// libc-free x86-64 Linux multi-threading example
//   $ cc -nostdlib stack_head.c
// Ref: https://nullprogram.com/blog/2023/03/23/
// This is free and unencumbered software released into the public domain.

#include "clone_args.h"

#define SYS_write      1
#define SYS_mmap       9
#define SYS_nanosleep  35
#define SYS_clone      56
#define SYS_exit       60
#define SYS_futex      202
#define SYS_exit_group 231

#define FUTEX_WAIT 0
#define FUTEX_WAKE 1

#define SYSCALL1(n, a) \
    syscall6(n,(long)(a),0,0,0,0,0)
#define SYSCALL2(n, a, b) \
    syscall6(n,(long)(a),(long)(b),0,0,0,0)
#define SYSCALL3(n, a, b, c) \
    syscall6(n,(long)(a),(long)(b),(long)(c),0,0,0)
#define SYSCALL4(n, a, b, c, d) \
    syscall6(n,(long)(a),(long)(b),(long)(c),(long)(d),0,0)
#define SYSCALL5(n, a, b, c, d, e) \
    syscall6(n,(long)(a),(long)(b),(long)(c),(long)(d),(long)(e),0)
#define SYSCALL6(n, a, b, c, d, e, f) \
    syscall6(n,(long)(a),(long)(b),(long)(c),(long)(d),(long)(e),(long)(f))

static long syscall6(long n, long a, long b, long c, long d, long e, long f) {
  register long ret;
  register long r10 asm("r10") = d;
  register long r8  asm("r8")  = e;
  register long r9  asm("r9")  = f;
  __asm volatile (
      "syscall"
      : "=a"(ret)
      : "a"(n), "D"(a), "S"(b), "d"(c), "r"(r10), "r"(r8), "r"(r9)
      : "rcx", "r11", "memory"
  );
  return ret;
}

static void millisleep(int ms) {
  long ts[] = {ms/1000, ms%1000 * 1000000L};
  SYSCALL2(SYS_nanosleep, ts, ts);
}

/* int num = 65; */ // Ignore this, used for my own personal test

static long fullwrite(int fd, void *buf, long len) {
  for (long off = 0; off < len;) {
      long r = SYSCALL3(SYS_write, fd, buf+off, len-off);
      if (r < 0) { return r; }
      off += r;
  }

  return len;
}

__attribute((noreturn)) static void exit(int status) {
  SYSCALL1(SYS_exit, status);
  __builtin_unreachable();
}

__attribute((noreturn)) static void exit_group(int status) {
  SYSCALL1(SYS_exit_group, status);
  __builtin_unreachable();
}

static void futex_wait(int *futex, int expect) {
  SYSCALL4(SYS_futex, futex, FUTEX_WAIT, expect, 0);
}

static void futex_wake(int *futex) {
  SYSCALL3(SYS_futex, futex, FUTEX_WAKE, 0x7fffffff);
}

/* The structure must have a 16-byte alignment on all architectures */
struct __attribute((aligned(16))) stack_head {
  void (*entry)(struct stack_head*); // The entry point pointer. Will receive a pointer to its own stack_head

  /* The rest of the arguments can be filled with any thread-local
     data we want. The following is just an example! */
  char *message;
  long message_length;
  int print_count;
  int join_futex; // Join "futex" to help us "wait" to "join" that thread later
};

typedef struct stack_head stack_head;

typedef struct {
    void       *tail;
    stack_head *head;
} thread_stack;

__attribute((naked)) static long newthread(CloneArgs* args) {
  __asm volatile (
    /* "args" is already in "rdi" */
    "mov  $88,  %%esi\n"  // arg2 = size (always "88" until further notice)
    "mov  $435, %%eax\n"  // SYS_clone
    "syscall\n"
    "mov  %%rsp, %%rdi\n"  // entry point argument
    "ret\n"
    : : : "rax", "rcx", "rsi", "rdi", "r11", "memory"
  );
}

static void threadentry(stack_head *stack) {
  char *message = stack->message;
  int length = stack->message_length;
  int count = stack->print_count;
  for (int i = 0; i < count; i++) {
    fullwrite(1, message, length);
    millisleep(25);
  }

  __atomic_store_n(&stack->join_futex, 1, __ATOMIC_SEQ_CST);
  futex_wake(&stack->join_futex);
  exit(0);
}

static thread_stack newstack(long size) {
  unsigned long p = SYSCALL6(SYS_mmap, 0, size, 3, 0x22, -1, 0);

  if (p > -4096UL) { /* Return "null" on failure */
    thread_stack stack = { (void*)0, (void*)0 };
    return stack;
  }

  long count = size / sizeof(thread_stack);
  thread_stack stack = { (void*)(p + count - 1), (void*)count };
  return stack;
}

__attribute((force_align_arg_pointer)) void _start(void) {
  thread_stack stack = newstack(1<<16);
  stack.head->entry = threadentry;

  // Thread data
  stack.head->message = "hello world\n";
  stack.head->message_length = 12;
  stack.head->print_count = 20;

  stack.head->join_futex = 0; // Set the "futex"

  CloneArgs args = {
    CLONE_VM,
    0, 0, 0, SIGCHLD /* I have also tried using "0" here */,
    (__aligned_u64)stack.tail, /* stack */
    (void *)stack.head - stack.tail, /* stack_size */
    0, 0, 0, 0
  };

  newthread(&args);

  futex_wait(&stack.head->join_futex, 0);
  exit_group(0);
}