target/108738 - limit STV chain discovery

The following puts a hard limit on the inherently quadratic STV chain
discovery.  Without a limit for the compiler.i testcase in PR26854
we see at -O2

 machine dep reorg                  : 574.45 ( 53%)

with release checking while with the proposed limit it's

 machine dep reorg                  :   2.86 (  1%)

	PR target/108738
	* config/i386/i386.opt (--param x86-stv-max-visits): New param.
	* doc/invoke.texi (--param x86-stv-max-visits): Document it.
	* config/i386/i386-features.h (scalar_chain::max_visits): New.
	(scalar_chain::build): Add bitmap parameter, return boolean.
	(scalar_chain::add_insn): Likewise.
	(scalar_chain::analyze_register_chain): Likewise.
	* config/i386/i386-features.cc (scalar_chain::scalar_chain):
	Initialize max_visits.
	(scalar_chain::analyze_register_chain): When we exhaust
	max_visits, abort.  Also abort when running into any
	disallowed insn.
	(scalar_chain::add_insn): Propagate abort.
	(scalar_chain::build): Likewise.  When aborting amend
	the set of disallowed insn with the insns set.
	(convert_scalars_to_vector): Adjust.  Do not convert aborted
	chains.

4 files changed, 70 insertions, 25 deletions

diff --git a/gcc/config/i386/i386-features.cc b/gcc/config/i386/i386-features.cc
index eff9130..c09abf8 100644
--- a/gcc/config/i386/i386-features.cc
+++ b/gcc/config/i386/i386-features.cc
@@ -296,6 +296,8 @@ scalar_chain::scalar_chain (enum machine_mode smode_, enum machine_mode vmode_)
 
   n_sse_to_integer = 0;
   n_integer_to_sse = 0;
+
+  max_visits = x86_stv_max_visits;
 }
 
 /* Free chain's data.  */
@@ -354,10 +356,12 @@ scalar_chain::mark_dual_mode_def (df_ref def)
 }
 
 /* Check REF's chain to add new insns into a queue
-   and find registers requiring conversion.  */
+   and find registers requiring conversion.  Return true if OK, false
+   if the analysis was aborted.  */
 
-void
-scalar_chain::analyze_register_chain (bitmap candidates, df_ref ref)
+bool
+scalar_chain::analyze_register_chain (bitmap candidates, df_ref ref,
+				      bitmap disallowed)
 {
   df_link *chain;
   bool mark_def = false;
@@ -371,6 +375,9 @@ scalar_chain::analyze_register_chain (bitmap candidates, df_ref ref)
       if (!NONDEBUG_INSN_P (DF_REF_INSN (chain->ref)))
 	continue;
 
+      if (--max_visits == 0)
+	return false;
+
       if (!DF_REF_REG_MEM_P (chain->ref))
 	{
 	  if (bitmap_bit_p (insns, uid))
@@ -381,6 +388,10 @@ scalar_chain::analyze_register_chain (bitmap candidates, df_ref ref)
 	      add_to_queue (uid);
 	      continue;
 	    }
+
+	  /* If we run into parts of an aborted chain discovery abort.  */
+	  if (bitmap_bit_p (disallowed, uid))
+	    return false;
 	}
 
       if (DF_REF_REG_DEF_P (chain->ref))
@@ -401,15 +412,19 @@ scalar_chain::analyze_register_chain (bitmap candidates, df_ref ref)
 
   if (mark_def)
     mark_dual_mode_def (ref);
+
+  return true;
 }
 
-/* Add instruction into a chain.  */
+/* Add instruction into a chain.  Return true if OK, false if the search
+   was aborted.  */
 
-void
-scalar_chain::add_insn (bitmap candidates, unsigned int insn_uid)
+bool
+scalar_chain::add_insn (bitmap candidates, unsigned int insn_uid,
+			bitmap disallowed)
 {
   if (!bitmap_set_bit (insns, insn_uid))
-    return;
+    return true;
 
   if (dump_file)
     fprintf (dump_file, "  Adding insn %d to chain #%d\n", insn_uid, chain_id);
@@ -426,22 +441,27 @@ scalar_chain::add_insn (bitmap candidates, unsigned int insn_uid)
   df_ref ref;
   for (ref = DF_INSN_UID_DEFS (insn_uid); ref; ref = DF_REF_NEXT_LOC (ref))
     if (!HARD_REGISTER_P (DF_REF_REG (ref)))
-      analyze_register_chain (candidates, ref);
+      if (!analyze_register_chain (candidates, ref, disallowed))
+	return false;
 
   /* The operand(s) of VEC_SELECT don't need to be converted/convertible.  */
   if (def_set && GET_CODE (SET_SRC (def_set)) == VEC_SELECT)
-    return;
+    return true;
 
   for (ref = DF_INSN_UID_USES (insn_uid); ref; ref = DF_REF_NEXT_LOC (ref))
     if (!DF_REF_REG_MEM_P (ref))
-      analyze_register_chain (candidates, ref);
+      if (!analyze_register_chain (candidates, ref, disallowed))
+	return false;
+
+  return true;
 }
 
 /* Build new chain starting from insn INSN_UID recursively
-   adding all dependent uses and definitions.  */
+   adding all dependent uses and definitions.  Return true if OK, false
+   if the chain discovery was aborted.  */
 
