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Message-ID: <20171126005916.GS1627@brightrain.aerifal.cx>
Date: Sat, 25 Nov 2017 19:59:16 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: Do not use 64 bit division if possible
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;
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