opt: Add local optimization `a * b [-1 <= b <= 0] -> -(a & b)`.

NEWS.md

@@ -45,6 +45,7 @@ libFirm 1.22.1 (2016-01-07)
 * amd64: Improve block schedule by removing empty basic blocks
 * amd64: Use cltd/cqto for sign extension before a division
 * amd64: Use store with immediate for call arguments
+* opt: Add local optimization `a * b [-1 <= b <= 0] -> -(a & b)`
 * Bugfixes
 
 libFirm 1.22.0 (2015-12-31)

ir/opt/iropt.c

@@ -3658,6 +3658,19 @@ static ir_node *can_negate_cheaply(dbg_info *const dbgi, ir_node *const node)
 	return NULL;
 }
 
+static bool is_zero_or_all_one(ir_node const *const n)
+{
+	if (is_Shrs(n)) {
+		ir_node *const r = get_Shrs_right(n);
+		if (is_Const(r)) {
+			long const v = get_Const_long(r);
+			if (v == (long)get_mode_size_bits(get_irn_mode(n)) - 1)
+				return true;
+		}
+	}
+	return false;
+}
+
 static ir_node *transform_node_shl_shr(ir_node *n);
 
 static ir_node *transform_node_Mul(ir_node *n)
@@ -3732,6 +3745,14 @@ static ir_node *transform_node_Mul(ir_node *n)
 			ir_node  *minus = new_rd_Minus(dbgi, block, bit);
 			return new_rd_And(dbgi, block, other, minus);
 		}
+
+		if (is_zero_or_all_one(a) || is_zero_or_all_one(b)) {
+			/* a * b [-1 <= b <= 0] -> -(a & b) */
+			dbg_info *const dbgi  = get_irn_dbg_info(n);
+			ir_node  *const block = get_nodes_block(n);
+			ir_node  *const and   = new_rd_And(dbgi, block, a, b);
+			return new_rd_Minus(dbgi, block, and);
+		}
 	}
 
 	/* distribute minus: