Re: [PATCH/RFC] thread-utils: better wrapper to avoid #ifdef NO_PTHREADS

> On Thu, Oct 18, 2018 at 08:05:22PM +0200, Nguyễn Thái Ngọc Duy wrote:
> 
>> On Thu, Oct 18, 2018 at 7:09 PM Jeff King <peff@peff.net> wrote:
>>>> In this particular case though I think we should be able to avoid so
>>>> much #if if we make a wrapper for pthread api that would return an
>>>> error or something when pthread is not available. But similar
>>>> situation may happen elsewhere too.
>>>
>>> Yeah, I think that is generally the preferred method anyway, just
>>> because of readability and simplicity.
>>
>> I've wanted to do this for a while, so let's test the water and see if
>> it's well received.
>>
>> This patch is a proof of concept that adds just enough macros so that
>> I can build index-pack.c on a single thread mode with zero #ifdef
>> related to NO_PTHREADS.
>>
>> Besides readability and simplicity, it reduces the chances of breaking
>> conditional builds (e.g. you rename a variable name but forgot that
>> the variable is in #if block that is not used by your
>> compiler/platform).
> 
> Yes, I love this. We're already halfway there with things like
> read_lock() in index-pack and elsewhere, which are conditionally no-ops.
> The resulting code is much easier to read, I think.
> 

>> Performance-wise I don't think there is any loss for single thread
>> mode. I rely on compilers recognizing HAVE_THREADS being a constant
>> and remove dead code or at least optimize in favor of non-dead code.
>>
>> Memory-wise, yes we use some more memory in single thread mode. But we
>> don't have zillions of mutexes or thread id, so a bit extra memory
>> does not worry me so much.
> 
> Yeah, I don't think carrying around a handful of ints is going to be a
> big deal.
> 

> I also think we may want to make a fundamental shift in our view of
> thread support. In the early days, it was "well, this is a thing that
> modern systems can take advantage of for certain commands". But these
> days I suspect it is more like "there are a handful of legacy systems
> that do not even support threads".
> 
> I don't think we should break the build on those legacy systems, but
> it's probably OK to stop thinking of it as "non-threaded platforms are
> the default and must pay zero cost" and more as "threaded platforms are
> the default, and non-threaded ones are OK to pay a small cost as long as
> they still work".
> 

>> @@ -74,4 +79,29 @@ int init_recursive_mutex(pthread_mutex_t *m)
>>   		pthread_mutexattr_destroy(&a);
>>   	}
>>   	return ret;
>> +#else
>> +	return ENOSYS;
>> +#endif
>> +}
> 
> I suspect some of these ENOSYS could just become a silent success.
> ("yep, I initialized your dummy mutex"). But it probably doesn't matter
> much either way, as we would not generally even bother checking this
> return.
> 
>> +#ifdef NO_PTHREADS
>> +int dummy_pthread_create(pthread_t *pthread, const void *attr,
>> +			 void *(*fn)(void *), void *data)
>> +{
>> +	return ENOSYS;
>>   }
> 
> Whereas for this one, ENOSYS makes a lot of sense (we should avoid the
> threaded code-path anyway when we see that online_cpus()==1, and this
> would let us know when we mess that up).
> 

>> +int dummy_pthread_init(void *data)
>> +{
>> +	/*
>> +	 * Do nothing.
>> +	 *
>> +	 * The main purpose of this function is to break compiler's
>> +	 * flow analysis or it may realize that functions like
>> +	 * pthread_mutex_init() is no-op, which means the (static)
>> +	 * variable is not used/initialized at all and trigger
>> +	 * -Wunused-variable
>> +	 */
>> +	return ENOSYS;
>> +}
> 
> It might be worth marking the dummy variables as MAYBE_UNUSED, exactly
> to avoid this kind of compiler complaint.
> 
> -Peff
>