From: Richard Henderson <richard.henderson@linaro.org>
To: Eric Blake <eblake@redhat.com>, qemu-devel@nongnu.org
Cc: Peter Maydell <peter.maydell@linaro.org>, berrange@redhat.com
Subject: Re: [PATCH] utils: Reduce chance of rounding inaccuracy in qemu_strtosz.
Date: Sat, 13 Mar 2021 15:48:43 -0600 [thread overview]
Message-ID: <f5648ec4-2a02-2c4e-a865-5c0fcda04436@linaro.org> (raw)
In-Reply-To: <20210311200702.1302855-1-eblake@redhat.com>
On 3/11/21 2:07 PM, Eric Blake wrote:
> + /*
> + * Add in a fudge-factor (2^53 when double is IEEE format) for
> + * all scales less than P (2^50), so that things like
> + * 12.345M with unit 1000 produce 12345000 instead of
> + * 12344999.
> + */
> + if (mul > 1e49) {
The comment says less than, the code says greater than.
> An alternative patch might be writing (uint64_t)(fraction * mul + 0.5)
> (that is, introduce the fudge factor after the multiplication instead
> of before). Preferences?
I think I would prefer this, or for further rounding error reduction,
fma(fraction, mul, 0.5).
r~
next prev parent reply other threads:[~2021-03-13 21:49 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-11 20:07 [PATCH] utils: Reduce chance of rounding inaccuracy in qemu_strtosz Eric Blake
2021-03-13 21:48 ` Richard Henderson [this message]
2021-03-15 11:33 ` Eric Blake
2021-03-15 13:18 ` Richard Henderson
2021-03-15 18:07 ` Eric Blake
2021-03-15 11:28 ` Daniel P. Berrangé
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