[PATCH] Fix PR81488

[Bill Schmidt <wschmidt at linux dot vnet dot ibm dot com>]

Hi,

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81488 reports a problem with SLSR where
too many memory resources are required to complete SLSR processing of conditional
candidates.  The code in question contains large trees of PHI dependencies that must
be examined in order to find all candidates for replacement.  Not only are the
dependency chains deep, but many PHIs contain duplicate source operands arriving by
different paths, and SLSR isn't currently smart enough to avoid tracing them more
than once.  This leads to exponential behavior and a bad ending.

Even removing the exponential behavior is not sufficient to fix the problem.  The
dependencies are just too complex.  So it is also necessary to put a limit on how
much time we want to spend examining PHI nodes before giving up.  I've arbitrarily
chosen 16 as the maximum number of PHI nodes to visit for each candidate, which
seems likely to be sufficient in most cases.

A side benefit of removing the exponential behavior is better accuracy in making
cost-model decisions.  With tracing through the same PHI dependencies more than
once, the insertion (+) and replacement (-) costs are overcounted.  This should
now be improved.

The original bug went latent on the trunk after it was reported, but I was able
to reproduce with an older revision and verify that the following patch fixes
the problem.  I've also bootstrapped and tested it on powerpc64le-linux-gnu with
no regressions.  Is this ok for trunk?

Thanks,
Bill


2017-08-18  Bill Schmidt  <wschmidt@linux.vnet.ibm.com>

	PR tree-optimization/81488
	* gimple-ssa-strength-reduction (struct slsr_cand_d): Add visited
	and cached_basis fields.
	(MAX_SPREAD): New constant.
	(alloc_cand_and_find_basis): Initialize new fields.
	(clear_visited): New function.
	(create_phi_basis_1): Rename from create_phi_basis, set visited
	and cached_basis fields.
	(create_phi_basis): New wrapper function.
	(phi_add_costs_1): Rename from phi_add_costs, add spread
	parameter, set visited field, short-circuit when limits reached.
	(phi_add_costs): New wrapper function.
	(record_phi_increments_1): Rename from record_phi_increments, set
	visited field.
	(record_phi_increments): New wrapper function.
	(phi_incr_cost_1): Rename from phi_incr_cost, set visited field.
	(phi_incr_cost): New wrapper function.
	(all_phi_incrs_profitable_1): Rename from
	all_phi_incrs_profitable, set visited field.
	(all_phi_incrs_profitable): New wrapper function.


Index: gcc/gimple-ssa-strength-reduction.c
===================================================================
--- gcc/gimple-ssa-strength-reduction.c	(revision 251159)
+++ gcc/gimple-ssa-strength-reduction.c	(working copy)
@@ -281,6 +281,14 @@ struct slsr_cand_d
   /* Savings that can be expected from eliminating dead code if this
      candidate is replaced.  */
   int dead_savings;
+
+  /* For PHI candidates, use a visited flag to keep from processing the
+     same PHI twice from multiple paths.  */
+  int visited;
+
+  /* We sometimes have to cache a phi basis with a phi candidate to
+     avoid processing it twice.  Valid only if visited==1.  */
+  tree cached_basis;
 };
 
 typedef struct slsr_cand_d slsr_cand, *slsr_cand_t;
@@ -369,7 +377,11 @@ enum count_phis_status
   DONT_COUNT_PHIS = 0,
   COUNT_PHIS = 1
 };
- 
+
+/* Constrain how many PHI nodes we will visit for a conditional
+   candidate (depth and breadth).  */
+const int MAX_SPREAD = 16;
+
 /* Pointer map embodying a mapping from statements to candidates.  */
 static hash_map<gimple *, slsr_cand_t> *stmt_cand_map;
 
@@ -671,6 +683,8 @@ alloc_cand_and_find_basis (enum cand_kind kind, gi
   c->sibling = 0;
   c->def_phi = kind == CAND_MULT ? find_phi_def (base) : 0;
   c->dead_savings = savings;
+  c->visited = 0;
+  c->cached_basis = NULL_TREE;
 
   cand_vec.safe_push (c);
 
@@ -2317,19 +2331,33 @@ create_add_on_incoming_edge (slsr_cand_t c, tree b
   return lhs;
 }
 
