chibicc - Simple c compiler if statement

What is changed ?

For parser, new node type called ND_IF is introduced. Three new nodes are introduced for Node type in parser. They are called cond, then, els which corresponds to code in if(cond){ } else {}. New production rule is introduced to deal with if statement

// stmt = "return" expr ";" 
//        | "{" compound_stmt
//        | expr_stmt
//        | "if" "(" expr ")" stmt ("else" stmt)?
static Node* stmt(Token**rest, Token* tok) {
  if(equal(tok, "return")) {
    Node* node = new_unary(ND_RETURN, expr(&tok, tok->next));
    *rest = skip(tok, ";");
    return node;
  }

  if(equal(tok, "{")) {
    return compound_stmt(rest, tok->next);
  }

  if(equal(tok, "if")) {
    tok = skip(tok->next, "(");
    Node* node = new_node(ND_IF);
    node->cond = expr(&tok, tok);
    tok = skip(tok, ")");
    node->then = stmt(&tok, tok);
    if(equal(tok, "else")) {
      node->els = stmt(&tok, tok->next);
    }
    *rest = tok;
    return node;
  }

  return expr_stmt(rest, tok);
}

For code generator, assembly code generation for if condition is introduced in gen_stmt. First we generate assembly code for cond node, and then we generate je $0, %rax to check condtion of cond node, and call jump .L.else.%d to do them instruction jump, %d is used to uniquly identify each else block. Because multiple if statement can be nested at the same time.

What is changed ?

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