jitproto.c

// jitproto.c
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>

typedef long(*fn)(long);

fn compile_identity(void) {
  // Allocate some memory and set its permissions correctly. In particular, we
  // need PROT_EXEC (which isn't normally enabled for data memory, e.g. from
  // malloc()), which tells the processor it's ok to execute it as machine
  // code.
  char *memory = mmap(NULL,             // address
                      4096,             // size
                      PROT_READ | PROT_WRITE | PROT_EXEC,
                      MAP_PRIVATE | MAP_ANONYMOUS,
                      -1,               // fd (not used here)
                      0);               // offset (not used here)
  if (!memory) {
    perror("failed to allocate memory");
    exit(1);
  }

  int i = 0;

  // mov %rdi, %rax
  memory[i++] = 0x48;           // REX.W prefix
  memory[i++] = 0x8b;           // MOV opcode, register/register
  memory[i++] = 0xc7;           // MOD/RM byte for %rdi -> %rax

  // ret
  memory[i++] = 0xc3;           // RET opcode

  return (fn) memory;
}

int main() {
  fn f = compile_identity();
  int i;
  for (i = 0; i < 10; ++i)
    printf("f(%d) = %ld\n", i, (*f)(i));
  munmap(f, 4096);
  return 0;
}