-/* Given a candidate C with BASIS_NAME being the LHS of C's basis which
-   is hidden by the phi node FROM_PHI, create a new phi node in the same
-   block as FROM_PHI.  The new phi is suitable for use as a basis by C,
-   with its phi arguments representing conditional adjustments to the
-   hidden basis along conditional incoming paths.  Those adjustments are
-   made by creating add statements (and sometimes recursively creating
-   phis) along those incoming paths.  LOC is the location to attach to
-   the introduced statements.  KNOWN_STRIDE is true iff C's stride is a
-   constant.  */
+/* Clear the visited field for a tree of PHI candidates.  */
 
+static void
+clear_visited (gphi *phi)
+{
+  unsigned i;
+  slsr_cand_t phi_cand = *stmt_cand_map->get (phi);
+
+  if (phi_cand->visited)
+    {
+      phi_cand->visited = 0;
+
+      for (i = 0; i < gimple_phi_num_args (phi); i++)
+	{
+	  tree arg = gimple_phi_arg_def (phi, i);
+	  gimple *arg_def = SSA_NAME_DEF_STMT (arg);
+	  if (gimple_code (arg_def) == GIMPLE_PHI)
+	    clear_visited (as_a <gphi *> (arg_def));
+	}
+    }
+}
+
+/* Recursive helper function for create_phi_basis.  */
+
 static tree
-create_phi_basis (slsr_cand_t c, gimple *from_phi, tree basis_name,
-		  location_t loc, bool known_stride)
+create_phi_basis_1 (slsr_cand_t c, gimple *from_phi, tree basis_name,
+		    location_t loc, bool known_stride)
 {
   int i;
   tree name, phi_arg;
@@ -2340,6 +2368,10 @@ static tree
   slsr_cand_t phi_cand = *stmt_cand_map->get (from_phi);
   auto_vec<tree> phi_args (nargs);
 
+  if (phi_cand->visited)
+    return phi_cand->cached_basis;
+  phi_cand->visited = 1;
+
   /* Process each argument of the existing phi that represents
      conditionally-executed add candidates.  */
   for (i = 0; i < nargs; i++)
@@ -2367,8 +2399,8 @@ static tree
 	     process it in the same fashion to ensure that all basis
 	     adjustments are made along its incoming edges.  */
 	  if (gimple_code (arg_def) == GIMPLE_PHI)
-	    feeding_def = create_phi_basis (c, arg_def, basis_name,
-					    loc, known_stride);
+	    feeding_def = create_phi_basis_1 (c, arg_def, basis_name,
+					      loc, known_stride);
 	  else
 	    {
 	      slsr_cand_t arg_cand = base_cand_from_table (arg);
@@ -2403,9 +2435,31 @@ static tree
       print_gimple_stmt (dump_file, phi, 0);
     }
 
+  phi_cand->cached_basis = name;
   return name;
 }
 
+/* Given a candidate C with BASIS_NAME being the LHS of C's basis which
+   is hidden by the phi node FROM_PHI, create a new phi node in the same
+   block as FROM_PHI.  The new phi is suitable for use as a basis by C,
+   with its phi arguments representing conditional adjustments to the
+   hidden basis along conditional incoming paths.  Those adjustments are
+   made by creating add statements (and sometimes recursively creating
+   phis) along those incoming paths.  LOC is the location to attach to
+   the introduced statements.  KNOWN_STRIDE is true iff C's stride is a
+   constant.  */
+
+static tree
+create_phi_basis (slsr_cand_t c, gimple *from_phi, tree basis_name,
+		  location_t loc, bool known_stride)
+{
+  tree retval = create_phi_basis_1 (c, from_phi, basis_name, loc,
+				    known_stride);
+  gcc_assert (retval);
+  clear_visited (as_a <gphi *> (from_phi));
+  return retval;
+}
+
 /* Given a candidate C whose basis is hidden by at least one intervening
    phi, introduce a matching number of new phis to represent its basis
    adjusted by conditional increments along possible incoming paths.  Then
@@ -2429,6 +2483,7 @@ replace_conditional_candidate (slsr_cand_t c)
   loc = gimple_location (c->cand_stmt);
   name = create_phi_basis (c, lookup_cand (c->def_phi)->cand_stmt,
 			   basis_name, loc, KNOWN_STRIDE);
+
   /* Replace C with an add of the new basis phi and a constant.  */
   widest_int bump = c->index * wi::to_widest (c->stride);
 
@@ -2435,19 +2490,22 @@ replace_conditional_candidate (slsr_cand_t c)
   replace_mult_candidate (c, name, bump);
 }
 
