linux-kernel - Re: Multiple local register variables w/ same register

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Date:	Tue, 19 Nov 2013 22:25:18 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Richard Henderson <rth@...ddle.net>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Will Deacon <will.deacon@....com>,
	linux-kernel@...r.kernel.org,
	Catalin Marinas <Catalin.Marinas@....com>,
	lttng-dev@...ts.lttng.org, Nathan Lynch <Nathan_Lynch@...tor.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jakub Jelinek <jakub@...hat.com>, gcc@....gnu.org
Subject: Re: Multiple local register variables w/ same register

----- Original Message -----
> From: "Richard Henderson" <rth@...ddle.net>
> To: "Peter Zijlstra" <peterz@...radead.org>, "Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>
> Cc: "Will Deacon" <will.deacon@....com>, linux-kernel@...r.kernel.org, "Catalin Marinas" <Catalin.Marinas@....com>,
> lttng-dev@...ts.lttng.org, "Nathan Lynch" <Nathan_Lynch@...tor.com>, "Paul E. McKenney"
> <paulmck@...ux.vnet.ibm.com>, "Linus Torvalds" <torvalds@...ux-foundation.org>, "Andrew Morton"
> <akpm@...ux-foundation.org>, "Jakub Jelinek" <jakub@...hat.com>, 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
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