglis
2023-07-08 19:22 ` Brian Inglis
2023-07-08 20:59 ` Mark Geisert
2023-07-08 21:53 ` Mark Geisert
2023-07-09 4:27 ` Brian Inglis
2023-07-09 7:58 ` Mark Geisert
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=ZKfs5ivJ94sVv77t@calimero.vinschen.de \
--to=corinna-cygwin@cygwin.com \
--cc=cygwin-patches@cygwin.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).