Hello,

see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80887#c10 for the 
context. FRE can go into an infinite recursion with some match.pd 
simplifications (that have been temporarily reverted).

Limiting the depth of recursive calls is not a great solution, but it is 
simple and I don't have a good idea for an alternate solution that does 
not disable a lot of desirable optimizations.

There are many ways to write the limiter, I went with

   depth_limiter d;
   if (d > 100) return false;

but I could also have written the class so the use would look like

   depth_limiter d(100);
   if (!d) return false;

for instance.

100 was picked arbitrarily, I don't think it is worth having a param for 
it, but we could certainly use a different value.

Bootstrap and testsuite on powerpc64le-unknown-linux-gnu.

2017-06-19  Marc Glisse  <marc.glisse@inria.fr>

 	* gimple-match-head.c (depth_limiter): New class.
 	* genmatch.c (decision_tree::gen): Use it.

-- 
Marc Glisse