Part 47: A Subset of sizeof

In a real C compiler, the sizeof() operator gives the size in bytes of:

• a type definition, and
• the type of an expression

I looked at the code in our compiler and I’m only using sizeof() for the first of the two options above, so I’m only going to implement the first one. This makes things a bit easier as we can assume that the tokens inside the sizeof() are a type definition.

New Token and Keyword

We need a “sizeof” keyword and a new token, T_SIZEOF. As per usual, I’ll let you look at the changes to scan.c.

Now, when adding new tokens, we also have to update the:

// List of token strings, for debugging purposes
char *Tstring[] = {
  "EOF", "=", "+=", "-=", "*=", "/=",
  "||", "&&", "|", "^", "&",
  "==", "!=", ",", ">", "<=", ">=", "<<", ">>",
  "+", "-", "*", "/", "++", "--", "~", "!",
  "void", "char", "int", "long",
  "if", "else", "while", "for", "return",
  "struct", "union", "enum", "typedef",
  "extern", "break", "continue", "switch",
  "case", "default", "sizeof",
  "intlit", "strlit", ";", "identifier",
  "{", "}", "(", ")", "[", "]", ",", ".",
  "->", ":"
};

I initially forgot to do this, and when debugging I was seeing the “wrong” token description for the tokens after “default”. Oops!

Changes to the Parser

The sizeof() operator is part of expression parsing, as it takes an expression and returns a new value. We can do things like:

  int x= 43 + sizeof(char);

Thus, we are going to modify expr.c to add sizeof(). It isn’t a binary operator, and it’s not a prefix or postfix operator, so the best place to add sizeof() is as part of parsing primary expressions.

In fact, once I found my silly bugs, the amount of new code to do sizeof() was small. Here it is:

// Parse a primary factor and return an
// AST node representing it.
static struct ASTnode *primary(void) {
  struct ASTnode *n;
  int id;
  int type=0;
  int size, class;
  struct symtable *ctype;

  switch (Token.token) {
  case T_SIZEOF:
    // Skip the T_SIZEOF and ensure we have a left parenthesis
    scan(&Token);
    if (Token.token != T_LPAREN)
      fatal("Left parenthesis expected after sizeof");
    scan(&Token);

    // Get the type inside the parentheses
    type= parse_stars(parse_type(&ctype, &class));
    // Get the type's size
    size= typesize(type, ctype);
    rparen();
    // Return a leaf node int literal with the size
    return (mkastleaf(A_INTLIT, P_INT, NULL, size));
    ...
  }
  ...
}

We already have a parse_type() function to parse a type definition, and we already have a parse_stars() function to parse any following asterisks. Finally, we already have a typesize() function which returns the number of bytes in a type. All we have to do is scan the tokens in, call these three functions, build a leaf AST node with an integer literal in it, and return it.

Yes, I know there are a bunch of subtleties that go with sizeof(), but I’m following the “KISS principle” and doing enough to make our compiler self-compiling.

Testing the New Code

The file tests/input115.c has a set of tests for the primitive types, a pointer and for the structures in our compiler:

struct foo { int x; char y; long z; }; 
typedef struct foo blah;

int main() {
  printf("%ld\n", sizeof(char));
  printf("%ld\n", sizeof(int));
  printf("%ld\n", sizeof(long));
  printf("%ld\n", sizeof(char *));
  printf("%ld\n", sizeof(blah));
  printf("%ld\n", sizeof(struct symtable));
  printf("%ld\n", sizeof(struct ASTnode));
  return(0);
}

At present, the output from our compiler is:

1
4
8
8
13
64
48

I’m wondering if we need to pad the struct foo struct to be 16 bytes instead of 13. We’ll cross that bridge when we get to it.

Conclusion and What’s Next

Well, sizeof() turned out to be simple, at least for the functionality that we need for our compiler. In reality, sizeof() is quite complicated for a full-blown production C compiler.

In the next part of our compiler writing journey, I will tackle static.