Re: [PATCH, GCC, stage1] Fallback to copy-prop if constant-prop not possible

[Steven Bosscher <stevenb.gcc@gmail.com>]

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On Tue, Feb 17, 2015 at 3:51 AM, Thomas Preud'homme wrote:
>> > -      else if (REG_P (src)
>> > -              && REGNO (src) >= FIRST_PSEUDO_REGISTER
>> > -              && REGNO (src) != regno)
>> > -       {
>> > -         if (try_replace_reg (reg_used, src, insn))
>> > +         else if (src_reg && REG_P (src_reg)
>> > +                  && REGNO (src_reg) >= FIRST_PSEUDO_REGISTER
>> > +                  && REGNO (src_reg) != regno
>> > +                  && try_replace_reg (reg_used, src_reg, insn))
>>
>> Likewise for the REG_P and ">= FIRST_PSEUDO_REGISTER" tests here
>> (with
>> the equivalent and IMHO preferable HARD_REGISTER_P test in
>> find_avail_set()).
>
> I'm not sure I follow you here. First, it seems to me that the equivalent
> test is rather REG_P && !HARD_REGISTER_P since here it checks if it's
> a pseudo register.
>
> Then, do you mean the test can be simply removed because of the
> REG_P && !HARD_REGISTER_P in hash_scan_set () called indirectly by
> compute_hash_table () when called in one_cprop_pass () before any
> cprop_insn ()? Or do you mean I should move the check in
> find_avail_set ()?


What I meant, is that I believe the tests are already done in
hash_scan_set and should be redundant in cprop_insn (i.e. the test can
be replaced with gcc_[checking_]assert).

I've attached a patch with some changes to it: introduce cprop_reg_p()
to get rid of all the "REG_P && regno > FIRST_PSEUDO_REGISTER" tests.
I still have the cprop_constant_p and cprop_reg_p tests in cprop_insn
but this weekend I'll try with gcc_checking_asserts instead. Please
have a look at the patch and let me know if you like it (given it's
mostly yours I hope you do like it ;-)

Bootstrapped & tested on powerpc64-unknown-linux-gnu. In building all
of cc1, 35 extra copies are propagated with the patch.

Ciao!
Steven

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Index: cprop.c
===================================================================
--- cprop.c	(revision 221523)
+++ cprop.c	(working copy)
@@ -285,6 +285,15 @@ cprop_constant_p (const_rtx x)
   return CONSTANT_P (x) && (GET_CODE (x) != CONST || shared_const_p (x));
 }
 
+/* Determine whether the rtx X should be treated as a register that can
+   be propagated.  Any pseudo-register is fine.  */
+
+static bool
+cprop_reg_p (const_rtx x)
+{
+  return REG_P (x) && !HARD_REGISTER_P (x);
+}
+
 /* Scan SET present in INSN and add an entry to the hash TABLE.
    IMPLICIT is true if it's an implicit set, false otherwise.  */
 
@@ -295,8 +304,7 @@ hash_scan_set (rtx set, rtx_insn *insn, struct has
   rtx src = SET_SRC (set);
   rtx dest = SET_DEST (set);
 
-  if (REG_P (dest)
-      && ! HARD_REGISTER_P (dest)
+  if (cprop_reg_p (dest)
       && reg_available_p (dest, insn)
       && can_copy_p (GET_MODE (dest)))
     {
@@ -321,9 +329,8 @@ hash_scan_set (rtx set, rtx_insn *insn, struct has
 	src = XEXP (note, 0), set = gen_rtx_SET (VOIDmode, dest, src);
 
       /* Record sets for constant/copy propagation.  */
-      if ((REG_P (src)
+      if ((cprop_reg_p (src)
 	   && src != dest
-	   && ! HARD_REGISTER_P (src)
 	   && reg_available_p (src, insn))
 	  || cprop_constant_p (src))
 	insert_set_in_table (dest, src, insn, table, implicit);
@@ -821,15 +828,15 @@ try_replace_reg (rtx from, rtx to, rtx_insn *insn)
   return success;
 }
 
-/* Find a set of REGNOs that are available on entry to INSN's block.  Return
-   NULL no such set is found.  */
+/* Find a set of REGNOs that are available on entry to INSN's block.  If found,
+   SET_RET[0] will be assigned a set with a register source and SET_RET[1] a
+   set with a constant source.  If not found the corresponding entry is set to
+   NULL.  */
 
-static struct cprop_expr *
-find_avail_set (int regno, rtx_insn *insn)
+static void
+find_avail_set (int regno, rtx_insn *insn, struct cprop_expr *set_ret[2])
 {
-  /* SET1 contains the last set found that can be returned to the caller for
-     use in a substitution.  */
-  struct cprop_expr *set1 = 0;
+  set_ret[0] = set_ret[1] = NULL;
 
   /* Loops are not possible here.  To get a loop we would need two sets
      available at the start of the block containing INSN.  i.e. we would
@@ -869,8 +876,10 @@ try_replace_reg (rtx from, rtx to, rtx_insn *insn)
          If the source operand changed, we may still use it for the next
          iteration of this loop, but we may not use it for substitutions.  */
 
-      if (cprop_constant_p (src) || reg_not_set_p (src, insn))
-	set1 = set;
+      if (cprop_constant_p (src))
+	set_ret[1] = set;
+      else if (reg_not_set_p (src, insn))
+	set_ret[0] = set;
 
