get_gcov_type() vs. -fprofile-update=atomic

[Sebastian Huber]

On 09/08/2021 12:22, Richard Biener wrote:
> On Mon, Aug 9, 2021 at 8:52 AM Sebastian Huber
> <sebastian.huber@embedded-brains.de>  wrote:
>> Hello,
>>
>> I would like to use gcov for a multi-threaded program running on an SMP
>> machine using a 32-bit SPARC/LEON3 target. This target supports
>> HAVE_atomic_compare_and_swapsi but not HAVE_atomic_compare_and_swapdi.
>> Unfortunately we have:
>>
>> /* Return the type node for gcov_type.  */
>>
>> tree
>> get_gcov_type (void)
>> {
>>     scalar_int_mode mode
>>       = smallest_int_mode_for_size (LONG_LONG_TYPE_SIZE > 32 ? 64 : 32);
>>     return lang_hooks.types.type_for_mode (mode, false);
>> }
>>
>> The long long type is 64-bit, the get_gcov_type() returns a 64-bit type.
>> This disables the atomic support in tree_profiling().
>>
>> For what is the gcov type used? Could we add an option to force it to
>> 32-bit? What would be the consequences?
> It's a generic counter used for number of CFG edge traversal tracking.

So I guess if this counter overflows, then you get inconsistent 
coverage/profiling information?

> 
> Can you not implement 64bit atomic support for 32bit SPARC somehow?

The 32-bit SPARC/LEON3 has only a 32-bit compare and swap instruction 
(gcc/config/sparc/sync.md). I don't know how you could implement a 
64-bit atomic support using this without spin locks (this is how 
libatomic support for RTEMS works).

-- 
embedded brains GmbH
Herr Sebastian HUBER
Dornierstr. 4
82178 Puchheim
Germany
email: sebastian.huber@embedded-brains.de
phone: +49-89-18 94 741 - 16
fax:   +49-89-18 94 741 - 08

Registergericht: Amtsgericht München
Registernummer: HRB 157899
Vertretungsberechtigte Geschäftsführer: Peter Rasmussen, Thomas Dörfler
Unsere Datenschutzerklärung finden Sie hier:
https://embedded-brains.de/datenschutzerklaerung/