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Re: [PATCH] Invalid unpoisoning of stack redzones on ARM

On Tue, Oct 29, 2013 at 01:06:21PM +0000, Richard Sandiford wrote:
> > If it is a pseudo, it is certainly a pseudo that isn't used for
> > anything else, as it is the result of (base >> 3) + constant, if it isn't a
> > pseudo, then supposedly it is better not to just keep adding the offsets to
> > a base and thus not to use the end address from asan_clear_shadow.
> > Your patch would force using the end address even if shadow_base
> > is initially say some_reg + 32, I think it is better to use shadow_mem
> > some_reg + 72 next time instead of some_other_reg + 40.
> But I thought the register pressure thing was to avoid having the
> original base live across the loop.  Doesn't that apply for reg + offset
> as well as plain reg?  If we have to force some_reg + 32 into a
> temporary and then loop on it, using the new base should avoid keeping
> both it and some_reg live at the same time.  Plus smaller offsets are
> often better on some targets.

On the other side, if we force the address to be just register based when it
previously wasn't, it makes life harder for alias analysis etc., at least in
the past say on ia64 I remember the fact that it didn't allow anything but
registers as valid addresses often resulted in tons of missed
optimizations), so in the end I guess I have nothing against the original
patch (with whatever form of the copy to reg call is desirable).


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