From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:55000)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <rth7680@gmail.com>) id 1VeZ2F-0006LN-Ii
	for qemu-devel@nongnu.org; Thu, 07 Nov 2013 18:30:55 -0500
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <rth7680@gmail.com>) id 1VeZ26-0006NT-VE
	for qemu-devel@nongnu.org; Thu, 07 Nov 2013 18:30:47 -0500
Sender: Richard Henderson <rth7680@gmail.com>
Message-ID: <527C2297.7040407@twiddle.net>
Date: Fri, 08 Nov 2013 09:30:31 +1000
From: Richard Henderson <rth@twiddle.net>
MIME-Version: 1.0
References: <1383769916-5582-1-git-send-email-tommusta@gmail.com>	<1383769916-5582-13-git-send-email-tommusta@gmail.com>
	<527C221A.5000609@twiddle.net>
In-Reply-To: <527C221A.5000609@twiddle.net>
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [PATCH 12/14] VSX Stage 4: Add Scalar SP Fused
	Multiply-Adds
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Tom Musta <tommusta@gmail.com>, qemu-devel@nongnu.org
Cc: qemu-ppc@nongnu.org

On 11/08/2013 09:28 AM, Richard Henderson wrote:
> On 11/07/2013 06:31 AM, Tom Musta wrote:
>>          }                                                                     \
>> +                                                                              \
>> +        if (r2sp) {                                                           \
>> +            float32 tmp32 = float64_to_float32(xt_out.fld[i],                 \
>> +                                               &env->fp_status);              \
>> +            xt_out.fld[i] = float32_to_float64(tmp32, &env->fp_status);       \
>> +        }                                                                     \
>> +                                                                              \
> 
> You can't get correct results for a single-precision fma from a
> double-precision fma and merely rounding the results.
> 
> See e.g. glibc's sysdeps/ieee754/dbl-64/s_fmaf.c.

Blah, nevermind.  That would be using separate add+mul in double-precision, not
using a double-precision fma primitive.


r~