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xen-devel
[Xen-devel] Re: [patch] pae: tlbflush linear page table updates
On 12 Aug 2005, at 10:56, Andi Kleen wrote:
"early clobber" is good to prevent output alias as input, if that
output
may be clobbered before input is used. However there seems no point to
simply alias among inputs. Or else, the only way I can see is that
compiler insert some extra lines in the middle of inline asm... Any
benefit for this likelihood?
There can be cases e.g. when the inputs are dependent and you use
general enough constraints. e.g. one input can be %reg and the other
offset(%reg). In this case they will essentially alias.
That would be a nasty one to track down. :-)
It seems the general policy in Linux also is not to bother with '&' on
outputs that have a forced alias in the input list. I'd be happy to see
a cleanup patch that adds more '&'s to Xen though -- in cases where we
are relying on the forced alias then it's unlikely to change the
generated assembly code.
-- Keir
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