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Multiple local register variables w/ same register
- From: Richard Henderson <rth at twiddle dot net>
- To: Peter Zijlstra <peterz at infradead dot org>, Mathieu Desnoyers <mathieu dot desnoyers at efficios dot com>
- Cc: Will Deacon <will dot deacon at arm dot com>, linux-kernel at vger dot kernel dot org, Catalin Marinas <Catalin dot Marinas at arm dot com>, lttng-dev at lists dot lttng dot org, Nathan Lynch <Nathan_Lynch at mentor dot com>, "Paul E. McKenney" <paulmck at linux dot vnet dot ibm dot com>, Linus Torvalds <torvalds at linux-foundation dot org>, Andrew Morton <akpm at linux-foundation dot org>, Jakub Jelinek <jakub at redhat dot com>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
- Date: Wed, 20 Nov 2013 07:56:57 +1000
- Subject: Multiple local register variables w/ same register
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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");
extern void abort(void);
if (f() != 2)
Anyone see anything wrong with the testcase? Do we thing this sort of thing
ought to work, perhaps with scopes lengthened?