Re: [PATCH 15/16] commit-reach: make can_all_from_reach... linear

From: René Scharfe <l.s.r@web.de>
Date: Oct 5, 2018, 19:36 UTC
Message-ID: <c05f192b-4e89-48b0-1c23-b43ec6fdb74b@web.de>
In-Reply-To: <20181005190847.GC17482@sigill.intra.peff.net>

Am 05.10.2018 um 21:08 schrieb Jeff King:

Show 19 quoted lines

> On Fri, Oct 05, 2018 at 08:48:27PM +0200, René Scharfe wrote:
>> +#define DEFINE_SORT(name, type, compare)				\
>> +static int compare##_void(const void *one, const void *two)		\
>> +{									\
>> +	return compare(one, two);					\
>> +}									\
>> +static void name(type base, size_t nmemb)				\
>> +{									\
>> +	const type dummy = NULL;					\
>> +	if (nmemb > 1)							\
>> +		qsort(base, nmemb, sizeof(base[0]), compare##_void);	\
>> +	else if (0)							\
>> +		compare(dummy, dummy);					\
>> +}
> 
> I do like that this removes the need to have the code block aspart of
> the macro.
> 
> Did you measure to see if there is any runtime impact?

No, but I wouldn't expect any -- the generated code should be the same in most cases.

Here's an example: https://godbolt.org/z/gwXENy.

> As an aside, we may need to take a "scope" argument in case somebody
> wants to do this in a non-static way.

Sure. (They could easily wrap the static function, but a macro parameter is simpler still.)

> It would be nice if we could make
> this "static inline", but I don't think even a clever compiler would be
> able to omit the wrapper call.

It could, if it was to inline qsort(3). Current compilers don't do that AFAIK, but I wouldn't be too surprised if they started to.

The typed comparison function can be inlined into the one with the void pointers, though.

René