[cfe-users] how clang merge strings in .rodata section

[Jian, Xu via cfe-users]

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.

Thanks.

-----Original Message-----
From: hwennborg at google.com [mailto:hwennborg at google.com] On Behalf Of Hans Wennborg
Sent: Friday, July 6, 2018 3:54 PM
To: Jian, Xu
Cc: cfe-users at 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 at 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