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Re: [Xen-devel] [PATCH] x86: usercopy assembly constraints

  • To: Jan Beulich <jbeulich@xxxxxxxxxx>
  • From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
  • Date: Wed, 14 Nov 2007 11:10:01 +0000
  • Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
  • Delivery-date: Wed, 14 Nov 2007 03:11:13 -0800
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AcgmruZMJNuAHJKiEdypyAAX8io7RQ==
  • Thread-topic: [Xen-devel] [PATCH] x86: usercopy assembly constraints

On 14/11/07 07:37, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

>> What's the bug? Does some gcc allocate the 4th output parameter into one of
>> the other used registers (cx,si,di) if we don't use '=&'?
> No, the effect is an 'impossible constraint' error, so there's not much reason
> to be afraid I think. But as said, the constraint was wrong.

I thought '&' was an extra constraint, so I'm surprised it could fix an
'impossible constraint' problem. But anyway, yes, the code is saner and more
consistent with your patch.

 -- Keir

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