ARM: Fix occasional safepoint crash

[Andrew Dinn]

On 28/05/12 13:46, Andrew Haley wrote:
> We have been seeing occasional crashes, sometimes in the garbage
> collector, on ARM.  These tended to happen at times of high memory
> allocation and therefore lots of GC activity.
> 
> With help from Andrew Dinn I found the problem.  The frame walking
> code that finds roots during garbage collection wals the stack, so you
> have to make sure that the saved SP is correct.  It wasn't: the C++
> interpreter assumes that the TOS item is saved at sp[1], not sp[0].
> That is to say, the code used to get the address of the top item is:
> 
> inline intptr_t* frame::interpreter_frame_tos_address() const {
>   return get_interpreterState()->_stack + 1;
> }
> 
> It's easy enough to fix this: adjust the saved SP when we enter a
> safepoint.

This patch certainly looks fine by eyeball. I have successfully rebuilt
with this patch (I also included the previously posted patch which
ensures 64 bit long copies are atomic) and the rerun the SpecJVM tests
which previously manifested the failures -- specifically
compiler.compiler and compiler.sunflow -- and did not see the previous
SEGV errors.

I also reran these same tests with the tweaked VM which forces regular
and frequent garbage collections and once again no SEGV errors manifested.

Although this is clearly conclusive proof that the problem is solved I
think it is a good enough test to suggest that this patch should be
committed.

regards,


Andrew Dinn
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