From: Christophe Lyon <christophe.lyon@st.com>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>
Subject: Re: [Qemu-devel] [PATCH] rsqrte_f32: No need to copy sign bit.
Date: Mon, 10 Oct 2011 13:26:36 +0200 [thread overview]
Message-ID: <4E92D66C.1060903@st.com> (raw)
In-Reply-To: <CAFEAcA_h8N8tudY6cxedYP3qXKF-7fyrdxk0EAnxHhnccwgBMg@mail.gmail.com>
On 09.10.2011 00:57, Peter Maydell wrote:
> On 3 October 2011 15:28, Christophe Lyon<christophe.lyon@st.com> wrote:
>> Indeed, the result is known to be always positive.
>> - val = ((val64>> 63)& 0x80000000)
>> - | ((result_exp& 0xff)<< 23)
>> + val = ((result_exp& 0xff)<< 23)
>> | ((val64>> 29)& 0x7fffff);
>> return make_float32(val);
> So we weren't generating incorrect results, we were just doing
> slightly more work than we really needed, right? I'm curious
> what prompted this patch :-)
>
Exactly. And no way to expose a bug :-(
I was reading 2 revisions of the ARM ARM, and noticed erratas in the descriptions of FPRecipEstimate and FPRSqrtEstimate.
Sign propagation has been removed in the former, and not in the later, so I re-read both functions carefully as well as qemu's implementation and came to this conclusion :-)
I have contacted ARM support and suggested them to fix both function accordingly :-)
Christophe.
next prev parent reply other threads:[~2011-10-10 11:26 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-10-03 14:28 [Qemu-devel] [PATCH] rsqrte_f32: No need to copy sign bit Christophe Lyon
2011-10-08 22:57 ` Peter Maydell
2011-10-10 11:26 ` Christophe Lyon [this message]
2011-10-10 12:26 ` Peter Maydell
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