rtl-optimization/105231 - distribute_notes and REG_EH_REGION

The following mitigates a problem in combine distribute_notes which
places an original REG_EH_REGION based on only may_trap_p which is
good to test whether a non-call insn can possibly throw but not if
actually it does or we care.  That's something we decided at RTL
expansion time where we possibly still know the insn evaluates
to a constant.

In fact, the REG_EH_REGION note with lp > 0 can only come from the
original i3 and an assert is added to that effect.  That means we only
need to retain the note on i3 or, if that cannot trap, drop it but we
should never move it to i2.

The following places constraints on the insns to combine with
non-call exceptions since we cannot handle the case where we
have more than one EH side-effect in the IL.  The patch also
makes sure we can accumulate that on i3 and do not split
a possible exception raising part of it to i2.  As a special
case we do not place any restriction on all externally
throwing insns when there is no REG_EH_REGION present.

2022-04-22  Richard Biener  <rguenther@suse.de>

	PR rtl-optimization/105231
	* combine.cc (distribute_notes): Assert that a REG_EH_REGION
	with landing pad > 0 is from i3.  Put any REG_EH_REGION note
	on i3 or drop it if the insn can not trap.
	(try_combine): Ensure that we can merge REG_EH_REGION notes
	with non-call exceptions.  Ensure we are not splitting a
	trapping part of an insn with non-call exceptions when there
	is any REG_EH_REGION note to preserve.

	* gcc.dg/torture/pr105231.c: New testcase.

gcc/combine.cc

@@ -2569,6 +2569,7 @@ try_combine (rtx_insn *i3, rtx_insn *i2, rtx_insn *i1, rtx_insn *i0,
   rtx new_other_notes;
   int i;
   scalar_int_mode dest_mode, temp_mode;
+  bool has_non_call_exception = false;
 
   /* Immediately return if any of I0,I1,I2 are the same insn (I3 can
      never be).  */
@@ -2951,6 +2952,32 @@ try_combine (rtx_insn *i3, rtx_insn *i2, rtx_insn *i1, rtx_insn *i0,
       return 0;
     }
 
+  /* With non-call exceptions we can end up trying to combine multiple
+     insns with possible EH side effects.  Make sure we can combine
+     that to a single insn which means there must be at most one insn
+     in the combination with an EH side effect.  */
+  if (cfun->can_throw_non_call_exceptions)
+    {
+      if (find_reg_note (i3, REG_EH_REGION, NULL_RTX)
+	  || find_reg_note (i2, REG_EH_REGION, NULL_RTX)
+	  || (i1 && find_reg_note (i1, REG_EH_REGION, NULL_RTX))
+	  || (i0 && find_reg_note (i0, REG_EH_REGION, NULL_RTX)))
+	{
+	  has_non_call_exception = true;
+	  if (insn_could_throw_p (i3)
+	      + insn_could_throw_p (i2)
+	      + (i1 ? insn_could_throw_p (i1) : 0)
+	      + (i0 ? insn_could_throw_p (i0) : 0) > 1)
+	    {
+	      if (dump_file && (dump_flags & TDF_DETAILS))
+		fprintf (dump_file, "Can't combine multiple insns with EH "
+			 "side-effects\n");
+	      undo_all ();
+	      return 0;
+	    }
+	}
+    }
+
   /* Record whether i2 and i3 are trivial moves.  */
   i2_was_move = is_just_move (i2);
   i3_was_move = is_just_move (i3);
@@ -3685,7 +3712,13 @@ try_combine (rtx_insn *i3, rtx_insn *i2, rtx_insn *i1, rtx_insn *i0,
 	      || !modified_between_p (*split, i2, i3))
 	  /* We can't overwrite I2DEST if its value is still used by
 	     NEWPAT.  */
-	  && ! reg_referenced_p (i2dest, newpat))
+	  && ! reg_referenced_p (i2dest, newpat)
+	  /* We should not split a possibly trapping part when we
+	     care about non-call EH and have REG_EH_REGION notes
+	     to distribute.  */
+	  && ! (cfun->can_throw_non_call_exceptions
+		&& has_non_call_exception
+		&& may_trap_p (*split)))
 	{
 	  rtx newdest = i2dest;
 	  enum rtx_code split_code = GET_CODE (*split);
@@ -14175,23 +14208,35 @@ distribute_notes (rtx notes, rtx_insn *from_insn, rtx_insn *i3, rtx_insn *i2,
 	  break;
 
 	case REG_EH_REGION:
-	  /* These notes must remain with the call or trapping instruction.  */
-	  if (CALL_P (i3))
-	    place = i3;
-	  else if (i2 && CALL_P (i2))
-	    place = i2;
-	  else
-	    {
-	      gcc_assert (cfun->can_throw_non_call_exceptions);
-	      if (may_trap_p (i3))
-		place = i3;
-	      else if (i2 && may_trap_p (i2))
-		place = i2;
-	      /* ??? Otherwise assume we've combined things such that we
-		 can now prove that the instructions can't trap.  Drop the
-		 note in this case.  */
-	    }
-	  break;
+	  {
+	    /* The landing pad handling needs to be kept in sync with the
+	       prerequisite checking in try_combine.  */
+	    int lp_nr = INTVAL (XEXP (note, 0));
+	    /* A REG_EH_REGION note transfering control can only ever come
+	       from i3.  */
+	    if (lp_nr > 0)
+	      gcc_assert (from_insn == i3);
+	    /* We are making sure there is a single effective REG_EH_REGION
+	       note and it's valid to put it on i3.  */
+	    if (!insn_could_throw_p (from_insn))
+	      /* Throw away stra notes on insns that can never throw.  */
+	      ;
+	    else
+	      {
+		if (CALL_P (i3))
+		  place = i3;
+		else
+		  {
+		    gcc_assert (cfun->can_throw_non_call_exceptions);
+		    /* If i3 can still trap preserve the note, otherwise we've
+		       combined things such that we can now prove that the
+		       instructions can't trap.  Drop the note in this case.  */
+		    if (may_trap_p (i3))
+		      place = i3;
+		  }
+	      }
+	    break;
+	  }
 
 	case REG_ARGS_SIZE:
 	  /* ??? How to distribute between i3-i1.  Assume i3 contains the

gcc/testsuite/gcc.dg/torture/pr105231.c

@@ -0,0 +1,15 @@
+/* { dg-do compile } */
+/* { dg-require-effective-target lp64 } */
+/* { dg-require-effective-target dfp } */
+/* { dg-additional-options "-fsanitize-coverage=trace-pc -fnon-call-exceptions --param=max-cse-insns=1 -frounding-math" } */
+/* { dg-additional-options "-mstack-arg-probe" { target x86_64-*-* i?86-*-* } } */
+
+void baz (int *);
+void bar (double, double, _Decimal64);
+
+void
+foo (void)
+{
+  int s __attribute__((cleanup (baz)));
+  bar (0xfffffffffffffffe, 0xebf3fff2fbebaf7f, 0xffffffffffffff);
+}