RFC: Patch for switch elimination (PR 54742)

[Steve Ellcey <sellcey at mips dot com>]

After talking to Jeff Law at the GCC Cauldron I have updated my switch
shortcut plugin pass to try and address this optimization in the hopes of
getting it added to GCC as a static pass.  I fixed the code to build with
the various C++ changes that have been happening in GCC but the current
version I have included in this email is not working yet.  When I run it
on coremark I get errors like:

core_state.c: In function 'core_bench_state':
core_state.c:43:8: error: size of loop 16 should be 13, not 5
 ee_u16 core_bench_state(ee_u32 blksize, ee_u8 *memblock, 
        ^
core_state.c:43:8: error: bb 15 does not belong to loop 16
core_state.c:43:8: error: bb 113 does not belong to loop 16
core_state.c:43:8: error: bb 118 does not belong to loop 16
core_state.c:43:8: error: bb 117 does not belong to loop 16
(etc) 

Apparently there have been some changes to the loop information that
is built since GCC 4.9.  I had hoped that adding a call to fix_loop_structure
after recalculating the dominance information would fix this but it didn't.

Does anyone have any ideas on how to fix up the loop information that my
optimization has messed as it duplicates blocks?  I tried adding a call
'loop_optimizer_init (LOOPS_NORMAL)' before the fix_loop_structure but that
did not seem to have any affect.

Steve Ellcey
sellcey@mips.com

2014-08-12  Steve Ellcey  <sellcey@mips.com>

	PR tree-opt/54742
	* Makefile.in (OBJS): Add tree-switch-shortcut.o.
	* common.opt (ftree-switch-shortcut): New.
	* opts.c (default_options_table): Add OPT_ftree_switch_shortcut.
	* params.def (PARAM_MAX_SWITCH_INSNS): New.
	(PARAM_MAX_SWITCH_PATHS): New.
	* passes.def (pass_tree_switch_shortcut): New.
	* timevar.def (TV_TREE_SWITCH_SHORTCUT): New.
	* tree-pass.h (make_pass_tree_switch_shortcut): New.
	* tree-switch-shortcut.c: New.

diff --git a/gcc/Makefile.in b/gcc/Makefile.in
index 31c1f4d..94e8ec4 100644
--- a/gcc/Makefile.in
+++ b/gcc/Makefile.in
@@ -1411,6 +1411,7 @@ OBJS = \
 	tree-scalar-evolution.o \
 	tree-sra.o \
 	tree-switch-conversion.o \
+	tree-switch-shortcut.o \
 	tree-ssa-address.o \
 	tree-ssa-alias.o \
 	tree-ssa-ccp.o \
diff --git a/gcc/common.opt b/gcc/common.opt
index 0c4f86b..fe0664a 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -2249,6 +2249,10 @@ ftree-sra
 Common Report Var(flag_tree_sra) Optimization
 Perform scalar replacement of aggregates
 
+ftree-switch-shortcut
+Common Report Var(flag_tree_switch_shortcut) Init(0) Optimization
+Convert jumps to switch statements into jumps to case statement.
+
 ftree-ter
 Common Report Var(flag_tree_ter) Optimization
 Replace temporary expressions in the SSA->normal pass
diff --git a/gcc/opts.c b/gcc/opts.c
index be1867c..f1ac2e5 100644
--- a/gcc/opts.c
+++ b/gcc/opts.c
@@ -514,6 +514,7 @@ static const struct default_options default_options_table[] =
     { OPT_LEVELS_3_PLUS, OPT_fvect_cost_model_, NULL, VECT_COST_MODEL_DYNAMIC },
     { OPT_LEVELS_3_PLUS, OPT_fipa_cp_clone, NULL, 1 },
     { OPT_LEVELS_3_PLUS, OPT_ftree_partial_pre, NULL, 1 },
+    { OPT_LEVELS_3_PLUS, OPT_ftree_switch_shortcut, NULL, 1 },
 
     /* -Ofast adds optimizations to -O3.  */
     { OPT_LEVELS_FAST, OPT_ffast_math, NULL, 1 },
diff --git a/gcc/params.def b/gcc/params.def
index cad00e2..65377d3 100644
--- a/gcc/params.def
+++ b/gcc/params.def
@@ -1058,6 +1058,20 @@ DEFPARAM (PARAM_MAX_SLSR_CANDIDATE_SCAN,
 	  "strength reduction",
 	  50, 1, 999999)
 
