tree-optimization/108352 - FSM threads creating irreducible loops

The following relaxes a heuristic that prevents creating irreducible
loops from FSM threads not covering multi-way branches.  Instead of
allowing threads that adhere to

      && (n_insns * (unsigned) param_fsm_scale_path_stmts
          > (m_path.length () *
             (unsigned) param_fsm_scale_path_blocks))

with reasoning "We also consider it worth creating an irreducible inner loop if
the number of copied statement is low relative to the length of the path --
in that case there's little the traditional loop optimizer would have done
anyway, so an irreducible loop is not so bad." that I cannot make much
sense of the following patch changes that to only allow those after
loop optimization and when they are (scaled) short:

      && (!(cfun->curr_properties & PROP_loop_opts_done)
          || (m_n_insns * param_fsm_scale_path_stmts
              >= param_max_jump_thread_duplication_stmts)))

This allows us to get rid of --param fsm-scale-path-blocks which
previous to the bisected revision allowed an enlarged path covering
the original allowance (but we do not consider that enlarged path
now because enlarging it doesn't add any information).

	PR tree-optimization/108352
	* tree-ssa-threadbackward.cc
	(back_threader_profitability::profitable_path_p): Adjust
	heuristic that allows non-multi-way branch threads creating
	irreducible loops.
	* doc/invoke.texi (--param fsm-scale-path-blocks): Remove.
	(--param fsm-scale-path-stmts): Adjust.
	* params.opt (--param=fsm-scale-path-blocks=): Remove.
	(-param=fsm-scale-path-stmts=): Adjust description.

	* gcc.dg/tree-ssa/ssa-thread-21.c: New testcase.
	* gcc.dg/tree-ssa/vrp46.c: Remove --param fsm-scale-path-blocks=1.

gcc/doc/invoke.texi

@@ -15981,16 +15981,13 @@ Max. size of loc list for which reverse ops should be added.
 
 @item fsm-scale-path-stmts
 Scale factor to apply to the number of statements in a threading path
-when comparing to the number of (scaled) blocks.
+crossing a loop backedge when comparing to
+@option{--param=max-jump-thread-duplication-stmts}.
 
 @item uninit-control-dep-attempts
 Maximum number of nested calls to search for control dependencies
 during uninitialized variable analysis.
 
-@item fsm-scale-path-blocks
-Scale factor to apply to the number of blocks in a threading path
-when comparing to the number of (scaled) statements.
-
 @item sched-autopref-queue-depth
 Hardware autoprefetcher scheduler model control flag.
 Number of lookahead cycles the model looks into; at '

gcc/params.opt

@@ -134,13 +134,9 @@ Maximum number of basic blocks before EVRP uses a sparse cache.
 Common Joined UInteger Var(param_evrp_switch_limit) Init(50) Optimization Param
 Maximum number of outgoing edges in a switch before EVRP will not process it.
 
--param=fsm-scale-path-blocks=
-Common Joined UInteger Var(param_fsm_scale_path_blocks) Init(3) IntegerRange(1, 10) Param Optimization
-Scale factor to apply to the number of blocks in a threading path when comparing to the number of (scaled) statements.
-
 -param=fsm-scale-path-stmts=
 Common Joined UInteger Var(param_fsm_scale_path_stmts) Init(2) IntegerRange(1, 10) Param Optimization
-Scale factor to apply to the number of statements in a threading path when comparing to the number of (scaled) blocks.
+Scale factor to apply to the number of statements in a threading path crossing a loop backedge when comparing to max-jump-thread-duplication-stmts.
 
 -param=gcse-after-reload-critical-fraction=
 Common Joined UInteger Var(param_gcse_after_reload_critical_fraction) Init(10) Param Optimization

gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-21.c

@@ -0,0 +1,26 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-thread2-stats -fdump-tree-optimized" } */
+
+long a;
+int b;
+void bar64_(void);
+void foo();
+int main() {
+  char c = 0;
+  unsigned d = 10;
+  int e = 2;
+  for (; d; d--) {
+    bar64_();
+    b = d;
+    e && (c = (e = 0) != 4) > 1;
+  }
+  if (c < 1)
+    foo();
+  a = b;
+}
+
+/* We need to perform a non-multi-way branch FSM thread creating an
+   irreducible loop in thread2 to allow followup threading to
+   remove the call to foo ().  */
+/* { dg-final { scan-tree-dump "Jumps threaded: 1" "thread2" } } */
+/* { dg-final { scan-tree-dump-not "foo" "optimized" } } */

gcc/testsuite/gcc.dg/tree-ssa/vrp46.c

@@ -1,5 +1,5 @@
 /* { dg-do compile } */
-/* { dg-options "-O2 -fdump-tree-vrp1 --param fsm-scale-path-blocks=1" } */
+/* { dg-options "-O2 -fdump-tree-vrp1" } */
 
 int func_81 (int);
 int func_98 (int);

gcc/tree-ssa-threadbackward.cc

@@ -868,22 +868,18 @@ back_threader_profitability::profitable_path_p (const vec<basic_block> &m_path,
      a multiway branch, in which case we have deemed it worth losing
      other loop optimizations later.
 
-     We also consider it worth creating an irreducible inner loop if
-     the number of copied statement is low relative to the length of
-     the path -- in that case there's little the traditional loop
-     optimizer would have done anyway, so an irreducible loop is not
-     so bad.  */
+     We also consider it worth creating an irreducible inner loop after
+     loop optimizations if the number of copied statement is low.  */
   if (!m_threaded_multiway_branch
       && *creates_irreducible_loop
-      && (m_n_insns * (unsigned) param_fsm_scale_path_stmts
-	  > (m_path.length () *
-	     (unsigned) param_fsm_scale_path_blocks)))
-
+      && (!(cfun->curr_properties & PROP_loop_opts_done)
+	  || (m_n_insns * param_fsm_scale_path_stmts
+	      >= param_max_jump_thread_duplication_stmts)))
     {
       if (dump_file && (dump_flags & TDF_DETAILS))
 	fprintf (dump_file,
-		 "  FAIL: Would create irreducible loop without threading "
-		 "multiway branch.\n");
+		 "  FAIL: Would create irreducible loop early without "
+		 "threading multiway branch.\n");
       /* We compute creates_irreducible_loop only late.  */
       return false;
     }