Re: [RFC PATCH v7 7/7] Restartable sequences: self-tests

----- On Jul 24, 2016, at 2:01 PM, Dave Watson davejwatson-b10kYP2dOMg@public.gmane.org wrote:

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+static inline __attribute__((always_inline))
+bool rseq_finish(struct rseq_lock *rlock,
+ intptr_t *p, intptr_t to_write,
+ struct rseq_state start_value)

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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:

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void push(void *obj) {
if (index < maxlen) {
freelist[index++] = obj;
}
}

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would be more efficiently implemented with a two-write rseq_finish:

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rseq_finish2(&freelist[index], obj, // first write
&index, index + 1, // second write
...);

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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