Re: [PATCH 1/5] add SWAP macro

From: René Scharfe <l.s.r@web.de>
Date: Jan 31, 2017, 21:02 UTC
Message-ID: <676ed19c-0c4e-341e-ba30-1f4a23440088@web.de>
In-Reply-To: <alpine.DEB.2.20.1701311305030.3469@virtualbox>

Am 31.01.2017 um 13:13 schrieb Johannes Schindelin:

Show 16 quoted lines

> Hi René,
>
> On Mon, 30 Jan 2017, René Scharfe wrote:
>
>> Am 30.01.2017 um 21:48 schrieb Johannes Schindelin:
>>
>>> The commit you quoted embarrasses me, and I have no excuse for it. I
>>> would love to see that myswap() ugliness fixed by replacing it with a
>>> construct that is simpler, and generates good code even without any
>>> smart compiler.
>>
>> I don't see a way to do that without adding a type parameter.
>
> Exactly. And you know what? I would be very okay with that type parameter.
>
> Coccinelle [*1*] should be able to cope with that, too, mehtinks.

Yes, a semantic patch can turn the type of the temporary variable into a macro parameter. Programmers would have to type the type, though, making the macro only half as good.

> It would be trivially "optimized" out of the box, even when compiling with
> Tiny C or in debug mode.

Such a compiler is already slowed down by memset(3) calls for initializing objects and lack of other optimizations. I doubt a few more memcpy(3) calls would make that much of a difference.

NB: git as compiled with TCC fails several tests, alas.  Builds wickedly 
fast, though.

Show 20 quoted lines

> And it would even allow things like this:
>
> #define SIMPLE_SWAP(T, a, b) do { T tmp_ = a; a = b; b = tmp_; } while (0)
> ...
> 	uint32_t large;
> 	char nybble;
>
> 	...
>
> 	if (!(large & ~0xf)) {
> 		SIMPLE_SWAP(char, nybble, large);
> 		...
> 	}
>
> i.e. mixing types, when possible.
>
> And while I do not necessarily expect that we need anything like this
> anytime soon, merely the fact that it allows for this flexibility, while
> being very readable at the same time, would make it a pretty good design
> in my book.

Such a skinny macro which only hides repetition is kind of attractive due to its simplicity; I can't say the same about the mixed type example above, though.

The fat version isn't that bad either even without inlining, includes a few safety checks and doesn't require us to tell the compiler something it already knows very well. I'd rather let the machine do the work.

René