Re: [Xen-devel] [PATCH v8 02/17] Add cmpxchg16b support for x86-64

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 14.10.15 at 11:29, <feng.wu@xxxxxxxxx> wrote:
> I understand your comments above. My understanding about the gcc
> online doc is we don't need to add memory clobber when the variable
> is either an input _or_ output one. However, seems your options is
> memory clobber can be removed only when it is an input _and_ output
> one. It would be highly appreciated if you can correct me if I understand
> the doc incorrectly.

Yes, I'm pretty convinced you read it wrong: "performs memory
reads or writes to items other than those listed in the input and
output operands" means _any_ operation not expressed by the
ams operands. The two sentence following that actually explain
this in more detail. IOW - unless your operands correctly express
_everything_ the assembly instruction(s) do(es), you need a
clobber (and this is not limited to memory).

Jan


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