linux-kernel - Re: [PATCH] mul_u64_u64_div

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Date: Fri, 28 Jun 2024 15:15:55 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Nicolas Pitre <npitre@...libre.com>
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 17:46:33 -0400 (EDT) Nicolas Pitre <npitre@...libre.com> wrote:

> 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?

Well, the fancy way would be as a new lib/test_XXX.c.  There's a ton of
precedent for that.  A less fancy way would be to paste the code into
this email thread ;)