Re: [Xen-devel] [PATCH] x86: fix wait code asm() constraints

[Keir Fraser <keir.xen@xxxxxxxxx>]

On 03/08/2012 12:36, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:

>> I'm confused. The registers have the same values at the start and the end of
>> the asm statement. How can it possibly matter, even in theory, whether they
>> temporarily change in the middle? Is this fairly strong assumption written
>> down in the gcc documentation anywhere?
> 
> It's in the specification of the & modifier:
> 
> "?&¹ Means (in a particular alternative) that this operand is an
>  earlyclobber operand, which is modified before the instruction
>  is finished using the input operands. Therefore, this operand
>  may not lie in a register that is used as an input operand or as
>  part of any memory address."
> 
> Of course, here we're not having any other operands, which
> is why at least at present getting this wrong does no harm.

Yep, okay, that makes sense. Especially the use of an input operand to form
a memory address, although of course that cannot happen in our specific case
here. I have acked your patch, although I'd like an update to the patch
comment.

 Thanks,
 -- Keir



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