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From: "René Scharfe" <l.s.r@web.de>
To: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Cc: Git List <git@vger.kernel.org>, Junio C Hamano <gitster@pobox.com>
Subject: Re: [PATCH 1/5] add SWAP macro
Date: Tue, 31 Jan 2017 22:02:54 +0100	[thread overview]
Message-ID: <676ed19c-0c4e-341e-ba30-1f4a23440088@web.de> (raw)
In-Reply-To: <alpine.DEB.2.20.1701311305030.3469@virtualbox>

Am 31.01.2017 um 13:13 schrieb Johannes Schindelin:
> 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.

> 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

  reply	other threads:[~2017-01-31 21:03 UTC|newest]

Thread overview: 50+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-01-28 21:13 [PATCH 0/5] introduce SWAP macro René Scharfe
2017-01-28 21:38 ` [PATCH 1/5] add " René Scharfe
2017-01-30 15:39   ` Johannes Schindelin
2017-01-30 16:48     ` René Scharfe
2017-01-30 20:48       ` Johannes Schindelin
2017-01-30 21:46         ` René Scharfe
2017-01-31 12:13           ` Johannes Schindelin
2017-01-31 21:02             ` René Scharfe [this message]
2017-02-01  0:44               ` Ramsay Jones
2017-02-01 11:39               ` Johannes Schindelin
2017-01-30 16:01   ` Johannes Schindelin
2017-01-30 16:59     ` René Scharfe
2017-01-30 18:41     ` Johannes Sixt
2017-01-30 21:03       ` Johannes Schindelin
2017-01-30 22:09         ` René Scharfe
2017-01-30 22:21           ` Brandon Williams
2017-01-31 21:03             ` René Scharfe
2017-01-31 21:35               ` Jeff King
2017-01-31 22:29                 ` Junio C Hamano
2017-01-31 22:36                   ` Jeff King
2017-02-01 11:28                 ` Johannes Schindelin
2017-02-01 11:47                   ` Jeff King
2017-02-01 18:06                     ` René Scharfe
2017-02-01 18:33                       ` Junio C Hamano
2017-02-07 22:04                         ` René Scharfe
2017-02-07 22:30                           ` Junio C Hamano
2017-02-08 15:14                             ` Johannes Schindelin
2017-01-31 12:03           ` Johannes Schindelin
2017-04-24 11:29   ` Jeff King
2017-04-24 11:49     ` Jeff King
2017-04-24 13:13       ` Duy Nguyen
2017-04-28 17:04     ` René Scharfe
2017-04-28 21:49       ` Jeff King
2017-04-29 18:16         ` René Scharfe
2017-04-30  3:11           ` Jeff King
2017-05-02  5:29             ` René Scharfe
2017-01-28 21:40 ` [PATCH 2/5] apply: use " René Scharfe
2017-01-28 21:40 ` [PATCH 3/5] " René Scharfe
2017-01-30 16:03   ` Johannes Schindelin
2017-01-30 17:18     ` René Scharfe
2017-01-30 22:22   ` Junio C Hamano
2017-01-31 21:02     ` René Scharfe
2017-01-28 21:41 ` [PATCH 4/5] diff: " René Scharfe
2017-01-30 16:04   ` Johannes Schindelin
2017-01-30 17:26     ` René Scharfe
2017-01-30 22:22   ` Junio C Hamano
2017-01-28 21:42 ` [PATCH 5/5] graph: " René Scharfe
2017-01-30 16:16   ` Johannes Schindelin
2017-01-30 17:41     ` René Scharfe
2017-01-30 23:20 ` [PATCH 0/5] introduce " Junio C Hamano

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