Date: Sun, 26 Nov 2017 02:12:58 +0100 From: David Guillen Fandos <david@...idgf.es> To: musl@...ts.openwall.com Subject: Re: Do not use 64 bit division if possible 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
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