----- On Jul 24, 2016, at 2:01 PM, Dave Watson davejwatson@xxxxxx wrote:
>>> +static inline __attribute__((always_inline))
>>> +bool rseq_finish(struct rseq_lock *rlock,
>>> + intptr_t *p, intptr_t to_write,
>>> + struct rseq_state start_value)
>
>>> This ABI looks like it will work fine for our use case. I don't think it
>>> has been mentioned yet, but we may still need multiple asm blocks
>>> for differing numbers of writes. For example, an array-based freelist push:
>
>>> void push(void *obj) {
>>> if (index < maxlen) {
>>> freelist[index++] = obj;
>>> }
>>> }
>
>>> would be more efficiently implemented with a two-write rseq_finish:
>
>>> rseq_finish2(&freelist[index], obj, // first write
>>> &index, index + 1, // second write
>>> ...);
>
>> Would pairing one rseq_start with two rseq_finish do the trick
>> there ?
>
> Yes, two rseq_finish works, as long as the extra rseq management overhead
> is not substantial.
The different is actually not negligible. On x86-64
Intel(R) Xeon(R) CPU E5-2630 v3 @ 2.40GHz:
(counter increment benchmark (single-thread))
* Single store per increment: 3.6 ns
* Two rseq_finish() per increment: 5.2 ns
* rseq_finish2() with two mov instructions per rseq_finish2(): 4.0 ns
And I expect the difference to be even larger on non-x86 architectures.
I'll try to figure out a way to do rseq_finish() and rseq_finish2()
without duplicating the code. Perhaps macros will be helpful there.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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