[PATCH] Fix PR54492

[William J. Schmidt]

Richard found some N^2 behavior in SLSR that has to be suppressed.
Searching for the best possible basis is overkill when there are
hundreds of thousands of possibilities.  This patch constrains the
search to "good enough" in such cases.

Bootstrapped and tested on powerpc64-unknown-linux-gnu with no
regressions.  Ok for trunk?

Thanks,
Bill


2012-08-10  Bill Schmidt  <wschmidt@linux.vnet.ibm.com>

	* gimple-ssa-strength-reduction.c (find_basis_for_candidate): Limit
	the time spent searching for a basis.


Index: gcc/gimple-ssa-strength-reduction.c
===================================================================
--- gcc/gimple-ssa-strength-reduction.c	(revision 191135)
+++ gcc/gimple-ssa-strength-reduction.c	(working copy)
@@ -353,10 +353,14 @@ find_basis_for_candidate (slsr_cand_t c)
   cand_chain_t chain;
   slsr_cand_t basis = NULL;
 
+  // Limit potential of N^2 behavior for long candidate chains.
+  int iters = 0;
+  const int MAX_ITERS = 50;
+
   mapping_key.base_expr = c->base_expr;
   chain = (cand_chain_t) htab_find (base_cand_map, &mapping_key);
 
-  for (; chain; chain = chain->next)
+  for (; chain && iters < MAX_ITERS; chain = chain->next, ++iters)
     {
       slsr_cand_t one_basis = chain->cand;