Hi Hans,
Thank you very much for your support.
It should not be a clang problem.
It is a problem that variable string (date and build host) is injected into ELF.
In zile/src/help.c:
DEFUN ("zile-version", zile_version)
/*+
Show the zile version.
+*/
{
minibuf_write ("Zile " VERSION " of " CONFIGURE_DATE " on "
CONFIGURE_HOST);
return TRUE;
}
END_DEFUN
This result in .rodata diffs between two build:
***************
*** 1 ****
! [ 1d1a] Zile 2.2.59 of Wed Nov 01 2017 on host-10
--- 1 ----
! [ 1d1a] Zile 2.2.59 of Wed Jul 04 2018 on host-04
"4" is a constant string defined in source code:
In zile/src/variables.c:
/*
* Default variables values table.
*/
static struct var_entry
{
char \*var; /* Variable name. */
char \*val; /* Default value. */
int local; /* If true, becomes local when set. */
} def_vars[] =
{
#define X(var, val, local, doc) { var, val, local },
#include "tbl_vars.h"
#undef X
};
In zile/src/tbl_vars.h:
X ("standard-indent", "4", FALSE, "\
"4" point at the end of " \F4" in one build, and point at the end of "Zile
2.2.59 of Wed Jul 04 2018 on host-04" in another build, thus after linking
cause ELF .data section diffs.
-----Original Message-----
From: hwennb...@google.com [mailto:hwennb...@google.com] On Behalf Of Hans
Wennborg
Sent: Friday, July 6, 2018 5:01 PM
To: Jian, Xu
Cc: cfe-users@lists.llvm.org
Subject: Re: [cfe-users] how clang merge strings in .rodata section
On Fri, Jul 6, 2018 at 10:22 AM, Jian, Xu
<xu.j...@dell.com<mailto:xu.j...@dell.com>> wrote:
> Hi Hans,
> We need to compare whether ELF files of two builds are identical.
> Because of string merge, the comparison has some trouble.
>
> For example in case following code lines (may be in different files):
> ---------------------------------------------------------------
> const char* s_array[1]="s";
> const char *first_s="this first bigger s"; const char *second_s="this
> second bigger s";
> ---------------------------------------------------------------
>
> After clang build ELF out, sometimes the s_array[1] contail the position of
> the tail of first_s in .rodata second, while sometimes second_s.
> This lead to .data section diff since s_array is in it.
> The ELF diffs, while nothing changed from functionality point of view.
Did the inputs change? If Clang is sometimes using the tail of first_s and
sometimes second_s, for the same input, that's a bug. The compilation should be
deterministic.
Can you provide sample input files and command lines that show this problem?
Thanks,
Hans
> -----Original Message-----
> From: hwennb...@google.com<mailto:hwennb...@google.com>
> [mailto:hwennb...@google.com] On Behalf Of
> Hans Wennborg
> Sent: Friday, July 6, 2018 3:54 PM
> To: Jian, Xu
> Cc: cfe-users@lists.llvm.org<mailto:cfe-users@lists.llvm.org>
> Subject: Re: [cfe-users] how clang merge strings in .rodata section
>
> On Thu, Jul 5, 2018 at 3:18 AM, Jian, Xu via cfe-users
> <cfe-users@lists.llvm.org<mailto:cfe-users@lists.llvm.org>> wrote:
>> Hi,
>>
>> The following c source code abc.c:
>>
>> #include <stdio.h>
>>
>> int g_val=10;
>>
>> const char *g_str="abc";
>>
>> const char *g_str1="c";
>>
>> int main(void)
>>
>> {
>>
>> printf("%s %s: %d\n",g_str,g_str1,g_val);
>>
>> return 0;
>>
>> }
>>
>>
>>
>> When compile with “clang abc.c -o abc” then dump .rodata section:
>>
>> # readelf -p .rodata abc
>>
>>
>>
>> String dump of section '.rodata':
>>
>> [ 0] abc
>>
>> [ 4] %s %s: %d
>>
>>
>>
>> When compile with “gcc abc.c -o abc” then dump .rodata section:
>>
>> $ readelf -p .rodata abc
>>
>>
>>
>> String dump of section '.rodata':
>>
>> [ 10] abc
>>
>> [ 14] c
>>
>> [ 16] %s %s: %d^J
>>
>>
>>
>> clang is able to merge short string (“c”) into the tail of a long
>> string (“abc”), while gcc will not.
>>
>> Does anybody know how to disable this behavior (make it similar to gcc) ?
>
> I don't think there is a way to disable it.
>
> Why do you want to disable this behaviour?
>
> - Hans
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