re PR middle-end/55492 (__atomic_load doesn't match ACQUIRE memory model)

2012-09-11  Bill Schmidt  <wschmidt@linux.vnet.ibm.com>

	PR tree-optimization/55492
	* doc/invoke.texi (max-slsr-cand-scan): New description.
	* gimple-ssa-strength-reduction.c (find_basis_for_candidate): Limit
	the time spent searching for a basis.
	* params.def (PARAM_MAX_SLSR_CANDIDATE_SCAN): New param.

From-SVN: r191178

gcc/ChangeLog

+2012-09-11  Bill Schmidt  <wschmidt@linux.vnet.ibm.com>
+
+	PR tree-optimization/55492
+	* doc/invoke.texi (max-slsr-cand-scan): New description.
+	* gimple-ssa-strength-reduction.c (find_basis_for_candidate): Limit
+	the time spent searching for a basis.
+	* params.def (PARAM_MAX_SLSR_CANDIDATE_SCAN): New param.
+
 2012-09-11  Richard Guenther  <rguenther@suse.de>
 
 	* gimple.h (gimple_register_type): Remove.

gcc/doc/invoke.texi

@@ -9407,6 +9407,11 @@ having a regular register file and accurate register pressure classes.
 See @file{haifa-sched.c} in the GCC sources for more details.
 
 The default choice depends on the target.
+
+@item max-slsr-cand-scan
+Set the maximum number of existing candidates that will be considered when
+seeking a basis for a new straight-line strength reduction candidate.
+
 @end table
 @end table
 

gcc/gimple-ssa-strength-reduction.c

@@ -54,6 +54,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "domwalk.h"
 #include "pointer-set.h"
 #include "expmed.h"
+#include "params.h"
 
 /* Information about a strength reduction candidate.  Each statement
    in the candidate table represents an expression of one of the
@@ -353,10 +354,14 @@ 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;
+  int max_iters = PARAM_VALUE (PARAM_MAX_SLSR_CANDIDATE_SCAN);
+
   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;
 

gcc/params.def

@@ -979,6 +979,13 @@ DEFPARAM (PARAM_SCHED_PRESSURE_ALGORITHM,
 	  "Which -fsched-pressure algorithm to apply",
 	  1, 1, 2)
 
+/* Maximum length of candidate scans in straight-line strength reduction.  */
+DEFPARAM (PARAM_MAX_SLSR_CANDIDATE_SCAN,
+	  "max-slsr-cand-scan",
+	  "Maximum length of candidate scans for straight-line "
+	  "strength reduction",
+	  50, 1, 999999)
+
 /*
 Local variables:
 mode:c