Add flag to disable jumptable optimization

This is useful for coverage. While it is currently safe to just
skip over the SWITCH_* opcodes, this may not be true in the future
due to opcache optimizations, so it's safer to disable emission of
SWITCH_* opcodes entirely.

Author: nikic

Zend/zend_compile.c

@@ -4907,6 +4907,10 @@ static zend_uchar determine_switch_jumptable_type(zend_ast_list *cases) {
 }
 
 static zend_bool should_use_jumptable(zend_ast_list *cases, zend_uchar jumptable_type) {
+	if (CG(compiler_options) & ZEND_COMPILE_NO_JUMPTABLES) {
+		return 0;
+	}
+
 	/* Thresholds are chosen based on when the average switch time for equidistributed
 	 * input becomes smaller when using the jumptable optimization. */
 	if (jumptable_type == IS_LONG) {

Zend/zend_compile.h

@@ -1040,6 +1040,9 @@ END_EXTERN_C()
 /* disable builtin special case function calls */
 #define ZEND_COMPILE_NO_BUILTINS				(1<<10)
 
+/* disable jumptable optimization for switch statements */
+#define ZEND_COMPILE_NO_JUMPTABLES				(1<<11)
+
 /* The default value for CG(compiler_options) */
 #define ZEND_COMPILE_DEFAULT					ZEND_COMPILE_HANDLE_OP_ARRAY