linux-kernel - Re: [PATCH v7 2/2] tty: add rpmsg driver

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Date:   Thu, 26 Mar 2020 11:59:28 +0100
From:   Arnaud POULIQUEN <arnaud.pouliquen@...com>
To:     Joe Perches <joe@...ches.com>, Jiri Slaby <jslaby@...e.cz>,
        Ohad Ben-Cohen <ohad@...ery.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        <linux-kernel@...r.kernel.org>, <linux-remoteproc@...r.kernel.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>
CC:     Suman Anna <s-anna@...com>,
        Fabien DESSENNE <fabien.dessenne@...com>,
        <linux-stm32@...md-mailman.stormreply.com>,
        xiang xiao <xiaoxiang781216@...il.com>
Subject: Re: [PATCH v7 2/2] tty: add rpmsg driver



On 3/26/20 1:01 AM, Joe Perches wrote:
> On Wed, 2020-03-25 at 14:31 +0100, Jiri Slaby wrote:
>> The question was exactly about that: can a compiler optimize it to a
>> bare number or will strlen call remain there?
> 
> $ cat str.c
> #include <string.h>
> 
> int foo(void)
> {
> 	return strlen("abc");
> }
> 
> $ gcc -c -O2 str.c
> $ objdump -d str.o
> str.o:     file format elf64-x86-64
> 
> 
> Disassembly of section .text:
> 
> 0000000000000000 <foo>:
>    0:	f3 0f 1e fa          	endbr64 
>    4:	b8 03 00 00 00       	mov    $0x3,%eax
>    9:	c3                   	retq   
> 
> 
same result with  arm gcc using  -O1 or -Og:

str.o:     file format elf32-littlearm


Disassembly of section .text:

00000000 <foo>:
   0:	e3a00003 	mov	r0, #3
   4:	e12fff1e 	bx	lr

So in conclusion replacing sizeof by srlen even if not optimized in -o0, right?

Thanks,
Arnaud