[Bug middle-end/58479] [4.9 Regression] slow var-tracking on x86_64-linux at -O1 (and above) with -g, but checking disabled

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58479

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
Or rather -fvar-tracking-assignments.  The slowness creeps in

 trivially dead code     :   0.37 (13%) usr   0.00 ( 0%) sys   0.37 (10%) wall 
     0 kB ( 0%) ggc
 complete unrolling      :   0.36 (13%) usr   0.53 (55%) sys   0.81 (22%) wall 
211254 kB (38%) ggc
 expand                  :   0.29 (10%) usr   0.09 ( 9%) sys   0.38 (10%) wall 
346562 kB (62%) ggc

var-tracking itself doesn't enter the picture.

main ()
{
  <bb 2>:
  # DEBUG D#1 => f
  # DEBUG t$0$0$0 => D#1
  # DEBUG t$0$0$1 => D#1
  # DEBUG t$0$0$2 => D#1
... 3.3 million (!) similar lines follow ...
  e = 8;
  d = 8;
  c = 8;
  b = 8;
  a = 8;
  return 0;

}

seems to be support for aggregate piece debug values makes this blow up
totally.  Nobody is going to p t[][][] here (and I suspect gdb support
isn't ready here either).