From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.org/gmane.linux.lib.musl.general/12129 Path: news.gmane.org!.POSTED!not-for-mail From: Rich Felker Newsgroups: gmane.linux.lib.musl.general Subject: Re: Do not use 64 bit division if possible Date: Sat, 25 Nov 2017 19:49:19 -0500 Message-ID: <20171126004919.GR1627@brightrain.aerifal.cx> References: <424674f0-8460-7807-7366-a87d8588e8bc@davidgf.es> <9716E0B3-B86C-4CFF-8636-6DE4BAA0D716@mac.com> <5575a0c9-0f53-f8e7-e0dc-6c1ff2b594f7@davidgf.es> <20171125235333.GQ1627@brightrain.aerifal.cx> Reply-To: musl@lists.openwall.com NNTP-Posting-Host: blaine.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit X-Trace: blaine.gmane.org 1511657373 30963 195.159.176.226 (26 Nov 2017 00:49:33 GMT) X-Complaints-To: usenet@blaine.gmane.org NNTP-Posting-Date: Sun, 26 Nov 2017 00:49:33 +0000 (UTC) User-Agent: Mutt/1.5.21 (2010-09-15) To: musl@lists.openwall.com Original-X-From: musl-return-12145-gllmg-musl=m.gmane.org@lists.openwall.com Sun Nov 26 01:49:29 2017 Return-path: Envelope-to: gllmg-musl@m.gmane.org Original-Received: from mother.openwall.net ([195.42.179.200]) by blaine.gmane.org with smtp (Exim 4.84_2) (envelope-from ) id 1eIl8Q-0007Zw-Kc for gllmg-musl@m.gmane.org; Sun, 26 Nov 2017 01:49:26 +0100 Original-Received: (qmail 24284 invoked by uid 550); 26 Nov 2017 00:49:31 -0000 Mailing-List: contact musl-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Original-Received: (qmail 24263 invoked from network); 26 Nov 2017 00:49:31 -0000 Content-Disposition: inline In-Reply-To: Original-Sender: Rich Felker Xref: news.gmane.org gmane.linux.lib.musl.general:12129 Archived-At: On Sun, Nov 26, 2017 at 01:10:15PM +1300, Michael Clark wrote: > > > > On 26/11/2017, at 12:53 PM, Rich Felker wrote: > > > > On Sun, Nov 26, 2017 at 12:46:56AM +0100, David Guillen Fandos wrote: > >> Thanks for your response. > >> Please note that PAGE_SIZE is not a constant but an alias to > >> libc.page_size which is a variable of type size_t (signed). > >> That's why at O1+ gcc doesn't generate a shift. > > > > Indeed; this varies by arch. > > Oh, I wasn’t aware of that. > > >> I also created a patch to include libc.page_shift, but as far as I > >> can see no other functions would benefit from it, since there's no > >> other divides there (only negations, additions and subtractions). > > > > Adding infrastructure complexity except in cases where it makes a > > significant improvement to size or performance is generally not > > desirable. mmap() is one other place where, in principle, division by > > PAGE_SIZE might take place, but in practice the size is constant 4096 > > or 8192 on all archs. > > > >> And yeah I agree, a_ctz_l is not exactly inexpensive but I guess it > >> is better than full 64 bit signed division (that's why I cast > >> unsigned otherwise the shift right is not trivial due to the sign). > > > > The cost here is more a matter of adding a reading complexity > > dependency on musl internals (a_*) where it's not needed. I wonder if > > GCC could optimize it if we instead of /PAGE_SIZE wrote > > /(PAGE_SIZE&-PAGE_SIZE). Or if we did something like define PAGE_SIZE > > as ((libc.page_size&-libc.page_size)==libc.page_size ? libc.page_size > > : 1/0) so that "PAGE_SIZE is not a power of 2" would become an > > unreachable case. > > Interesting. It seems GCC figures out the division by zero is unreachable but the (n&-n) expression leads to a power of two, not to a log2 n so the ctz is still required. > > - https://cx.rv8.io/g/eHf2Ah > > One could do so once at initialisation time and add PAGE_SHIFT and on architectures with variable page sizes do this: > > #define PAGE_SHIFT libc.page_shift > > diff --git a/src/env/__libc_start_main.c b/src/env/__libc_start_main.c > index 2d758af..f24d10a 100644 > --- a/src/env/__libc_start_main.c > +++ b/src/env/__libc_start_main.c > @@ -29,6 +29,7 @@ void __init_libc(char **envp, char *pn) > __hwcap = aux[AT_HWCAP]; > __sysinfo = aux[AT_SYSINFO]; > libc.page_size = aux[AT_PAGESZ]; > + libc.page_shift = a_ctz_l(libc.page_size); > > if (!pn) pn = (void*)aux[AT_EXECFN]; > if (!pn) pn = ""; > > That isolates the a_ctz_l to one place. Is there a reason it makes a difference? The operation involves a syscall so the cost of a division is going to be dominated by the syscall. If you're calling this repeatedly/in a loop, your program is going to be super slow with or without the division. Rich