gcse fouling notes

[Alan Modra <amodra at bigpond dot net dot au>]

This "interesting" REG_EQUAL note, generated during gcse, causes
an abort in pa.c:output_global_address, which doesn't expect
a string of `const's like this.

(insn 424 421 428 (set (reg/s/f:SI 229)
        (plus:SI (reg/s/f:SI 225)
            (reg/s:SI 227))) 166 {addsi3} (nil)
    (expr_list:REG_EQUAL (const:SI (const:SI (const:SI (plus:SI (symbol_ref:SI ("game"))
                        (const_int 131072 [0x20000])))))
        (nil)))


In try_replace_reg, we start with:

(gdb) p debug_rtx (from)

(reg/s:SI 227)

(gdb) p debug_rtx (to)

(const_int 131072 [0x20000])

(gdb) p debug_rtx (insn)

(insn 424 421 428 (set (reg/s/f:SI 229)
        (plus:SI (reg/s/f:SI 225)
            (reg/s:SI 227))) 166 {addsi3} (nil)
    (expr_list:REG_EQUAL (const:SI (plus:SI (symbol_ref:SI ("game"))
                (const_int 131072 [0x20000])))
        (nil)))

and validate_replace_rtx_1 eventually gets to work on the note.
<case PLUS> turns 
(plus:SI (symbol_ref:SI ("game"))
    (const_int 131072 [0x20000]))

into 
(const:SI (plus:SI (symbol_ref:SI ("game"))
        (const_int 131072 [0x20000])))


I think this should fix the problem, but there are a number of
other possible solutions.  eg. Teaching simplify_rtx to remove
outermost `const's in a string of `const's.

	* recog.c (validate_replace_rtx_1): Exit early if nothing changed
	in args of commutative or comparison operations.

This one affects the branch too, but I don't have a simple testcase.

-- 
Alan Modra

--- gcc/recog.c.orig	Tue Jun 12 20:40:33 2001
+++ gcc/recog.c	Tue Jun 12 21:42:14 2001
@@ -437,7 +437,15 @@ validate_replace_rtx_1 (loc, from, to, o
 
       validate_replace_rtx_1 (&XEXP (x, 0), from, to, object);
       validate_replace_rtx_1 (&XEXP (x, 1), from, to, object);
-      if (prev_changes != num_changes && CONSTANT_P (XEXP (x, 0)))
+      /* If nothing changed, we can exit now.  In fact, continuing on
+	 into the switch statement below can be bad, eg. turning
+	 (plus (symbol_ref) (const_int)) into
+	 (const (plus (symbol_ref) (const_int))).  This might not seem
+	 so bad, but the first rtx is already enclosed in `const', so
+	 we get a string of (const (const (const...))).  */
+      if (prev_changes == num_changes)
+	return;
+      if (CONSTANT_P (XEXP (x, 0)))
 	{
 	  validate_change (object, loc,
 			   gen_rtx_fmt_ee (GET_RTX_CLASS (code) == 'c' ? code