From: Matt <matt@use.net>
To: Ian Lance Taylor <iant@google.com>
Cc: gcc-help@gcc.gnu.org
Subject: Re: bootstrap comparison failure with bootstrap-lto
Date: Sun, 20 Nov 2011 06:51:00 -0000 [thread overview]
Message-ID: <Pine.NEB.4.64.1111191638350.15450@cesium.clock.org> (raw)
In-Reply-To: <mcrpqgolxw0.fsf@dhcp-172-18-216-180.mtv.corp.google.com>
On Fri, 18 Nov 2011, Ian Lance Taylor wrote:
> Matt <matt@use.net> writes:
>
>> Should I file a bug about the multiple runs being necessary? I poked
>> around a bit and it seemed that different -j options results in a
>> different number of tries. That being said, even -j1 required multiple
>> tries. (This may be a symptom of one of the issues that causes my
>> builds to fail spectacularly every blue moon when doing -jN>6.)
>
> Sure, go ahead and file a bug. It would be nice to get these things
> cleaned up.
Done. http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51232
I added you on the cc for it, hope you don't mind.
> Great. I think that now we've cleared through the weeds and gotten to
> the interesting part: why does it crash? I think the failing test is
> gcc_AC_INITFINI_ARRAY in gcc/acinclude.m4. It is intended to see
> whether the host linker correctly supports a mix of .init_array and
> .ctors sections with priorities. This test is intended to be more or
> less independent of the compiler, and is intended to only test the
> linker. Both the host gcc used to build stage1 and the stage1 gcc
> itself are presumably using the same linker (right?). So, why would the
> test pass with the host gcc and fail with the stage1 gcc?
>
> If you can trace the points at which the static variable "count"
> changes, that may help us pin down what is going wrong.
Pin down, indeed :) When I try to watch or print 'count', I get this from
GDB:
Reading symbols from /home/matt/src/gcc-trunk/obj/conftest...done.
(gdb) watch count
Watchpoint 1: count
(gdb) run
Starting program: /home/matt/src/gcc-trunk/obj/conftest
Error evaluating expression for watchpoint 1
value has been optimized out
Watchpoint 1 deleted.
So, it looks like an optimization bug of a sort to my naive eyes. As such,
I started trying different combinations on the compile line, the original
was:
/home/matt/src/gcc-trunk/obj/./prev-gcc/xgcc
-B/home/matt/src/gcc-trunk/obj/./prev-gcc/
-B/home/matt/x86_64-unknown-linux-gnu/bin/
-B/home/matt/x86_64-unknown-linux-gnu/bin/
-B/home/matt/x86_64-unknown-linux-gnu/lib/ -isystem
/home/matt/x86_64-unknown-linux-gnu/include -isystem
/home/matt/x86_64-unknown-linux-gnu/sys-include -o conftest -g -O2
-flto=jobserver -frandom-seed=1 -static-libstdc++ -static-libgcc ~/finitest.c
If I take out the -flto=jobserver, the test no longer crashes and the
variable is no longer optimized out incorrectly. Trying other -flto
variants all elicited the original problem. When I tested compilation
using the trunk *or* the system compiler (GCC 4.6.1-based) with
-O2 -flto, I was able to repeat the problem exactly.
Is this a wrong-code bug, and/or should that test never be
compiled with -flto?
--
tangled strands of DNA explain the way that I behave.
http://www.clock.org/~matt
next prev parent reply other threads:[~2011-11-20 0:48 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-11-14 22:23 Matt
2011-11-15 5:56 ` Ian Lance Taylor
2011-11-16 21:55 ` Matt
2011-11-17 5:43 ` Ian Lance Taylor
2011-11-18 6:35 ` Matt
2011-11-18 13:47 ` Ian Lance Taylor
2011-11-18 21:08 ` Matt
2011-11-19 1:11 ` Ian Lance Taylor
2011-11-19 2:41 ` Matt
2011-11-19 15:37 ` Ian Lance Taylor
2011-11-20 6:51 ` Matt [this message]
2011-11-20 7:32 ` Ian Lance Taylor
2011-11-20 20:23 ` Matt Hargett
2011-11-21 17:27 ` Ian Lance Taylor
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