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Re: [Qemu-devel] [PATCH v1 08/33] s390x/tcg: Implement VECTOR LOAD

From: David Hildenbrand
Subject: Re: [Qemu-devel] [PATCH v1 08/33] s390x/tcg: Implement VECTOR LOAD
Date: Thu, 28 Feb 2019 17:40:52 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0

On 28.02.19 17:34, Richard Henderson wrote:
> On 2/27/19 11:48 PM, David Hildenbrand wrote:
>> I think that would be wrong. It is only an alignment hint.
>> "Setting the alignment hint to a non-zero value
>> that doesn’t correspond to the alignment of the second operand may
>> reduce performance on some models."
>> So we must not inject an exception when unaligned. This, however would
>> be the result of MO_ALIGN,, right?
> Ah, I didn't get that an alignment exception is not raised.  (I do find that
> odd.  If the user is asserting a given alignment, why would we not tell him if
> he is wrong?)

I was wondering the same thing. Most probably because they didn't
specify that that field has to contain 0 when introducing the
instruction. And as they added the alignment constraint only on new
hardware generations (z14), it could result for some instructions where
stuff "used to work" to suddenly report an exception.

> So, yes, ignore all of this from me -- leave MO_ALIGN off.
> r~



David / dhildenb

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