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Re: [Xen-devel] [PATCH-WIP 01/13] xen/arm: use r12 to pass the hypercall number to the hypervisor

On Fri, 9 Mar 2012, Dave Martin wrote:

> Register variables feel like a red herring though.  We're only using
> those because we can't do the needful thing and actually desscribe
> these constraints in the asm constraints (which would seem to be the
> right place).  We specifically don't care where those values are
> except at the boundaries of the asm block itself.


> Is there a reason why ARM gcc doesn't provide the ability to specify
> such exact-register constraints, or is this more for historical
> reasons?  It is possible?

I don't know how much things have changed since I last looked at the gcc 
code, but implementing this seemed to be pretty trivial at the time.  
The problem would be to determine a good letter scheme to map to actual 


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