From: Jiri Slaby <jslaby@suse.cz>
To: Joe Perches <joe@perches.com>,
Arnaud POULIQUEN <arnaud.pouliquen@st.com>,
Ohad Ben-Cohen <ohad@wizery.com>,
Bjorn Andersson <bjorn.andersson@linaro.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
linux-kernel@vger.kernel.org, linux-remoteproc@vger.kernel.org,
Mathieu Poirier <mathieu.poirier@linaro.org>
Cc: Suman Anna <s-anna@ti.com>,
Fabien DESSENNE <fabien.dessenne@st.com>,
linux-stm32@st-md-mailman.stormreply.com,
xiang xiao <xiaoxiang781216@gmail.com>
Subject: Re: [PATCH v7 2/2] tty: add rpmsg driver
Date: Thu, 26 Mar 2020 07:38:29 +0100 [thread overview]
Message-ID: <a06f559c-3507-210c-d743-765163900935@suse.cz> (raw)
In-Reply-To: <f97d9dc54178e4344512bc7986265f101f4639c6.camel@perches.com>
On 26. 03. 20, 1:01, 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
Perfect, but:
* that is userspace (different strlen).
* what about other archs -- AFAIR, i386 implements strlen in asm in the
kernel
Maybe compilers use intrinsics and only after optimizations, they put a
call to strlen?
/me digging
Yeah, even gimple already contains:
int D.2094;
D.2094 = 3;
return D.2094;
That means, even -O0 should generate 3 instead of the call:
$ echo -e '#include <string.h>\nint f() { return strlen("abc"); }' | gcc
-S -x c - -o - -O0
...
f:
.LFB0:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movl $3, %eax
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
So fine, use strlen :).
thanks,
--
js
suse labs
next prev parent reply other threads:[~2020-03-26 6:38 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-24 17:04 [PATCH v7 0/2] Add rpmsg tty driver Arnaud Pouliquen
2020-03-24 17:04 ` [PATCH v7 1/2] rpmsg: core: add API to get MTU Arnaud Pouliquen
2020-03-31 17:36 ` Mathieu Poirier
2020-04-01 6:28 ` Jiri Slaby
2020-04-01 6:29 ` Jiri Slaby
2020-04-01 11:34 ` Arnaud POULIQUEN
2020-03-24 17:04 ` [PATCH v7 2/2] tty: add rpmsg driver Arnaud Pouliquen
2020-03-24 17:18 ` Greg Kroah-Hartman
2020-03-25 11:34 ` Arnaud POULIQUEN
2020-03-24 17:23 ` Joe Perches
2020-03-25 11:36 ` Arnaud POULIQUEN
2020-03-24 17:44 ` Randy Dunlap
2020-03-25 8:10 ` Jiri Slaby
2020-03-25 11:39 ` Arnaud POULIQUEN
2020-03-24 20:52 ` Bjorn Andersson
2020-03-25 16:57 ` Arnaud POULIQUEN
2020-04-06 14:18 ` Arnaud POULIQUEN
2020-05-06 2:54 ` Bjorn Andersson
2020-05-06 10:21 ` Arnaud POULIQUEN
2020-03-25 8:45 ` Jiri Slaby
2020-03-25 13:15 ` Arnaud POULIQUEN
2020-03-25 13:31 ` Jiri Slaby
2020-03-26 0:01 ` Joe Perches
2020-03-26 6:38 ` Jiri Slaby [this message]
2020-03-26 10:59 ` Arnaud POULIQUEN
2020-03-26 12:31 ` Jiri Slaby
2020-03-26 11:40 ` Arnaud POULIQUEN
2020-03-26 11:45 ` Jiri Slaby
2020-04-01 18:06 ` Mathieu Poirier
2020-04-02 15:25 ` Arnaud POULIQUEN
2020-04-03 20:18 ` Mathieu Poirier
2020-07-15 16:06 ` [PATCH v7 0/2] Add rpmsg tty driver Arnaud POULIQUEN
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