Re: Do not use 64 bit division if possible

[Rich Felker <dalias@libc.org>]

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;