From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pa0-x230.google.com ([2607:f8b0:400e:c03::230]) by bombadil.infradead.org with esmtps (Exim 4.80.1 #2 (Red Hat Linux)) id 1Waonk-0007DV-6f for linux-mtd@lists.infradead.org; Thu, 17 Apr 2014 16:04:36 +0000 Received: by mail-pa0-f48.google.com with SMTP id hz1so521989pad.35 for ; Thu, 17 Apr 2014 09:04:13 -0700 (PDT) Date: Thu, 17 Apr 2014 23:57:59 +0800 From: Huang Shijie To: Gerhard Sittig Subject: Re: [PATCH] mtd: spi-nor: fix the wrong dummy value Message-ID: <20140417155758.GA14819@localhost.localdomain> References: <1397636299-2390-1-git-send-email-b32955@freescale.com> <20140416200849.GO3528@book.gsilab.sittig.org> <20140417134127.GA3980@localhost.localdomain> <20140417155507.GT3528@book.gsilab.sittig.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140417155507.GT3528@book.gsilab.sittig.org> Cc: marex@denx.de, Huang Shijie , computersforpeace@gmail.com, linux-mtd@lists.infradead.org, dwmw2@infradead.org List-Id: Linux MTD discussion mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Thu, Apr 17, 2014 at 05:55:07PM +0200, Gerhard Sittig wrote: > On Thu, 2014-04-17 at 21:41 +0800, Huang Shijie wrote: > > > > The disassemble code for "int dummy = 8; dummy /= 8;" is: > > -------------------------------------------------- > > 83a6: 2308 movs r3, #8 > > 83a8: 607b str r3, [r7, #4] > > 83aa: 687b ldr r3, [r7, #4] > > 83ac: 1dda adds r2, r3, #7 > > 83ae: 2b00 cmp r3, #0 > > 83b0: bfb4 ite lt > > 83b2: 4613 movlt r3, r2 > > 83b4: 461b movge r3, r3 > > 83b6: 10db asrs r3, r3, #3 > > 83b8: 607b str r3, [r7, #4] > > -------------------------------------------------- > > > > The disassemble code for "int dummy = 8; dummy >>= 3;" is: > > -------------------------------------------------- > > 83a6: 2308 movs r3, #8 > > 83a8: 607b str r3, [r7, #4] > > 83aa: 687b ldr r3, [r7, #4] > > 83ac: 10db asrs r3, r3, #3 > > 83ae: 607b str r3, [r7, #4] > > -------------------------------------------------- > > > > Obviously, the "dummy >>= 3" is faster then "dummy /= 8". > > That is because of signedness. Both forms of "/= 8" and ">>= 3" > should be identical to the compiler, and generate the same > output. Compilers know that division by powers of two can be > done with a shift. > > Signedness apparently makes a difference. If you know that the > number of clocks always is non-negative, use appropriate data > types. Or let the compiler carry out the correct instructions > for the very data type that was declared. Pick one, don't > violate abstractions. > > Counter example, matching the expectation: > > unsigned int u_div(unsigned int v) { > return v / 8; > } > > unsigned int u_shift(unsigned int v) { > return v >> 3; > } > > 0000001c : > 1c: e1a001a0 lsr r0, r0, #3 > 20: e12fff1e bx lr > > 00000024 : > 24: e1a001a0 lsr r0, r0, #3 > 28: e12fff1e bx lr > > > Anyway, source code should be written for humans, as it gets read > more often than written, and maintenance is hard enough already. > Try to come up with a text search pattern to catch both the 3 and > 8 values at the same time. Or try to easily see how they are the > same when there is no comment. Is the code path so hot that > single instructions count so badly, that the downsides should be > considered acceptable? okay. I will change to "dummy /= 8" in the next version. thanks Huang Shijie