[Bug rtl-optimization/56069] [4.9/5/6/7 Regression] RA pessimization

law at redhat dot com gcc-bugzilla@gcc.gnu.org
Thu Jun 23 15:51:00 GMT 2016


https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56069

Jeffrey A. Law <law at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://sourceware.org/bugz
                   |                            |illa/show_bug.cgi?id=20295

--- Comment #15 from Jeffrey A. Law <law at redhat dot com> ---
On 04/27/2016 05:33 AM, Bernd Schmidt wrote:
> On 04/27/2016 06:02 AM, Jeff Law wrote:
>> AFAICT the sra-1.c expects to see the incremented value and I'm at a
>> loss to understand what's really going on here.  Can you give more
>> details?
> 
> Yeah, maybe my first impression wasn't very accurate.
> 
> When I try to run gdb manually, it just crashes:
> 
> (gdb) show version
> GNU gdb (Gentoo 7.10.1 vanilla) 7.10.1
> (gdb) b 43
> Breakpoint 1 at 0x40059b: file sra-1.c, line 43.
> (gdb) run
> Starting program: /local/src/egcs/bscommit/gcc/a.out
> 
> Breakpoint 1, f3 (k=<optimized out>) at sra-1.c:43
> 43      bar (a.j);        /* { dg-final { gdb-test 43 "a.j" "14" } } */
> (gdb) p a.j
> Segmentation fault (core dumped)
> 
[ ... ]

> 
> 
> I don't really understand the var-tracking stuff too well, so no idea
> where to go from here. I suppose I'm withdrawing my patch.
Based on the above, there's some kind of GDB bug.  So your patch may still be a
good thing.

I did a build on F23 which has effectively the same version of gdb and can
reproduce the gdb segfault.  It also reproduces on F24 which has gdb-7.11.1

AFAICT gdb thinks the value of "a" has been optimized out, but goes absolutely
bananas and segfaults if you try to examine a field within "a".

[law@torsion gcc]$ ./xgcc -B./ -g sra-1.c -O2
[law@torsion gcc]$ gdb ./a.out
GNU gdb (GDB) Fedora 7.10.1-30.fc23
[ ... ]
(gdb) b 43
Breakpoint 1 at 0x40056b: file sra-1.c, line 43.
(gdb) r
Starting program: /opt/notnfs/law/gcc-testing/obj/gcc/a.out 
Missing separate debuginfos, use: dnf debuginfo-install
glibc-2.22-10.fc23.x86_64

Breakpoint 1, f3 (k=<optimized out>) at sra-1.c:43
43        bar (a.j);            /* { dg-final { gdb-test 43 "a.j" "14" } } */
(gdb) p a
$1 = <optimized out>
(gdb) p a.i
Segmentation fault (core dumped)


My gdb skills are far too rusty to take this further.  I've filed an upstream
report (BZ20295 in the gdb tracker).  Probably not much we can do until the GDB
side gets fixed.

I'm going to attach this to 56069 for future reference.
jeff


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