9ccs

9ccs is 9cc-syakyou edition. Syakyou means copy with hands in Japanese.

See https://www.sigbus.info/compilerbook

How to run

make test

Pit fall points

Debugging with gdb

gcc -g3 9ccs.c
gdb a.out

ref: 478b796bc2854ce3d1e6737d2bdd7e18943254c3 remove tokenize function call and compile, then run result SIGSEGV with Address boundary error! https://qiita.com/arene-calix/items/a08363db88f21c81d351

Increment and assign

ref: e45cec0fc2283111391bc78810b114d26e9912d6

This cause bug token->str has no multiple operator. Ex, '5==6' -> token->str is '6'

            cur = new_token(TK_RESERVED, cur, p+=2, 2);
            cur = new_token(TK_RESERVED, cur, p++, 1);

x=0;
y = x++;
->
x = 1, y = 0

x=0;
y= x+=1
->
x=1, y=1

gcc withc -Wl option

Incorrect

gcc --static -Wl, -N -o test test.o
/usr/bin/ld: cannot find : No such file or directory
collect2: error: ld returned 1 exit status

correct

gcc --static -Wl,-N -o test test.o

Split source files cause undefined reference

Incorrect and emit undefined reference to "user_input"

int main(int argc, char **argv) {
    if (argc != 2) {
        fprintf(stderr, "The number of arguments is incorrect.\n");
        return 1;
    }

    user_input = argv[1];
    token = tokenize(user_input);
    Node *node = expr();

    printf(".intel_syntax noprefix\n");
    printf(".globl main\n");
    printf("main: \n");

    gen(node);

    printf("    pop rax\n");
    printf("    ret\n");
    return 0;
}

Correct

char *user_input;
Token *token;

int main(int argc, char **argv) {
    ...
}

Implement return stmt

Encounter this crash.

./test.sh
return 2;
 ^ Not a number

This is caused by consume function take into consideration only for TK_RESERVED. Adding for TK_RETURN fix this issue.

Implement if and else stmt

Following is work fine and return 2

.intel_syntax noprefix
.globl main
main: 
    push 2
    push 3
    pop rax
    pop rax
    ret

SIGSEGV with address boundary error

.intel_syntax noprefix
.globl main
main: 
    push 2
    push 3
    pop rax
    ret

We need to clear arranged registers (or stacks?)

Implement for

We need to handle a = 1; for (;;) return a;. If cond is null, generate push 0\n for cmp always is true.

Impl function call

Missing #define _GNU_SOURCE and occured SIGSEGV.

https://stackoverflow.com/questions/5582211/what-does-define-gnu-source-imply

Impl function definition

stack and queue

Input main() { a = 52; a; }

I implemented gen(vec_pop(node->stmts)); for ND_FUNC at codegen.c

But this input AST is [[{kind: ND_ASSIGN}], [{kind: ND_LVAR}]]

pop op break the order of node. For my case, compiler generate ND_LVAR first and then generate for ND_ASSIGN.

I fixed it with gen(node->stmts->data[i]).

Difference with 9ccs and real C semantics

All statements automatically return each value in my own C with 9ccs until now. For example, main() { 2; } return 2.

This cause problem with foo() { 2; } main() { foo(); }. I need this return 2.

But this code emits following and the return is 0.

.intel_syntax noprefix
.globl buzz
buzz: 
    push rbp
    mov rbp, rsp
    sub rsp, 0 # prologue
    push 3
    pop rax
    mov rsp, rbp # epilogue
    pop rbp
    ret
.globl main
main: 
    push rbp
    mov rbp, rsp
    sub rsp, 0 # prologue
    call buzz # <- RAX is 3
    pop rax # <- CAUSE: 3 is popped and RAX is 0
    mov rsp, rbp # epilogue
    pop rbp
    ret # <- return is 0

The pop rax after call buzz is unnecessary. It seems bad to prevent pop rax when ND_CALL because the AST is same between foo() { 2; } and main() {foo();}.

In real C, int foo() { 2; } void main() { printf("foo: %i", foo())} emit foo: 0.

So, C needs return statement to return value. Without it, no value returned. For example, https://godbolt.org/z/nvbdEf8fM

From now on, my own C need to follow real C semantics.

ND_RETURN assumption

            if (node->lhs->kind != ND_CALL) {
                // All operations except function call left calculated value at stack top.
                // function call already left return value at RAX, so not pop stack to RAX.
                printf("    pop rax\n");
            }

arg is expression

Remember foo(2 - 1);. Arg is not primary, it's expr. If arg is primary, the parser go infinite loop and never end.

error_at broken

error_at function broken at e9434af401b6b9841eeb2110525c79ea898fee7c.

It cause infinite loop when error_at calling because token->str doesn't have all entire string after token but only token str.

Implement pointer + number or pointer - number

This is difficult and learn a lot from https://github.com/rui314/9cc/commit/19ab268effbc7d5439545589373e7cf7fd7305ac

• You need ty in almost every node
• If you want to allocate value in linear without array, you use array in test_func (this compiled by gcc).
• Because array is pointer, you need int **p to handle it as function arg

*** stack smashing detected *** means overflow. This is caused by int arr[2]; arr[3] = 1;

Implement variable definition

When I variable definition, I mistook that variable definition is treated same as variable use (ND_VAR).

I noticed this because the deref code is not work well and debug the assembly step by step.

Exmple.

int main() {
    int *p;
    alloc1(&p, 3, 6);
    return *(p + 1); // -> 6 😊
}

int main() {
    int *p;
    alloc1(&p, 3, 6);
    int q;
    q = *(p + 1);
    return q; // -> 0 🤔
}

Stack and heap

Without static keyword in C lang, I fell into the pit holl about the difference between stack and heap.

See my blog post in Japanese

Implement global variables

Use Assembler Directives for global variable

.zero is assembler directive.

See https://docs.oracle.com/cd/E26502_01/html/E28388/eoiyg.html

What is PIE?

https://qiita.com/0yoyoyo/items/85122f31ba8d14332e3d https://tokyodebian-team.pages.debian.net/pdf2017/debianmeetingresume201712.pdf

What is .data, .bss and .text section?

https://karino2.github.io/c-lesson/casm_link_load.html http://www.ertl.jp/~takayuki/readings/info/no02.html

What is -fno-common option?

http://solid.kmckk.com/doc/skit/current/solid_toolchain/overview.html https://qiita.com/yasuhirokimura/items/d5337d73a016502b9d54

What R prefix stands for?

https://stackoverflow.com/questions/43933379/what-do-the-e-and-r-prefixes-stand-for-in-the-names-of-intel-32-bit-and-64-bit-r

assert is not func, but macro

https://godbolt.org/z/cWj48TzKs

Errata of compiler book

Compiler error

foo.c:10: x = y + + 5;
                  ^ 式ではありません

The above is valid expression in C. See https://godbolt.org/z/W73W3arnY

unary EBNF

expr    = mul ("+" mul | "-" mul)*
mul     = unary ("*" unary | "/" unary)*
unary   = ("+" | "-")? primary
primary = num | "(" expr ")"

But, the reference implementation says following.

// unary = ("+" | "-")? unary
//       | primary

Mac

docker build -t compilerbook https://www.sigbus.info/compilerbook/Dockerfile

cd to cloned dir then run following.

docker run --rm -v $PWD:/9ccs -w /9ccs compilerbook make test