-void
-scalar_chain::build (bitmap candidates, unsigned insn_uid)
+bool
+scalar_chain::build (bitmap candidates, unsigned insn_uid, bitmap disallowed)
 {
   queue = BITMAP_ALLOC (NULL);
   bitmap_set_bit (queue, insn_uid);
@@ -454,7 +474,17 @@ scalar_chain::build (bitmap candidates, unsigned insn_uid)
       insn_uid = bitmap_first_set_bit (queue);
       bitmap_clear_bit (queue, insn_uid);
       bitmap_clear_bit (candidates, insn_uid);
-      add_insn (candidates, insn_uid);
+      if (!add_insn (candidates, insn_uid, disallowed))
+	{
+	  /* If we aborted the search put sofar found insn on the set of
+	     disallowed insns so that further searches reaching them also
+	     abort and thus we abort the whole but yet undiscovered chain.  */
+	  bitmap_ior_into (disallowed, insns);
+	  if (dump_file)
+	    fprintf (dump_file, "Aborted chain #%d discovery\n", chain_id);
+	  BITMAP_FREE (queue);
+	  return false;
+	}
     }
 
   if (dump_file)
@@ -478,6 +508,8 @@ scalar_chain::build (bitmap candidates, unsigned insn_uid)
     }
 
   BITMAP_FREE (queue);
+
+  return true;
 }
 
 /* Return a cost of building a vector costant
@@ -2282,6 +2314,7 @@ convert_scalars_to_vector (bool timode_p)
 
   for (unsigned i = 0; i <= 2; ++i)
     {
+      auto_bitmap disallowed;
       bitmap_tree_view (&candidates[i]);
       while (!bitmap_empty_p (&candidates[i]))
 	{
@@ -2296,14 +2329,14 @@ convert_scalars_to_vector (bool timode_p)
 	  /* Find instructions chain we want to convert to vector mode.
 	     Check all uses and definitions to estimate all required
 	     conversions.  */
-	  chain->build (&candidates[i], uid);
-
-	  if (chain->compute_convert_gain () > 0)
-	    converted_insns += chain->convert ();
-	  else
-	    if (dump_file)
-	      fprintf (dump_file, "Chain #%d conversion is not profitable\n",
-		       chain->chain_id);
+	  if (chain->build (&candidates[i], uid, disallowed))
+	    {
+	      if (chain->compute_convert_gain () > 0)
+		converted_insns += chain->convert ();
+	      else if (dump_file)
+		fprintf (dump_file, "Chain #%d conversion is not profitable\n",
+			 chain->chain_id);
+	    }
 
 	  delete chain;
 	}
diff --git a/gcc/config/i386/i386-features.h b/gcc/config/i386/i386-features.h
index 00c2c5e..72a9f54 100644
--- a/gcc/config/i386/i386-features.h
+++ b/gcc/config/i386/i386-features.h
@@ -148,12 +148,15 @@ class scalar_chain
   /* Registers used in both vector and sclar modes.  */
   bitmap defs_conv;
 
+  /* Limit on chain discovery.  */
+  unsigned max_visits;
+
   bitmap insns_conv;
   hash_map<rtx, rtx> defs_map;
   unsigned n_sse_to_integer;
   unsigned n_integer_to_sse;
 
-  void build (bitmap candidates, unsigned insn_uid);
+  bool build (bitmap candidates, unsigned insn_uid, bitmap disallowed);
   virtual int compute_convert_gain () = 0;
   int convert ();
 
@@ -168,8 +171,9 @@ class scalar_chain
   void convert_registers ();
 
  private:
-  void add_insn (bitmap candidates, unsigned insn_uid);
-  void analyze_register_chain (bitmap candidates, df_ref ref);
+  bool add_insn (bitmap candidates, unsigned insn_uid, bitmap disallowed);
+  bool analyze_register_chain (bitmap candidates, df_ref ref,
+			       bitmap disallowed);
   virtual void convert_insn (rtx_insn *insn) = 0;
   virtual void convert_op (rtx *op, rtx_insn *insn) = 0;
 };
diff --git a/gcc/config/i386/i386.opt b/gcc/config/i386/i386.opt
index 7d57f61..94fdd63 100644
--- a/gcc/config/i386/i386.opt
+++ b/gcc/config/i386/i386.opt
@@ -599,6 +599,10 @@ Target Mask(STV) Save
 Disable Scalar to Vector optimization pass transforming 64-bit integer
 computations into a vector ones.
 
+-param=x86-stv-max-visits=
+Target Joined UInteger Var(x86_stv_max_visits) Init(10000) IntegerRange(1, 1000000) Param
+The maximum number of use and def visits when discovering a STV chain before the discovery is aborted.
+
 mdispatch-scheduler
 Target RejectNegative Var(flag_dispatch_scheduler)
 Do dispatch scheduling if processor is bdver1, bdver2, bdver3, bdver4
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 0045661..2da6880 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -16229,6 +16229,10 @@ The following choices of @var{name} are available on i386 and x86_64 targets:
 @item x86-stlf-window-ninsns
 Instructions number above which STFL stall penalty can be compensated.
 
+@item x86-stv-max-visits
+The maximum number of use and def visits when discovering a STV chain before
+the discovery is aborted.
+
 @end table
 
 @end table