[Bug 1420556] [NEW] gdb source tests are failing in Ubuntu 15.04

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---Problem Description---
gdb source tests are failing in Ubuntu 15.04
 
---uname output---
Linux ubuntu 3.18.0-12-generic #13-Ubuntu SMP Thu Jan 29 13:44:26 UTC 2015 ppc64le ppc64le ppc64le GNU/Linux
 
Machine Type = P8 
  
---Steps to Reproduce---
Install a Power VM LPAR with Ubuntu 15.04 ISO.
Then install gdb source and build and execute the same.

root at ubuntu:~# apt-get source gdb
root at ubuntu:~# cd gdb-7.8.2/
root at ubuntu:~/gdb-7.8.2# dpkg-buildpackage -b

                === gdb Summary ===

# of expected passes            25147
# of unexpected failures        280
# of unexpected successes       2
# of expected failures          66
# of unknown successes          1
# of known failures             59
# of unresolved testcases       11
# of untested testcases         21
# of unsupported tests          99
/root/gdb-7.8.2/build/objdir/gdb/testsuite/../../gdb/gdb version  7.8.2-0ubuntu1) -nw -nx -data-directory /root/gdb-7.8.2/build/objdir/gdb/testsuite/../data-directory

Package: gdb-7.8.2-0ubuntu1

I've reproduced the tests and looking at gdb.log, noticed that for some
reason breakpoints for main() are being hit in an unexpected place and
this is causing a lot of testcase failures. Also, this problem happens
only when using gcc 4.9 (Ubuntu 15.04 default); it's not reproducible on
gcc 4.8 or 5. This is still being investigated.

Moreover, in the gdb.python/python.exp specifically, there is this
commit 55cfb2c4c8accd5031fd7c8167988fc4624e847c from upstream gdb that
fixes the testcase, removing 19 of the unexpected failures.

I've created this testcase that exemplifies the issue with the
'breakpoint main' hit the unexpected place:

ubuntu at ubuntu:~$ cat array.c
int main (void)
{
  int array[1];

  return 0;
}
ubuntu at ubuntu:~$ gcc array.c -g -o array-4.9
ubuntu at ubuntu:~$ gdb -q ./array-4.9 -ex 'break main' -ex run
Reading symbols from ./array-4.9...done.
Breakpoint 1 at 0x1000060c: file array.c, line 3.
Starting program: /home/ubuntu/array-4.9 

Breakpoint 1, main () at array.c:3
3       {
(gdb) 

While with gcc-4.8, gdb stops at the expected place:

ubuntu at ubuntu:~$ gcc-4.8 array.c -g -o array-4.8
ubuntu at ubuntu:~$ gdb -q ./array-4.8 -ex 'break main' -ex run
Reading symbols from ./array-4.8...done.
Breakpoint 1 at 0x100005a0: file array.c, line 6.
Starting program: /home/ubuntu/array-4.8 

Breakpoint 1, main () at array.c:6
6         return 0;
(gdb) 

Comparing the assembly output generated by both gcc's, we can also
notice a repeated '.loc 1 3 0' in the code that seems to cause this
misplaced breakpoint:

ubuntu at ubuntu:~$ gcc array.c -g -o array-4.9.s -S
ubuntu at ubuntu:~$ cat array-4.9.s
(...)
main:   
.LFB0:
        .file 1 "array.c"
        .loc 1 3 0
        .cfi_startproc
0:      addis 2,12,.TOC.-0b at ha
        addi 2,2,.TOC.-0b at l
        .localentry     main,.-main
        mflr 0
        std 0,16(1)
        std 31,-8(1)
        stdu 1,-64(1)
        .cfi_def_cfa_offset 64
        .cfi_offset 65, 16
        .cfi_offset 31, -8
        mr 31,1
        .cfi_def_cfa_register 31
        .loc 1 3 0  <-- repeated .loc
        ld 9,-28688(13)
        std 9,40(31)
        li 9,0
        .loc 1 6 0
        li 9,0
        .loc 1 7 0
        mr 3,9   
        ld 10,40(31)
        ld 9,-28688(13)
        cmpld 7,10,9
        li 10,0  
        li 9,0   
        beq 7,.L3
        bl __stack_chk_fail
        nop
.L3:
(...)

If this additional .loc is removed, gdb is able to hit the breakpoint at the correct place.
Moreover, it might be worth mentioning that this additional .loc doesn't appear on gcc-5 output too, where this breakpoint issue is not reproducible as well.

** Affects: gdb (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: architecture-ppc64le bot-comment bugnameltc-121304 severity-high targetmilestone-inin1504
-- 
gdb source tests are failing in Ubuntu 15.04
https://bugs.launchpad.net/bugs/1420556
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