rtlanal: Tweak operator precedence for comparisons

One quirk of the commutative operand precedence rules is that:

- PLUS and MINUS have precedence 4
- other binary arithmetic operators have precedence 2
- NOT and NEG have precedence 1
- (binary) comparison operators have precedence 0

This means that the arithmetic inverse operator (NEG) has a lower
precedence than the binary arithmetic operators, but the logical
inverse operator (NOT) has a higher precedence than binary comparisons
that produce a logical result.  In other words, we have:

  some binary > some unary > some other binary

This patch shuffles the precedence values so that all binary operators
have precedence over all unary operators.  It means that existing
aarch64.md patterns such as:

  (define_insn "*add<mode>3_carryinC_zero"
    [(set (reg:CC_ADC CC_REGNUM)
	  (compare:CC_ADC
	    (plus:<DWI>
	      (match_operand:<DWI> 2 "aarch64_carry_operation" "")
	      (zero_extend:<DWI> (match_operand:GPI 1 "register_operand" "r")))
	    (match_operand 4 "const_scalar_int_operand" "")))
     (set (match_operand:GPI 0 "register_operand" "=r")
	  (plus:GPI (match_operand:GPI 3 "aarch64_carry_operation" "")
		    (match_dup 1)))]

are now canonical.

gcc/
	* rtlanal.cc (commutative_operand_precedence): Bump the precedence
	of non-commutative binary arithmetic to 3.  Give comparisons a
	precedence of 2.

diff --git a/gcc/rtlanal.cc b/gcc/rtlanal.cc
index 88561a54e5a09fba20e9a5921804cb713e4a8ac2..5274a5c59cf1dd43c8f82a6b5641a1c2fa8d667a 100644 (file)
--- a/gcc/rtlanal.cc
+++ b/gcc/rtlanal.cc
@@ -3829,6 +3829,13 @@ commutative_operand_precedence (rtx op)
       /* If only one operand is a binary expression, it will be the first
          operand.  In particular,  (plus (minus (reg) (reg)) (neg (reg)))
          is canonical, although it will usually be further simplified.  */
+      return 3;
+
+    case RTX_COMM_COMPARE:
+    case RTX_COMPARE:
+      /* Give comparisons a cost between the unary expressions below
+        and the other binary expressions above, so that we don't have
+        a situation where the canonical order is binary, unary, binary.  */
       return 2;
 
     case RTX_UNARY: