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Re: [Qemu-ppc] [Qemu-devel] [PATCH 00/14] VSX Stage 4
From: |
Tom Musta |
Subject: |
Re: [Qemu-ppc] [Qemu-devel] [PATCH 00/14] VSX Stage 4 |
Date: |
Fri, 08 Nov 2013 08:53:39 -0600 |
User-agent: |
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:24.0) Gecko/20100101 Thunderbird/24.1.0 |
On 11/7/2013 6:23 PM, Richard Henderson wrote:
> On 11/07/2013 06:31 AM, Tom Musta wrote:
>> The single-precision scalar arithmetic instructions all interpret the most
>> significant 64 bits of a VSR as a single precision floating point number
>> stored in double precision format (similar to the standard PowerPC floating
>> point single precision instructions). Thus a common theme in the supporting
>> code is rounding of an intermediate double-precision number to single
>> precision.
>
> Modulo my comments wrt the actual computation of fma, the patches all look
> fine.
>
> But I'd like to also mention a pre-existing flaw/niggle in the ppc port.
>
> The conversions to/from in-register representation for the single-precision
> values should never raise exceptions. Yet we always use
>
> d.d = float32_to_float64(f.f, &env->fp_status);
> f.f = float64_to_float32(d.d, &env->fp_status);
>
> The use of env->fp_status is either wrong or extremely misleading. It sure
> looks like the operation affects global cpu state. It may be that that state
> is never copied back to the "real" fpscr and so doesn't actually affect cpu
> state, but how can I see that for sure?
>
> I think it would be better to implement ConvertSPtoDP_NP and ConvertSP64toSP
> exactly as written in the spec.
>
> Or at minimum use a dummy fp_status that's not associated with env. It should
> not matter what the "real" rounding mode is in either case, since values that
> are not exactly representable as single-precision values give undefined
> results.
Richard:
Thanks for your comments. I concur with this comment on fp_status.
I am looking into the comments on fused multiply-add.
- [Qemu-ppc] [PATCH 13/14] VSX Stage 4: Add xscvsxdsp and xscvuxdsp, (continued)
- [Qemu-ppc] [PATCH 13/14] VSX Stage 4: Add xscvsxdsp and xscvuxdsp, Tom Musta, 2013/11/06
- [Qemu-ppc] [PATCH 12/14] VSX Stage 4: Add Scalar SP Fused Multiply-Adds, Tom Musta, 2013/11/06
- Re: [Qemu-ppc] [Qemu-devel] [PATCH 12/14] VSX Stage 4: Add Scalar SP Fused Multiply-Adds, Richard Henderson, 2013/11/07
- Re: [Qemu-ppc] [Qemu-devel] [PATCH 12/14] VSX Stage 4: Add Scalar SP Fused Multiply-Adds, Richard Henderson, 2013/11/07
- Re: [Qemu-ppc] [Qemu-devel] [PATCH 12/14] VSX Stage 4: Add Scalar SP Fused Multiply-Adds, Richard Henderson, 2013/11/07
- Re: [Qemu-ppc] [Qemu-devel] [PATCH 12/14] VSX Stage 4: Add Scalar SP Fused Multiply-Adds, Tom Musta, 2013/11/13
- Re: [Qemu-ppc] [Qemu-devel] [PATCH 12/14] VSX Stage 4: Add Scalar SP Fused Multiply-Adds, Richard Henderson, 2013/11/13
- Re: [Qemu-ppc] [Qemu-devel] [PATCH 12/14] VSX Stage 4: Add Scalar SP Fused Multiply-Adds, Tom Musta, 2013/11/14
[Qemu-ppc] [PATCH 14/14] VSX Stage 4: Add xxleqv, xxlnand and xxlorc, Tom Musta, 2013/11/06
Re: [Qemu-ppc] [Qemu-devel] [PATCH 00/14] VSX Stage 4, Richard Henderson, 2013/11/07
Re: [Qemu-ppc] [Qemu-devel] [PATCH 00/14] VSX Stage 4, Andreas Färber, 2013/11/08