       /* If the source of the set is anything except a register, then
 	 we have reached the end of the copy chain.  */
@@ -881,10 +890,6 @@ try_replace_reg (rtx from, rtx to, rtx_insn *insn)
 	 and see if we have an available copy into SRC.  */
       regno = REGNO (src);
     }
-
-  /* SET1 holds the last set that was available and anticipatable at
-     INSN.  */
-  return set1;
 }
 
 /* Subroutine of cprop_insn that tries to propagate constants into
@@ -1050,7 +1055,8 @@ cprop_insn (rtx_insn *insn)
   int changed = 0, changed_this_round;
   rtx note;
 
-retry:
+  do
+    {
   changed_this_round = 0;
   reg_use_count = 0;
   note_uses (&PATTERN (insn), find_used_regs, NULL);
@@ -1064,8 +1070,8 @@ cprop_insn (rtx_insn *insn)
     {
       rtx reg_used = reg_use_table[i];
       unsigned int regno = REGNO (reg_used);
-      rtx src;
-      struct cprop_expr *set;
+	  rtx src_cst = NULL, src_reg = NULL;
+	  struct cprop_expr *set[2];
 
       /* If the register has already been set in this block, there's
 	 nothing we can do.  */
@@ -1074,17 +1080,16 @@ cprop_insn (rtx_insn *insn)
 
       /* Find an assignment that sets reg_used and is available
 	 at the start of the block.  */
-      set = find_avail_set (regno, insn);
-      if (! set)
-	continue;
+	  find_avail_set (regno, insn, set);
+	  if (set[0])
+	    src_reg = set[0]->src;
+	  if (set[1])
+	    src_cst = set[1]->src;
 
-      src = set->src;
-
       /* Constant propagation.  */
-      if (cprop_constant_p (src))
+	  if (src_cst && cprop_constant_p (src_cst)
+	      && constprop_register (reg_used, src_cst, insn))
 	{
-          if (constprop_register (reg_used, src, insn))
-	    {
 	      changed_this_round = changed = 1;
 	      global_const_prop_count++;
 	      if (dump_file != NULL)
@@ -1093,19 +1098,17 @@ cprop_insn (rtx_insn *insn)
 			   "GLOBAL CONST-PROP: Replacing reg %d in ", regno);
 		  fprintf (dump_file, "insn %d with constant ",
 			   INSN_UID (insn));
-		  print_rtl (dump_file, src);
+		  print_rtl (dump_file, src_cst);
 		  fprintf (dump_file, "\n");
 		}
 	      if (insn->deleted ())
 		return 1;
 	    }
-	}
-      else if (REG_P (src)
-	       && REGNO (src) >= FIRST_PSEUDO_REGISTER
-	       && REGNO (src) != regno)
+	  /* Copy propagation.  */
+	  else if (src_reg && cprop_reg_p (src_reg)
+		   && REGNO (src_reg) != regno
+		   && try_replace_reg (reg_used, src_reg, insn))
 	{
-	  if (try_replace_reg (reg_used, src, insn))
-	    {
 	      changed_this_round = changed = 1;
 	      global_copy_prop_count++;
 	      if (dump_file != NULL)
@@ -1113,7 +1116,7 @@ cprop_insn (rtx_insn *insn)
 		  fprintf (dump_file,
 			   "GLOBAL COPY-PROP: Replacing reg %d in insn %d",
 			   regno, INSN_UID (insn));
-		  fprintf (dump_file, " with reg %d\n", REGNO (src));
+		  fprintf (dump_file, " with reg %d\n", REGNO (src_reg));
 		}
 
 	      /* The original insn setting reg_used may or may not now be
@@ -1123,12 +1126,10 @@ cprop_insn (rtx_insn *insn)
 		 and made things worse.  */
 	    }
 	}
-
+    }
       /* If try_replace_reg simplified the insn, the regs found
 	 by find_used_regs may not be valid anymore.  Start over.  */
-      if (changed_this_round)
-	goto retry;
-    }
+  while (changed_this_round);
 
   if (changed && DEBUG_INSN_P (insn))
     return 0;
@@ -1191,7 +1192,7 @@ do_local_cprop (rtx x, rtx_insn *insn)
   /* Rule out USE instructions and ASM statements as we don't want to
      change the hard registers mentioned.  */
   if (REG_P (x)
-      && (REGNO (x) >= FIRST_PSEUDO_REGISTER
+      && (cprop_reg_p (x)
           || (GET_CODE (PATTERN (insn)) != USE
 	      && asm_noperands (PATTERN (insn)) < 0)))
     {
@@ -1207,7 +1208,7 @@ do_local_cprop (rtx x, rtx_insn *insn)
 
 	  if (cprop_constant_p (this_rtx))
 	    newcnst = this_rtx;
-	  if (REG_P (this_rtx) && REGNO (this_rtx) >= FIRST_PSEUDO_REGISTER
+	  if (cprop_reg_p (this_rtx)
 	      /* Don't copy propagate if it has attached REG_EQUIV note.
 		 At this point this only function parameters should have
 		 REG_EQUIV notes and if the argument slot is used somewhere
@@ -1328,9 +1329,8 @@ implicit_set_cond_p (const_rtx cond)
   if (GET_CODE (cond) != EQ && GET_CODE (cond) != NE)
     return false;
 
-  /* The first operand of COND must be a pseudo-reg.  */
-  if (! REG_P (XEXP (cond, 0))
-      || HARD_REGISTER_P (XEXP (cond, 0)))
+  /* The first operand of COND must be a register we can propagate.  */
+  if (cprop_reg_p (XEXP (cond, 0)))
     return false;
 
   /* The second operand of COND must be a suitable constant.  */