From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753035AbbIQCFZ (ORCPT ); Wed, 16 Sep 2015 22:05:25 -0400 Received: from sender154-mail.zoho.com ([74.201.84.154]:27829 "EHLO sender154-mail.zoho.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753117AbbIQCFY (ORCPT ); Wed, 16 Sep 2015 22:05:24 -0400 Message-ID: <55FA1C36.7060007@innercoder.com> Date: Wed, 16 Sep 2015 20:49:42 -0500 From: Jaime Arrocha User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Icedove/31.4.0 MIME-Version: 1.0 To: David Laight , "'Austin S Hemmelgarn'" , Steve Calfee , Eric Curtin CC: Valentina Manea , "shuah.kh@samsung.com" , USB list , Kernel development list Subject: Re: First kernel patch (optimization) References: <1442346808-3784-1-git-send-email-ericcurtin17@gmail.com> <55F95671.6060405@gmail.com> <063D6719AE5E284EB5DD2968C1650D6D1CB97D13@AcuExch.aculab.com> In-Reply-To: <063D6719AE5E284EB5DD2968C1650D6D1CB97D13@AcuExch.aculab.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/16/2015 07:56 AM, David Laight wrote: > From: Austin S Hemmelgarn >> Sent: 16 September 2015 12:46 >> On 2015-09-15 20:09, Steve Calfee wrote: >>> On Tue, Sep 15, 2015 at 12:53 PM, Eric Curtin wrote: >>>> Signed-off-by: Eric Curtin >>>> >>>> diff --git a/tools/usb/usbip/src/usbip_detach.c b/tools/usb/usbip/src/usbip_detach.c >>>> index 05c6d15..9db9d21 100644 >>>> --- a/tools/usb/usbip/src/usbip_detach.c >>>> +++ b/tools/usb/usbip/src/usbip_detach.c >>>> @@ -47,7 +47,9 @@ static int detach_port(char *port) >>>> uint8_t portnum; >>>> char path[PATH_MAX+1]; >>>> >>>> - >>>> + unsigned int port_len = strlen(port); >>>> + >>>> + for (unsigned int i = 0; i < port_len; i++) >>>> if (!isdigit(port[i])) { >>>> err("invalid port %s", port); >>>> return -1; >>>> >>>> -- >>> Hi Eric, >>> >>> This is fine, but what kind of wimpy compiler optimizer will not move >>> the constant initializer out of the loop? I bet if you compare binary >>> sizes/code it will be exactly the same, and you added some characters >>> of code. Reorganizing code for readability is fine, but for compiler >>> (in)efficiency seems like a bad idea. >> While I agree with your argument, I would like to point out that it is a >> well established fact that GCC's optimizers are kind of brain-dead at >> times and need their hands held. >> >> I'd be willing to bet that the code will be marginally larger (because >> of adding another variable), but might run slightly faster too (because >> in my experience, GCC doesn't always catch things like this), and should >> compile a little faster (because the optimizers don't have to do as much >> work). > The compiler probably can't optimise the strlen(). > If isdigit() is a real function (the locale specific one probably is) > then the compile cannot assume that port[n] isn't changed by the call > to isdigit. > > A simpler change would be: > for (unsigned int i = 0; port[i] != 0; i++) > > Much better would be to use strtoul() instead of atoi(). > > David > I actually took some time to verify this. GCC makes this optimization with -O2 at least on gcc 4.7.2. One interesting observation I found was that in O0 and O2, it does make a call to strlen while in O1 it calculates the length of the string using: repnz scas %es:(%rdi),%al not %rcx sub $0x2,%rcx Why does it do that? Is the code above faster? If yes, why not do it in O2 too? Is this still a topic for this forum? gcc version 4.7.2 (Debian 4.7.2-5) code void conv_input(char *port) { int portnum; for(int i = 0; i