Porting libsanitizer to aarch64

[Richard Earnshaw]

On 23/05/13 17:36, Jakub Jelinek wrote:
> On Thu, May 23, 2013 at 05:28:29PM +0100, Richard Earnshaw wrote:
>>> FWIW, I would actually recommend against conditionalizing FRAME_GROWS_DOWNWARD
>>> for a new port.  Just make it _always_ grow down and save yourself the
>>> additional code bloat in the backend.
>>
>> Doing that would add significantly to the cost of setting up the frame.
>
> Why?
>

I just explained the overheads of laying out the frame from the other end.

>> FRAME_GROWS_DOWNWARD
>> Define this macro to nonzero value if the addresses of local
>> variable slots are at negative offsets from the frame pointer.
>
> Yes, but this usually talks about the eliminable soft frame pointer,
> something that can be eliminated later on to hard frame pointer or stack
> pointer.  So, if right now say you are eliminating soft frame pointer
> to hard frame pointer with offset zero, you could eliminate it to
> hard frame pointer minus frame size instead or vice versa (haven't looked
> what aarch64 does now).
>

So you're saying the documentation is wrong, or at the very least 
misleading...  What about dealing with address validation for soft 
accesses?  Surely we get better code if the SFP ~= HFP since we end up 
with fewer cases where we fall back to sub-optimal sequences.

R.