Date: Sun, 26 Nov 2017 01:49:09 +0100 From: David Guillen Fandos <david@...idgf.es> To: musl@...ts.openwall.com Subject: Re: Do not use 64 bit division if possible Hey, Wow that's an awesome optimization (the a&-a), didn't know gcc was smart enough to figure that out by itself :D 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; On 26/11/17 01:10, Michael Clark wrote: > > >> On 26/11/2017, at 12:53 PM, Rich Felker <dalias@...c.org> wrote: >> >> On Sun, Nov 26, 2017 at 12:46:56AM +0100, David Guillen Fandos wrote: >>> Thanks for your response. >>> Please note that PAGE_SIZE is not a constant but an alias to >>> libc.page_size which is a variable of type size_t (signed). >>> That's why at O1+ gcc doesn't generate a shift. >> >> Indeed; this varies by arch. > > Oh, I wasn’t aware of that. > >>> I also created a patch to include libc.page_shift, but as far as I >>> can see no other functions would benefit from it, since there's no >>> other divides there (only negations, additions and subtractions). >> >> Adding infrastructure complexity except in cases where it makes a >> significant improvement to size or performance is generally not >> desirable. mmap() is one other place where, in principle, division by >> PAGE_SIZE might take place, but in practice the size is constant 4096 >> or 8192 on all archs. >> >>> And yeah I agree, a_ctz_l is not exactly inexpensive but I guess it >>> is better than full 64 bit signed division (that's why I cast >>> unsigned otherwise the shift right is not trivial due to the sign). >> >> The cost here is more a matter of adding a reading complexity >> dependency on musl internals (a_*) where it's not needed. I wonder if >> GCC could optimize it if we instead of /PAGE_SIZE wrote >> /(PAGE_SIZE&-PAGE_SIZE). Or if we did something like define PAGE_SIZE >> as ((libc.page_size&-libc.page_size)==libc.page_size ? libc.page_size >> : 1/0) so that "PAGE_SIZE is not a power of 2" would become an >> unreachable case. > > Interesting. It seems GCC figures out the division by zero is unreachable but the (n&-n) expression leads to a power of two, not to a log2 n so the ctz is still required. > > - https://cx.rv8.io/g/eHf2Ah > > One could do so once at initialisation time and add PAGE_SHIFT and on architectures with variable page sizes do this: > > #define PAGE_SHIFT libc.page_shift > > diff --git a/src/env/__libc_start_main.c b/src/env/__libc_start_main.c > index 2d758af..f24d10a 100644 > --- a/src/env/__libc_start_main.c > +++ b/src/env/__libc_start_main.c > @@ -29,6 +29,7 @@ void __init_libc(char **envp, char *pn) > __hwcap = aux[AT_HWCAP]; > __sysinfo = aux[AT_SYSINFO]; > libc.page_size = aux[AT_PAGESZ]; > + libc.page_shift = a_ctz_l(libc.page_size); > > if (!pn) pn = (void*)aux[AT_EXECFN]; > if (!pn) pn = ""; > > That isolates the a_ctz_l to one place. >
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