1. C++ parse analysis Save and restore parser state
Kei Hasegawa

2. Example 1 void f() { } int (n); // Declaration statement ...
int (n+n); // Expression statement }

3. Derivation `int’ at Ex 1 (n+n); int (n); int simple-type-specifier
decl-specifier
If next token is ‘(‘ after simple-type-specifier,
it’s not obvious remained program text is
init-declarator-list or ‘(‘ expression-list ‘)’
decl-specifier-seq

4. Give priority to declaration rule
Report file of yacc/bison
State X1
R11 type_specifier: simple_type_specifier .
R12 postfix_expression: simple_type_specifier . '(' expression_list ')'
R | simple_type_specifier . '(' ')'
'(' shift, and go to state Y1
'(' [reduce using rule R11 (type_specifier)]
$default reduce using rule R11 (type_specifier)
This says that for ‘(‘ after simple-type-specifier, R11 is not used.

5. Give priority to declaration rule (continue)
Insert bellow code to `yyparse’ of appropriate location:
if ( yystate == X1 && yychar == ‘(‘ ) {
yyn = R11 + 1;
goto yyreduce; }
Of cause, this cause sytax error for function style cast.

6. Save parser state Again fix previous code:
if (yystate == X1 && yychar == ‘(‘ ) {
if (!retry[X1] ) { // At 1st time
save(yystate, ...); // Save parser state
yyn = R11 + 1; // parse as declaration
goto yyreduce; }

7. Restore parser state
Insert bellow code to `yyparse’ where calls `yyerror’:
if (parser states are saved) {
restore(&yystate, ...); // Restore parser state
++retry[yystate]; // Increment retry counter
goto yynewstate; }

8. Also save tokens which cause 1st syntax error
int get_token() {
int ret = ...
...
if (parser states are saved) {
// Save token for retrying
// Also save token’s property if it has. }
return ret;

9. Ex 2
struct T { ... };
int a;
...
T t1(); // Function declaration whose return type is `T’
T t2(a); // Type `T’ object definition initialized with `a’

10. Even though giving priority to functio declaration, ...
Report file of yacc/bison
State X2
R21 declarator: direct_declarator .
R22 direct_declarator: direct_declarator . '(' parameter_declaration_clause ')'
...
'(' shift, and go to state Y2
'(' [reduce using rule R21 (declarator)]
$default reduce using rule R21 (declarator)
This says that, for ‘(‘ after declarator, R21 is not used:
Object definition of Ex 2 cause syntax error.

11. Save parser state Insert code to `yyparse’ of appropriate location:
if (yystate == X2 && yychar == ‘(‘ ) {
if (!retry[X2] ) // At 1st time
save(yystate, ...); // Save parser state
else {
yyn = R21 + 1;
goto yyreduce; }

12. Ex 3
double f();
void g(int a) {
int(f())+a; }

13. int(f())+a; Reading ‘(‘ after `int’, yystate becomes X1.
Reading ‘(‘ after `f’, yystate becomes X2.
Reading ‘+‘, discard state saved at X2 , and restore state saved at X1 .
Note that it’s not correct to add `f’ into symbol table.

14. Discard the last saved state
Insert code to `yyparse’ of appropriate location:
if (yystate == X2) {
switch (yychar) {
case ‘(‘ : case ‘[‘ : ...
break; // Not cause syntax error as declaration
default:
if (The last state is saved at X2) {
Discard the last saved state;
goto yyerrlab; }