linux-kernel - Re: [PATCH] mul_u64_u64_div

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Message-ID: <9nr61164-o95o-s47s-33sq-ss86499p6sso@onlyvoer.pbz>
Date: Fri, 28 Jun 2024 17:46:33 -0400 (EDT)
From: Nicolas Pitre <npitre@...libre.com>
To: Andrew Morton <akpm@...ux-foundation.org>
cc: Uwe Kleine-König <u.kleine-koenig@...libre.com>, 
    linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mul_u64_u64_div_u64: make it precise always

On Fri, 28 Jun 2024, Andrew Morton wrote:

> On Fri, 28 Jun 2024 15:06:20 -0400 (EDT) Nicolas Pitre <npitre@...libre.com> wrote:
> 
> > Library facilities must always return exact results. If the caller may
> > be contented with approximations then it should do the approximation on
> > its own.
> > 
> > In this particular case the comment in the code says "the algorithm
> > ... might lose some precision". Well, if you try it with e.g.:
> > 
> > 	a = 18446462598732840960
> > 	b = 18446462598732840960
> > 	c = 18446462598732840961
> > 
> > then the produced answer is 0 whereas the exact answer should be
> > 18446462598732840959. This is _some_ precision loss indeed!
> > 
> > Let's reimplement this function so it always produces the exact result
> > regardless of its inputs while preserving existing fast paths
> > when possible.
> 
> I assume this was tested with some userspace harness?  It would be
> interesting to see that so that reviewers can see that suitable cases
> have been covered.

I do have a user space test tool but it isn't pretty looking.
How should this be acceptably presented for public consumption?


Nicolas