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@skeeto /jit.c
Last active Aug 20, 2017

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Basic JIT
/* http://redd.it/2z68di */
#define _BSD_SOURCE // MAP_ANONYMOUS
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <sys/mman.h>
#define PAGE_SIZE 4096
struct asmbuf {
uint8_t code[PAGE_SIZE - sizeof(uint64_t)];
uint64_t count;
};
struct asmbuf *
asmbuf_create(void)
{
int prot = PROT_READ | PROT_WRITE;
int flags = MAP_ANONYMOUS | MAP_PRIVATE;
return mmap(NULL, PAGE_SIZE, prot, flags, -1, 0);
}
void
asmbuf_free(struct asmbuf *buf)
{
munmap(buf, PAGE_SIZE);
}
void
asmbuf_finalize(struct asmbuf *buf)
{
mprotect(buf, PAGE_SIZE, PROT_READ | PROT_EXEC);
}
void
asmbuf_ins(struct asmbuf *buf, int size, uint64_t ins)
{
for (int i = size - 1; i >= 0; i--)
buf->code[buf->count++] = (ins >> (i * 8)) & 0xff;
}
void
asmbuf_immediate(struct asmbuf *buf, int size, const void *value)
{
memcpy(buf->code + buf->count, value, size);
buf->count += size;
}
int main(void)
{
/* Compile input program */
struct asmbuf *buf = asmbuf_create();
asmbuf_ins(buf, 3, 0x4889f8); // mov %rdi, %rax
int c;
while ((c = fgetc(stdin)) != '\n' && c != EOF) {
if (c == ' ')
continue;
char operator = c;
long operand;
scanf("%ld", &operand);
asmbuf_ins(buf, 2, 0x48bf); // mov operand, %rdi
asmbuf_immediate(buf, 8, &operand);
switch (operator) {
case '+':
asmbuf_ins(buf, 3, 0x4801f8); // add %rdi, %rax
break;
case '-':
asmbuf_ins(buf, 3, 0x4829f8); // sub %rdi, %rax
break;
case '*':
asmbuf_ins(buf, 4, 0x480fafc7); // imul %rdi, %rax
break;
case '/':
asmbuf_ins(buf, 3, 0x4831d2); // xor %rdx, %rdx
asmbuf_ins(buf, 3, 0x48f7ff); // idiv %rdi
break;
}
}
asmbuf_ins(buf, 1, 0xc3); // ret
asmbuf_finalize(buf);
long init;
unsigned long term;
scanf("%ld %lu", &init, &term);
long (*recurrence)(long) = (void *)buf->code;
for (unsigned long i = 0, x = init; i <= term; i++, x = recurrence(x))
fprintf(stderr, "Term %lu: %ld\n", i, x);
asmbuf_free(buf);
return 0;
}
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