Fix PR44281 (bad RA with global regs)

[Michael Matz]

Hi,

On Mon, 29 Feb 2016, Mikael Pettersson wrote:

> Well, almost.  While it is true that a signal handler cannot 
> *accidentally* clobber the register state of the interrupted thread, it 
> can in fact access and update any part of that state via the ucontext_t 
> passed to it.  Doing so is uncommon, but not unheard of and not even 
> that difficult -- I've done it myself in several different runtime 
> systems.

Yeah, well, sure.  That's not clobbering the registers directly though, 
but setting it up so that the kernel does it on return :)  If you do that, 
you have to have a special sig-handler anyway, lest it clobbers other 
registers that are currently in use by the interrupted piece of code.

> The code in a signal handler cannot assume that global register 
> variables are in sync with the interrupted thread, or that plain 
> assignments to them are reflected back, but that's not GCC's fault, nor 
> is it GCC's job to make that happen.

And it's documented to not happen (reliably anyway), so all is fine.


Ciai,
Michael.