-/* Compute the expected costs of inserting basis adjustments for
-   candidate C with phi-definition PHI.  The cost of inserting 
-   one adjustment is given by ONE_ADD_COST.  If PHI has arguments
-   which are themselves phi results, recursively calculate costs
-   for those phis as well.  */
+/* Recursive helper function for phi_add_costs.  SPREAD is a measure of
+   how many PHI nodes we have visited at this point in the tree walk.  */
 
 static int
-phi_add_costs (gimple *phi, slsr_cand_t c, int one_add_cost)
+phi_add_costs_1 (gimple *phi, slsr_cand_t c, int one_add_cost, int *spread)
 {
   unsigned i;
   int cost = 0;
   slsr_cand_t phi_cand = *stmt_cand_map->get (phi);
 
+  if (phi_cand->visited)
+    return 0;
+
+  phi_cand->visited = 1;
+  (*spread)++;
+
   /* If we work our way back to a phi that isn't dominated by the hidden
      basis, this isn't a candidate for replacement.  Indicate this by
      returning an unreasonably high cost.  It's not easy to detect
@@ -2469,7 +2527,12 @@ static int
 	  gimple *arg_def = SSA_NAME_DEF_STMT (arg);
 
 	  if (gimple_code (arg_def) == GIMPLE_PHI)
-	    cost += phi_add_costs (arg_def, c, one_add_cost);
+	    {
+	      cost += phi_add_costs_1 (arg_def, c, one_add_cost, spread);
+
+	      if (cost >= COST_INFINITE || *spread > MAX_SPREAD)
+		return COST_INFINITE;
+	    }
 	  else
 	    {
 	      slsr_cand_t arg_cand = base_cand_from_table (arg);
@@ -2483,6 +2546,20 @@ static int
   return cost;
 }
 
+/* Compute the expected costs of inserting basis adjustments for
+   candidate C with phi-definition PHI.  The cost of inserting 
+   one adjustment is given by ONE_ADD_COST.  If PHI has arguments
+   which are themselves phi results, recursively calculate costs
+   for those phis as well.  */
+
+static int
+phi_add_costs (gimple *phi, slsr_cand_t c, int one_add_cost)
+{
+  int spread = 0;
+  int retval = phi_add_costs_1 (phi, c, one_add_cost, &spread);
+  clear_visited (as_a <gphi *> (phi));
+  return retval;
+}
 /* For candidate C, each sibling of candidate C, and each dependent of
    candidate C, determine whether the candidate is dependent upon a 
    phi that hides its basis.  If not, replace the candidate unconditionally.
@@ -2648,18 +2725,18 @@ record_increment (slsr_cand_t c, widest_int increm
     }
 }
 
-/* Given phi statement PHI that hides a candidate from its BASIS, find
-   the increments along each incoming arc (recursively handling additional
-   phis that may be present) and record them.  These increments are the
-   difference in index between the index-adjusting statements and the
-   index of the basis.  */
+/* Recursive helper function for record_phi_increments.  */
 
 static void
-record_phi_increments (slsr_cand_t basis, gimple *phi)
+record_phi_increments_1 (slsr_cand_t basis, gimple *phi)
 {
   unsigned i;
   slsr_cand_t phi_cand = *stmt_cand_map->get (phi);
   
+  if (phi_cand->visited)
+    return;
+  phi_cand->visited = 1;
+
   for (i = 0; i < gimple_phi_num_args (phi); i++)
     {
       tree arg = gimple_phi_arg_def (phi, i);
@@ -2669,7 +2746,7 @@ static void
 	  gimple *arg_def = SSA_NAME_DEF_STMT (arg);
 
 	  if (gimple_code (arg_def) == GIMPLE_PHI)
-	    record_phi_increments (basis, arg_def);
+	    record_phi_increments_1 (basis, arg_def);
 	  else
 	    {
 	      slsr_cand_t arg_cand = base_cand_from_table (arg);
@@ -2680,6 +2757,19 @@ static void
     }
 }
 
+/* Given phi statement PHI that hides a candidate from its BASIS, find
+   the increments along each incoming arc (recursively handling additional
+   phis that may be present) and record them.  These increments are the
+   difference in index between the index-adjusting statements and the
+   index of the basis.  */
+
+static void
+record_phi_increments (slsr_cand_t basis, gimple *phi)
+{
+  record_phi_increments_1 (basis, phi);
+  clear_visited (as_a <gphi *> (phi));
+}
+
 /* Determine how many times each unique increment occurs in the set
    of candidates rooted at C's parent, recording the data in the
    increment vector.  For each unique increment I, if an initializer
@@ -2717,15 +2807,11 @@ record_increments (slsr_cand_t c)
     record_increments (lookup_cand (c->dependent));
 }
 
