[lttng-dev] Multiple local register variables w/ same register

[Mathieu Desnoyers]

----- Original Message -----
> From: "Richard Henderson" <rth at twiddle.net>
> To: "Peter Zijlstra" <peterz at infradead.org>, "Mathieu Desnoyers" <mathieu.desnoyers at efficios.com>
> Cc: "Will Deacon" <will.deacon at arm.com>, linux-kernel at vger.kernel.org, "Catalin Marinas" <Catalin.Marinas at arm.com>,
> lttng-dev at lists.lttng.org, "Nathan Lynch" <Nathan_Lynch at mentor.com>, "Paul E. McKenney"
> <paulmck at linux.vnet.ibm.com>, "Linus Torvalds" <torvalds at linux-foundation.org>, "Andrew Morton"
> <akpm at linux-foundation.org>, "Jakub Jelinek" <jakub at redhat.com>, gcc at gcc.gnu.org
> Sent: Tuesday, November 19, 2013 4:56:57 PM
> Subject: Multiple local register variables w/ same register
> 
> On 11/20/2013 03:33 AM, Peter Zijlstra wrote:
> > On Tue, Nov 19, 2013 at 05:02:20PM +0000, Mathieu Desnoyers wrote:
> >> Unfortunately I don't have a ARM cross-compiler setup ready. Nathan could
> >> test
> >> it for us though.
> >>
> >> It might shuffle things around enough to work around the issue, but with
> >> the
> >> approach you propose, I would be concerned about the compiler being within
> >> its rights to reorder the code into the following sequence:
> >>
> >> struct thread_info *ptra, *ptrb;
> >>
> >> ptra = current_thread_info();
> >> /*
> >>  * each current_thread_info() would have a clobber on *sp, which orders
> >>  * those two wrt each other.
> >>   */
> >> ptrb = current_thread_info();
> >>
> >> load from ptra->preempt_count;
> >> /*
> >>  * however, the following accesses that depend on ptra and ptrb could be
> >>  * reordered if the compiler has no way to know that ptra and ptrb are
> >>  * aliased.
> >>  */
> >> store to ptrb->preempt_count;
> >>
> >> One question that might be worth asking: with the local register variable
> >> extension
> >> (http://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/Local-Reg-Vars.html#Local-Reg-Vars)
> >> (thanks to Jakub for the pointer), should the compiler consider two
> >> variables
> >> bound to the same register as being aliased or not ? AFAIU, local reg vars
> >> appear
> >> to be architecture-specific, so maybe there is something fishy on ARM ?
> 
> It appears not:
> 
> int __attribute__((noinline)) f(void)
> {
>   {
>     register int x __asm__("eax");
>     x = 1;
>   }
>   {
>     register int y __asm__("eax");
>     return ++y;
>   }
> }
> 
> extern void abort(void);
> 
> int main(void)
> {
>   if (f() != 2)
>     abort();
>   return 0;
> }
> 
> Anyone see anything wrong with the testcase?

This testcase is targeting a general purpose register, whereas the issue I'm presenting gets the stack pointer as base address for many memory operations targeting the same offset from this base address. So strictly speaking, I think the two cases are slightly different.

Thanks,

Mathieu


> Do we thing this sort of thing
> ought to work, perhaps with scopes lengthened?
> 
> 
> r~
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com