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Date: Sun, 26 Nov 2017 13:10:15 +1300
From: Michael Clark <michaeljclark@....com>
To: musl@...ts.openwall.com
Subject: Re: Do not use 64 bit division if possible



> 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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