Re: Merging calls to `abort'

[James E Wilson <wilson at specifixinc dot com>]

Richard Stallman wrote:
> Currently, I believe, GCC combines various calls to abort in a single
> function, because it knows that none of them returns.

To give this request a little context, consider the attached example. 
If I compile this with -O2 -g, and run it under the debugger, it tells 
me that the program died at line 7 (j == 0) which is impossible.  It is 
easy to tell what went wrong in a trivial example like this, but in a 
big program like gcc this can be very confusing.  This reduces the 
usefulness of using abort in combination with -g and optimization.

The optimization that causes the problem is crossjumping.  If I compile 
with -O2 -fno-crossjumping I get the desired result.  So rms's request 
is essentially to disable crossjumping of abort calls.

Incidentally, we have already pushed crossjumping back from -O to -O2. 
If I use gcc-3.3, the example fails with -O -g.  But with gcc-4, it 
works at -O -g and fails only at -O2 (or -Os) -g.  Since -O -g works 
with current sources, perhaps this is a good enough solution.  Or 
perhaps just asking people to use -fno-crossjumping is a good enough 
solution.

Otherwise, we need to consider the merits of disabling an optimization 
to make debugging easier.  This is a difficult choice to make, but at 
-O2, I'd prefer that we optimize, and suggest other debugging techniques 
intead of relying on the line numbers of abort calls.  Such as using 
assert instead.

I suppose there is yet another alternative, which is to extend the debug 
info to indicate that the single abort call is both line 5 and line 7, 
and then extend gdb to handle this gracefully, but this does not seem to 
be a practical solution, at least not a practical short term one.
--
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com