gcov in infinite loop

gcov in infinite loop

[paulf]

Hi

I'm trying to use gcov. No problems building executables and running them. But when I get to running gcov, at least on one object file, gcov gets stuck in an infinite loop. I've tried running it overnight, but no sign of progress by the morning.

A few more details:
We mostly use RHEL 4.0 to 5.3. We're using GCC 4.5.1 built inhouse.

Any ideas where the problem might be? I haven't yet tried GCC 4.5.3.

A+
Paul Floyd

Re: gcov in infinite loop

[Andi Hellmund]

>
> Hi
>
> I'm trying to use gcov. No problems building executables and running them. But when I get to running gcov, at least on one object file, gcov gets stuck in an infinite loop. I've tried running it overnight, but no sign of progress by the morning.
>
> A few more details:
> We mostly use RHEL 4.0 to 5.3. We're using GCC 4.5.1 built inhouse.
>
> Any ideas where the problem might be? I haven't yet tried GCC 4.5.3.
>
>    
Hey Paul,

it's hard to say what could be wrong without more details. Testing 4.5.3 
would be indeed a good idea to check if this misbehavior has already 
been fixed.

If not, you should try to get and post a stack trace of the infinite 
loop, e.g. using gdb or pstack(1) ...

Which gcov options are you using, by the way?

Thanks,
Andi

Re: gcov in infinite loop

[Paul Floyd]

Hi

> Hey Paul,
> 
> it's hard to say what could be wrong without more details. Testing 4.5.3 
> would be indeed a good idea to check if this misbehavior has already 
> been fixed.

I've started.

> If not, you should try to get and post a stack trace of the infinite 
> loop, e.g. using gdb or pstack(1) ...

pstack is not very helpful:
#0  0x00000000412d22 in main ()

gdb is better

#0 accumulate_line_counts (argc=8, argv=0x7fffffffc7f8)
at /free/NEW_GNU_BUILD/devenv_g3_package/src/gcc-4.5.1/gcc/gcov.c:1737
#1 generate_results (argc=8, argv=0x7fffffffc7f8)
at /free/NEW_GNU_BUILD/devenv_g3_package/src/gcc-4.5.1/gcc/gcov.c:566
#2 main (argc=8, argv=0x7fffffffc7f8)
at /free/NEW_GNU_BUILD/devenv_g3_package/src/gcc-4.5.1/gcc/gcov.c:377

> Which gcov options are you using, by the way?

lcov -c -d . -o sim.info

wich according to ps runs gcov with

gcov adms_search.gcda -o sim -b -c -a -p

Trying those options one at a time, -b -c -p pass, -a doesn't.

Looking at the man page, -a determines block counts within a line. 
Thanks to our chronic macro abuse, we have some pretty enormous single 
lines, so it is conceivable that I'm hitting a performance problem and 
gcov isn't stuck in an infinite loop.

A+
Paul

Re: gcov in infinite loop

[Paul Floyd]

paulf@free.fr wrote:
> Hi
> 
> I'm trying to use gcov. No problems building executables and running them. But when I get to running gcov, at least on one object file, gcov gets stuck in an infinite loop. I've tried running it overnight, but no sign of progress by the morning.
> 

Hi

I built a version of gcov without inlining, and traced the problem. In 
the gcov source there's this comment:

           /* Find the loops. This uses the algorithm described in
	     Tiernan 'An Efficient Search Algorithm to Find the
	     Elementary Circuits of a Graph', CACM Dec 1970. We hold
	     the P array by having each block point to the arc that
	     connects to the previous block. The H array is implicitly
	     held because of the arc ordering, and the block's
	     previous arc pointer.

	     Although the algorithm is O(N^3) for highly connected
	     graphs, at worst we'll have O(N^2), as most blocks have
	     only one or two exits. Most graphs will be small.

	     For each loop we find, locate the arc with the smallest
	     transition count, and add that to the cumulative
	     count.  Decrease flow over the cycle and remove the arc
	     from consideration.  */


The point where gcov stops is a line containing a macro that expands to 
almost 30K characters. I didn't bother to confirm whether it was just 
the O(N^2)-ness or if it was really stuck. I just changed the Makefile 
to add -DNDEBUG for that file, and the long line (with a lot of asserts) 
shrunk to 2K characters, and gcov can handle it relatively easily.

A+
Paul