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Date: Sat, 25 Nov 2017 20:23:11 -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 02:12:58AM +0100, David Guillen Fandos wrote:
>
> 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!
If this is a real problem you're hitting, I'm interested in helping,
but it seems unlikely. If your program uses printf or other common
functions it will already be pulling in __divdi3 I think.
Rich
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