+/* Maximum number of instructions to duplicate when shortcutting a switch.  */
+DEFPARAM (PARAM_MAX_SWITCH_INSNS,
+	  "max-switch-insns",
+	  "Maximum number of instructions to duplicate when "
+	  "shortcutting a switch statement",
+	  100, 1, 999999)
+
+/* Maximum number of paths to duplicate when shortcutting a switch.  */
+DEFPARAM (PARAM_MAX_SWITCH_PATHS,
+	  "max-switch-paths",
+	  "Maximum number of new paths to create when"
+	  " shortcutting a switch statement",
+	  50, 1, 999999)
+
 DEFPARAM (PARAM_ASAN_STACK,
          "asan-stack",
          "Enable asan stack protection",
diff --git a/gcc/passes.def b/gcc/passes.def
index f13df6c..8bbf2d0 100644
--- a/gcc/passes.def
+++ b/gcc/passes.def
@@ -157,6 +157,7 @@ along with GCC; see the file COPYING3.  If not see
       NEXT_PASS (pass_cselim);
       NEXT_PASS (pass_copy_prop);
       NEXT_PASS (pass_tree_ifcombine);
+      NEXT_PASS (pass_tree_switch_shortcut);
       NEXT_PASS (pass_phiopt);
       NEXT_PASS (pass_tail_recursion);
       NEXT_PASS (pass_ch);
diff --git a/gcc/timevar.def b/gcc/timevar.def
index a04d05c..d9ee915 100644
--- a/gcc/timevar.def
+++ b/gcc/timevar.def
@@ -170,6 +170,7 @@ DEFTIMEVAR (TV_TREE_LOOP_IVCANON     , "tree canonical iv")
 DEFTIMEVAR (TV_SCEV_CONST            , "scev constant prop")
 DEFTIMEVAR (TV_TREE_LOOP_UNSWITCH    , "tree loop unswitching")
 DEFTIMEVAR (TV_COMPLETE_UNROLL       , "complete unrolling")
+DEFTIMEVAR (TV_TREE_SWITCH_SHORTCUT  , "switch statement shortcuts")
 DEFTIMEVAR (TV_TREE_PARALLELIZE_LOOPS, "tree parallelize loops")
 DEFTIMEVAR (TV_TREE_VECTORIZATION    , "tree vectorization")
 DEFTIMEVAR (TV_TREE_SLP_VECTORIZATION, "tree slp vectorization")
diff --git a/gcc/tree-pass.h b/gcc/tree-pass.h
index 1477d1f..f898e27 100644
--- a/gcc/tree-pass.h
+++ b/gcc/tree-pass.h
@@ -575,6 +575,7 @@ extern gimple_opt_pass *make_pass_early_inline (gcc::context *ctxt);
 extern gimple_opt_pass *make_pass_inline_parameters (gcc::context *ctxt);
 extern gimple_opt_pass *make_pass_update_address_taken (gcc::context *ctxt);
 extern gimple_opt_pass *make_pass_convert_switch (gcc::context *ctxt);
+extern gimple_opt_pass *make_pass_tree_switch_shortcut (gcc::context *ctxt);
 
