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From: Andrew Morton on 3 Aug 2010 18:30
On Mon, 2 Aug 2010 18:09:51 +0200
Oleg Nesterov <oleg(a)redhat.com> wrote:

> We have a bugreport which blames div64_u64() on 32bit platforms.
>
> However, the code obviously doesn't even try to pretend it can do
> the 64bit division precisely. If there is something in the high
> word of divisor, div64_u64() just shifts both arguments and throws
> out the low bits.
>
> Add a small comment to avoid the confusion.
>
> Signed-off-by: Oleg Nesterov <oleg(a)redhat.com>
> ---
>
> lib/div64.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> --- a/lib/div64.c
> +++ b/lib/div64.c
> @@ -77,7 +77,10 @@ s64 div_s64_rem(s64 dividend, s32 diviso
> EXPORT_SYMBOL(div_s64_rem);
> #endif
>
> -/* 64bit divisor, dividend and result. dynamic precision */
> +/*
> + * 64bit divisor, dividend and result. Dynamic precision, unless
> + * divisor fits in u32 result is not exactly correct.
> + */
> #ifndef div64_u64
> u64 div64_u64(u64 dividend, u64 divisor)
> {

Well that was a bit lazy of us - I wonder how hard it is to fix.

At present people will test their code on 64-bit only to find out later
that it doesn't work correctly on 32-bit. Bad. Perhaps we should
similarly break the 64-bit version :)

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From: Ben Woodard on 3 Aug 2010 20:20
Andrew,

On Tue, 2010-08-03 at 15:28 -0700, Andrew Morton wrote:
> Well that was a bit lazy of us - I wonder how hard it is to fix.
>
> At present people will test their code on 64-bit only to find out
> later
> that it doesn't work correctly on 32-bit. Bad. Perhaps we should
> similarly break the 64-bit version :)

I just talked to Brian. He's got an implementation that does the
division correctly. He's going to clean it up and send you a patch so
that it will work without losing precision on both 32b and 64b. It is
based upon public domain code from http://www.hackersdelight.org and
has been proven to be correct in all cases.

-ben


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