gen: optimize `str == ''` to `str.len == 0` for performance (#6527)

vlib/v/gen/cgen.v

@@ -2390,39 +2390,49 @@ fn (mut g Gen) infix_expr(node ast.InfixExpr) {
 		g.write(', ')
 		g.expr(node.right)
 		g.write(')')
-	} else if left_type == table.string_type_idx && node.op != .key_in && node.op != .not_in {
-		fn_name := match node.op {
-			.plus {
-				'string_add('
-			}
-			.eq {
-				'string_eq('
-			}
-			.ne {
-				'string_ne('
-			}
-			.lt {
-				'string_lt('
-			}
-			.le {
-				'string_le('
-			}
-			.gt {
-				'string_gt('
-			}
-			.ge {
-				'string_ge('
-			}
-			else {
-				verror('op error for type `$left_sym.name`')
-				'/*node error*/'
+	} else if left_type == table.string_type_idx && node.op !in [.key_in, .not_in] {
+		// `str == ''` -> `str.len == 0` optimization
+		if node.op in [.eq, .ne] &&
+			node.right is ast.StringLiteral && (node.right as ast.StringLiteral).val == '' {
+			arrow := if left_type.is_ptr() { '->' } else { '.' }
+			g.write('(')
+			g.expr(node.left)
+			g.write(')')
+			g.write('${arrow}len $node.op 0')
+		} else {
+			fn_name := match node.op {
+				.plus {
+					'string_add('
+				}
+				.eq {
+					'string_eq('
+				}
+				.ne {
+					'string_ne('
+				}
+				.lt {
+					'string_lt('
+				}
+				.le {
+					'string_le('
+				}
+				.gt {
+					'string_gt('
+				}
+				.ge {
+					'string_ge('
+				}
+				else {
+					verror('op error for type `$left_sym.name`')
+					'/*node error*/'
+				}
 			}
+			g.write(fn_name)
+			g.expr(node.left)
+			g.write(', ')
+			g.expr(node.right)
+			g.write(')')
 		}
-		g.write(fn_name)
-		g.expr(node.left)
-		g.write(', ')
-		g.expr(node.right)
-		g.write(')')
 	} else if node.op in [.eq, .ne] && left_sym.kind == .array && right_sym.kind == .array {
 		ptr_typ := g.gen_array_equality_fn(left_type)
 		if node.op == .eq {
@@ -2723,11 +2733,15 @@ fn (mut g Gen) match_expr(node ast.MatchExpr) {
 					}
 					g.expr(expr)
 				} else if type_sym.kind == .string {
-					g.write('string_eq(')
-					g.write(cond_var)
-					g.write(', ')
-					g.expr(expr)
-					g.write(')')
+					if (expr as ast.StringLiteral).val == '' {
+						g.write('${cond_var}.len == 0')
+					} else {
+						g.write('string_eq(')
+						g.write(cond_var)
+						g.write(', ')
+						g.expr(expr)
+						g.write(')')
+					}
 				} else if expr is ast.RangeExpr {
 					// if type is unsigned and low is 0, check is unneeded
 					mut skip_low := false