 /* Current optimization pass.  */
 extern opt_pass *current_pass;
diff --git a/gcc/tree-switch-shortcut.c b/gcc/tree-switch-shortcut.c
new file mode 100644
index 0000000..8ecb74b
--- /dev/null
+++ b/gcc/tree-switch-shortcut.c
@@ -0,0 +1,373 @@
+/* Switch shortcutting optimization for GNU C
+   Copyright (C) 2013 Free Software Foundation, Inc.
+   Contributed by Steve Ellcey (sellcey@imgtec.com).
+
+This file is part of GCC.
+
+GCC is free software; you can redistribute it and/or modify it under
+the terms of the GNU General Public License as published by the Free
+Software Foundation; either version 3, or (at your option) any later
+version.
+
+GCC is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or
+FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+for more details.
+
+You should have received a copy of the GNU General Public License
+along with GCC; see the file COPYING3.  If not see
+<http://www.gnu.org/licenses/>.  */
+
+/* This file implements an optimization where, when a variable is set
+   to a constant value and there is a path that leads from this definition
+   to a switch statement that uses that variable as its controlling expression
+   we duplicate the blocks on this path and change the switch goto to a
+   direct goto to the label of the switch block that control would goto based
+   on the value of the variable.  */
+
+#include "config.h"
+#include "system.h"
+#include "coretypes.h"
+#include "tm.h"
+#include "params.h"
+#include "flags.h"
+#include "tree.h"
+#include "tree-pass.h"
+#include "basic-block.h"
+#include "function.h"
+#include "hash-table.h"
+#include "tree-ssa-alias.h"
+#include "tree-cfg.h"
+#include "tree-ssa-operands.h"
+#include "tree-inline.h"
+#include "gimple-expr.h"
+#include "is-a.h"
+#include "gimple.h"
+#include "tree-phinodes.h"
+#include "gimple-iterator.h"
+#include "gimple-ssa.h"
+#include "ssa-iterators.h"
+#include "tree-into-ssa.h"
+#include "cfgloop.h"
+
+/* Helper function for find_path, visited_bbs is used to make sure we don't
+   fall into an infinite loop.  */
+
+static int
+find_path_1(basic_block start_bb, basic_block end_bb, hash_set<basic_block> *visited_bbs)
+{
+  edge_iterator ei;
+  edge e;
+
+  if (start_bb == end_bb) return 1;
+
+  if (!visited_bbs->add(start_bb))
+    {
+      FOR_EACH_EDGE (e, ei, start_bb->succs)
+	if (find_path_1 (e->dest, end_bb, visited_bbs))
+	  return 1;
+    }
+    return 0;
+}
+
+/* Return 1 if there is a path from start_bb to end_bb and 0 if there
+   is not.  There may be multiple paths from start_bb to end_bb.  */
+
+static int
+find_path(basic_block start_bb, basic_block end_bb)
+{
+  edge_iterator ei;
+  edge e;
+  hash_set<basic_block> visited_bbs;
+  int p = 0;
+
+  if (start_bb == end_bb) return 1;
+
+  if (!visited_bbs.add(start_bb))
+    {
+      FOR_EACH_EDGE (e, ei, start_bb->succs)
+	if (find_path_1 (e->dest, end_bb, &visited_bbs))
+	  {
+	    p = 1;
+	    break;
+	  }
+    }
+  return p;
+}
+
+
+/* We save the paths we want to copy in bbs_list_array.  n_bbs_list is the
+   number of paths saved, bbs_list_array[i] is the list of basic blocks in
+   one path.  Each path starts with the block where a variable is assigned
+   a constant value (bbs_list_array[i][0]) and ends with the switch statement
+   block (bbs_list_array[i][bbs_list_size[i]-2]) followed by the block that
+   the switch statement is going to go to given the constant value of the
+   variable (bbs_list_array[i][bbs_list_size[i]-1]).  */
+
+static basic_block **bbs_list_array;
+static int *val_array;
+static int *bbs_list_size;
+static int max_path_count;
+static int max_insn_count;
+static int n_bbs_list;
+
+/* bbs_list[0] is the block with the switch statement,
+   bbs_list[n-1] is the block where the switch statement variable is assigned
+     a constant value,
+   The entries in between make a (reverse) path between the two.
+
+   We don't want to change bb_list, we want to leave that alone and
+   and copy the path to bbs_list_array so that we wind up with a list (array)
+   of paths that we want to update.  We also want to add the block that the
+   switch is going to go to on to the list so that we know which exit from
+   the switch statement is important.  */
+
+static void
+save_new_path (basic_block *bbs_list, int n, tree val)
+{
+  int i;
+  int insn_count;
+  basic_block bb;
+  edge switch_taken_edge;
+  gimple_stmt_iterator gsi;
+
+  if (n <= 1) return;
+
+  if (n_bbs_list >= max_path_count)
+    return;
+
+  /* Put the blocks in 'correct' order and add in where we want to go after
+     the switch statement, We want to leave bbs_list untouched for future
+     calls.  */
+
+  bbs_list_array[n_bbs_list] = XNEWVEC (basic_block, n+1);
+  for (i = 0; i < n; i++)
+    bbs_list_array[n_bbs_list][i] = bbs_list[n-i-1];
+
+  switch_taken_edge = find_taken_edge (bbs_list[0], val);
+  bbs_list_array[n_bbs_list][n] = switch_taken_edge->dest;
+
+  bbs_list_size[n_bbs_list] = n + 1;
+  val_array[n_bbs_list] = (int) TREE_INT_CST_LOW (val);
+
+  /* Count how many instructions are in the blocks we are going to
+     duplicate and if there are too many do not save this path
+     (return without incrementing n_bbs_list).  */
+
+  insn_count = 0;
+  for (i = 1; i < n; i++)
+    {
+      bb = bbs_list_array[n_bbs_list][i];
+      for (gsi = gsi_start_bb (bb); !gsi_end_p (gsi); gsi_next (&gsi))
+	insn_count += estimate_num_insns (gsi_stmt (gsi), &eni_size_weights);
+    }
+
+  if (insn_count > max_insn_count)
+    return;
+
+  n_bbs_list = n_bbs_list + 1;
+}
+
+/* switch_stmt is a switch statement whose switch index expression
+   is the variable expr.  We trace the value of the variable back
+   through any phi nodes looking for places where it gets a constant
+   value and save the path in bbs_list.  Then we call save_new_path
+   to create a list of such paths.  */
+
+static void
+process_switch (tree expr, gimple switch_stmt,
+		hash_set<gimple> *visited_phis,
+	        basic_block *bbs_list, int n)
+{
+  gimple def_stmt;
+  tree var;
+  unsigned int i;
+  edge e;
+  edge_iterator ei;
+  basic_block bbx;
+  basic_block var_bb;
+  int e_count;
+
+  gcc_assert (gimple_code (switch_stmt) == GIMPLE_SWITCH);
+  var = SSA_NAME_VAR (expr);
+  def_stmt = SSA_NAME_DEF_STMT (expr);
+  var_bb = gimple_bb (def_stmt);
+
+  if (var == NULL || var_bb == NULL) return;
+
+  /* We have a variable definition (var) that is defined in var_bb,
+     We want to put the path from var_bb to the current bb into the
+     bbs_list.  If there is more then one path, skip this and don't
+     try to do the optimization.  */
+
+  bbx = bbs_list[n-1];
+  while (bbx != var_bb)
+   {
+     e_count = 0;
+     FOR_EACH_EDGE (e, ei, bbx->preds)
+       {
+         if (find_path (var_bb, e->src))
+	   {
+      	     bbs_list[n] = e->src;
+      	     n = n + 1;
+	     e_count = e_count + 1;
+    	   }
+       }
+     if (e_count != 1) return;
+     bbx = bbs_list[n-1];
+   }
+
+  if ((gimple_code (def_stmt) == GIMPLE_PHI)
+       && !visited_phis->add (def_stmt))
+    {
+      for (i = 0; i < gimple_phi_num_args (def_stmt); i++)
+	{
+	  tree arg = gimple_phi_arg_def (def_stmt, i);
+	  if (arg && (TREE_CODE (arg) == INTEGER_CST))
+	    {
+	      /* const char *name = IDENTIFIER_POINTER (DECL_NAME (var)); */
+	      bbs_list[n] = gimple_phi_arg_edge (def_stmt, i)->src;
+	      save_new_path(bbs_list, n + 1, arg);
+	    }
+	  else if (arg && (TREE_CODE (arg) == SSA_NAME))
+	    {
+		  bbs_list[n] = gimple_phi_arg_edge (def_stmt, i)->src;
+		  process_switch (arg, switch_stmt, visited_phis, bbs_list, n+1);
+	    }
+	}
+    }
+}
+
+/* Find paths that lead from blocks where a variable is assigned a constant
+   value to a switch statement where that variable is used as the switch
+   index.  Save the paths in bbs_list_array so that they can be processed
+   by copy_switch_paths.  */
+
+static unsigned int
+find_switch_shortcuts (function *fun)
+{
+  basic_block bb;
+  hash_set<gimple> visited_phis;
+  basic_block *bbs_list;
+  int n = 1;
+
+  bbs_list = XNEWVEC (basic_block, n_basic_blocks_for_fn (fun));
+  FOR_EACH_BB_FN (bb, fun)
+    {
+      gimple stmt = last_stmt (bb);
+      if (stmt && gimple_code (stmt) == GIMPLE_SWITCH)
+	{
+	  tree op = gimple_switch_index (stmt);
+	  tree var = SSA_NAME_VAR (op);
+	  if (var)
+	    {
+	      bbs_list[0] = bb;
+	      process_switch (op, stmt, &visited_phis, bbs_list, n);
+	    }
+	}
+    }
+  XDELETEVEC (bbs_list);
+  return 0;
+}
+
+/* Call gimple_duplicate_sese_region to douplicate the blocks in bb_list.
+   We free and recalculate all ssa and dominance information afterwords
+   because the region being copied is not really SESE and so we cannot
+   trust gimple_duplicate_sese_region to correctly update the dataflow
+   information.  */
+
+static void
+duplicate_blocks (basic_block *bb_list, int bb_count)
+{
+  edge orig_edge, exit_edge;
+
+  orig_edge = find_edge (bb_list[0], bb_list[1]);
+  exit_edge = find_edge (bb_list[bb_count-2], bb_list[bb_count-1]);
+  gimple_duplicate_sese_region (orig_edge, exit_edge, &bb_list[1], bb_count-2, NULL, false);
+  free_dominance_info (CDI_DOMINATORS);
+  update_ssa (TODO_update_ssa);
+  calculate_dominance_info (CDI_DOMINATORS);
+  fix_loop_structure (NULL);
+}
+
+/* Go through the paths saved in bbs_list_array and make copies of them.  */
+
+static void
+copy_switch_paths (void)
+{
+  int i;
+
+  /* Process each path in bbs_list_size.  */
+  for (i = 0; i < n_bbs_list; i++)
+    {
+    /* For each path in bbs_list_size loop through and copy each block in
+       the path (except the first on where the constant is assigned and
+       the final one where the switch statement goes to.  */
+
+    if (!single_pred_p (bbs_list_array[i][1]))
+      duplicate_blocks (bbs_list_array[i], bbs_list_size[i]);
+    }
+}
+
+
+/* Main entry for the tree if-conversion pass.  */
+
+namespace {
+
+const pass_data pass_data_tree_switch_shortcut =
+{
+  GIMPLE_PASS, /* type */
+  "switch_shortcut", /* name */
+  OPTGROUP_NONE, /* optinfo_flags */
+  TV_TREE_SWITCH_SHORTCUT, /* tv_id */
+  ( PROP_cfg | PROP_ssa ), /* properties_required */
+  0, /* properties_provided */
+  0, /* properties_destroyed */
+  0, /* todo_flags_start */
+  TODO_update_ssa, /* todo_flags_finish */
+};
+
+class pass_tree_switch_shortcut : public gimple_opt_pass
+{
+public:
+  pass_tree_switch_shortcut (gcc::context *ctxt)
+    : gimple_opt_pass (pass_data_tree_switch_shortcut, ctxt)
+  {}
+
+  /* opt_pass methods: */
+  virtual bool gate (function *)
+    {
+      return flag_tree_switch_shortcut;
+    }
+  virtual unsigned int execute (function *);
+
+}; // class pass_tree_switch_shortcut
+
+unsigned int
+pass_tree_switch_shortcut::execute (function *fun)
+{
+  int i;
+
+  n_bbs_list = 0;
+  max_insn_count = PARAM_VALUE (PARAM_MAX_SWITCH_INSNS);
+  max_path_count = PARAM_VALUE (PARAM_MAX_SWITCH_PATHS);
+  val_array = XNEWVEC (int, max_path_count);
+  bbs_list_size = XNEWVEC (int, max_path_count); 
+  bbs_list_array = XNEWVEC (basic_block *, max_path_count);
+  find_switch_shortcuts (fun);
+  copy_switch_paths ();
+  XDELETEVEC (val_array);
+  XDELETEVEC (bbs_list_size);
+  for (i = 0; i < n_bbs_list; i++)
+    XDELETEVEC (bbs_list_array[i]); 
+  XDELETEVEC (bbs_list_array);
+  return 0;
+}
+
+} // anon namespace
+
+gimple_opt_pass *
+make_pass_tree_switch_shortcut (gcc::context *ctxt)
+{
+  return new pass_tree_switch_shortcut (ctxt);
+}