-/* Add up and return the costs of introducing add statements that
-   require the increment INCR on behalf of candidate C and phi
-   statement PHI.  Accumulate into *SAVINGS the potential savings
-   from removing existing statements that feed PHI and have no other
-   uses.  */
+/* Recursive helper function for phi_incr_cost.  */
 
 static int
-phi_incr_cost (slsr_cand_t c, const widest_int &incr, gimple *phi,
-	       int *savings)
+phi_incr_cost_1 (slsr_cand_t c, const widest_int &incr, gimple *phi,
+		 int *savings)
 {
   unsigned i;
   int cost = 0;
@@ -2732,6 +2818,10 @@ static int
   slsr_cand_t basis = lookup_cand (c->basis);
   slsr_cand_t phi_cand = *stmt_cand_map->get (phi);
 
+  if (phi_cand->visited)
+    return 0;
+  phi_cand->visited = 1;
+
   for (i = 0; i < gimple_phi_num_args (phi); i++)
     {
       tree arg = gimple_phi_arg_def (phi, i);
@@ -2744,7 +2834,7 @@ static int
 	    {
 	      int feeding_savings = 0;
 	      tree feeding_var = gimple_phi_result (arg_def);
-	      cost += phi_incr_cost (c, incr, arg_def, &feeding_savings);
+	      cost += phi_incr_cost_1 (c, incr, arg_def, &feeding_savings);
 	      if (uses_consumed_by_stmt (feeding_var, phi))
 		*savings += feeding_savings;
 	    }
@@ -2768,6 +2858,21 @@ static int
   return cost;
 }
 
+/* Add up and return the costs of introducing add statements that
+   require the increment INCR on behalf of candidate C and phi
+   statement PHI.  Accumulate into *SAVINGS the potential savings
+   from removing existing statements that feed PHI and have no other
+   uses.  */
+
+static int
+phi_incr_cost (slsr_cand_t c, const widest_int &incr, gimple *phi,
+	       int *savings)
+{
+  int retval = phi_incr_cost_1 (c, incr, phi, savings);
+  clear_visited (as_a <gphi *> (phi));
+  return retval;
+}
+
 /* Return the first candidate in the tree rooted at C that has not
    already been replaced, favoring siblings over dependents.  */
 
@@ -3309,16 +3414,21 @@ insert_initializers (slsr_cand_t c)
     }
 }
 
-/* Return TRUE iff all required increments for candidates feeding PHI
-   are profitable (and legal!) to replace on behalf of candidate C.  */
+/* Recursive helper function for all_phi_incrs_profitable.  */
 
 static bool
-all_phi_incrs_profitable (slsr_cand_t c, gphi *phi)
+all_phi_incrs_profitable_1 (slsr_cand_t c, gphi *phi, int *spread)
 {
   unsigned i;
   slsr_cand_t basis = lookup_cand (c->basis);
   slsr_cand_t phi_cand = *stmt_cand_map->get (phi);
 
+  if (phi_cand->visited)
+    return true;
+
+  phi_cand->visited = 1;
+  (*spread)++;
+
   /* If the basis doesn't dominate the PHI (including when the PHI is
      in the same block as the basis), we won't be able to create a PHI
      using the basis here.  */
@@ -3346,7 +3456,9 @@ static bool
 
 	  if (gimple_code (arg_def) == GIMPLE_PHI)
 	    {
-	      if (!all_phi_incrs_profitable (c, as_a <gphi *> (arg_def)))
+	      if (!all_phi_incrs_profitable_1 (c, as_a <gphi *> (arg_def),
+					       spread)
+		  || *spread > MAX_SPREAD)
 		return false;
 	    }
 	  else
@@ -3388,6 +3500,18 @@ static bool
   return true;
 }
   
+/* Return TRUE iff all required increments for candidates feeding PHI
+   are profitable (and legal!) to replace on behalf of candidate C.  */
+
+static bool
+all_phi_incrs_profitable (slsr_cand_t c, gphi *phi)
+{
+  int spread = 0;
+  bool retval = all_phi_incrs_profitable_1 (c, phi, &spread);
+  clear_visited (phi);
+  return retval;
+}
+
 /* Create a NOP_EXPR that copies FROM_EXPR into a new SSA name of
    type TO_TYPE, and insert it in front of the statement represented
    by candidate C.  Use *NEW_VAR to create the new SSA name.  Return