Re: [Xen-devel] [PATCH] add canonical address checks to HVM

[Keir Fraser <keir@xxxxxxxxxxxxx>]

>> Is there any guarantee that right-shift is signed when using gcc?
> 
> I suppose so, I believe this is assumed to be that way in various other
> places.
> However, I'm not sure I have an idea where I could look up implementation
> defined behavior for gcc.

Looking into it a bit I think a compiler has to be consistent (i.e., it is
implementation *defined*) and gcc makes reasonable effort to do signed
shifts on architectures that have ISA support for it. I may add a boot-time
BUG_ON() just as a sanity check. :-)

> Sure, should work too, but would incur more overhead. I was actually trying
> to even avoid the two shifts, but I wasn't able to find something that would
> use just one *and* would be faster than the version I submitted.

I don't think it's possible. Two shifts and a compare is pretty tight.

 -- Keir

> 
> Jan
> 
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