From: David Guillen Fandos <david@davidgf.es>
To: musl@lists.openwall.com
Subject: Re: Do not use 64 bit division if possible
Date: Sun, 26 Nov 2017 02:12:58 +0100 [thread overview]
Message-ID: <fa58abc1-b011-7f6f-2e02-b4e7fb1fecd2@davidgf.es> (raw)
In-Reply-To: <20171126005916.GS1627@brightrain.aerifal.cx>
On 26/11/17 01:59, Rich Felker wrote:
> On Sun, Nov 26, 2017 at 01:49:09AM +0100, David Guillen Fandos wrote:
>> Hey,
>>
>> Wow that's an awesome optimization (the a&-a), didn't know gcc was
>> smart enough to figure that out by itself :D
>
> It doesn't seem to be doing any optimizing for me. What it *should* do
> is optimize the div to ctz+shift.
>
> BTW please don't top-reply; it makes threads hard to follow and hard
> to meaningfully reply to inline.
>
> Rich
>
>
>> I just realized that PAGE_SIZE seems indeed to be defined to a
>> constant for some architectures, did not notice since I was running
>> on MIPS which has a page size different for each uarch.
>>
>> I'd say the (a&-a) is a very simple optimization and we should use
>> it, since it adds almost no complexity and sames some cycles and
>> some .text bytes, which is sometimes a bit tight.
>>
>> Something like this? Doesn't hurt constants, improves some arches :)
>>
>> diff --git a/src/conf/sysconf.c b/src/conf/sysconf.c
>> index b8b761d0..aa9fc9d1 100644
>> --- a/src/conf/sysconf.c
>> +++ b/src/conf/sysconf.c
>> @@ -206,7 +206,7 @@ long sysconf(int name)
>> if (name==_SC_PHYS_PAGES) mem = si.totalram;
>> else mem = si.freeram + si.bufferram;
>> mem *= si.mem_unit;
>> - mem /= PAGE_SIZE;
>> + mem /= (unsigned)(PAGE_SIZE & -PAGE_SIZE);
>> return (mem > LONG_MAX) ? LONG_MAX : mem;
>> case JT_ZERO & 255:
>> return 0;
Sorry for that, default settings you know :)
Well the main reason is cause in MIPS it requires adding __divdi3 which
is around 1KB of code, which hey, it's not much, but why would we need
it right? It makes a difference in embedded tools with statically linked
musl.
Thanks for your interest!
David
next prev parent reply other threads:[~2017-11-26 1:12 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-11-25 20:52 David Guillen Fandos
2017-11-25 23:15 ` Michael Clark
2017-11-25 23:46 ` David Guillen Fandos
2017-11-25 23:53 ` Rich Felker
2017-11-26 0:10 ` Michael Clark
2017-11-26 0:49 ` David Guillen Fandos
2017-11-26 0:59 ` Rich Felker
2017-11-26 1:12 ` David Guillen Fandos [this message]
2017-11-26 1:23 ` Rich Felker
2017-11-26 1:40 ` David Guillen Fandos
2017-11-26 0:49 ` Rich Felker
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=fa58abc1-b011-7f6f-2e02-b4e7fb1fecd2@davidgf.es \
--to=david@davidgf.es \
--cc=musl@lists.openwall.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.
Code repositories for project(s) associated with this public inbox
https://git.vuxu.org/mirror/musl/
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 NNTP newsgroup(s).