linux-kernel - Re: [PATCH] mul_u64_u64_div

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Date: Fri, 28 Jun 2024 14:20:25 -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 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.