dynasm-driver.c

// Driver file for DynASM-based JITs.

#include <assert.h>
#include <stdio.h>
#include <sys/mman.h>

#include "dynasm/dasm_proto.h"

#if defined(__x86_64__) || defined(__i386)
#include "dynasm/dasm_x86.h"
#elif defined(__arm__)
#include "dynasm/dasm_arm.h"
#else
#error "The architecture is not supported"
#endif

void initjit(dasm_State **state, const void *actionlist);
void *jitcode(dasm_State **state);
void free_jitcode(void *code);

#include JIT

void initjit(dasm_State **state, const void *actionlist)
{
    dasm_init(state, 1);
    dasm_setup(state, actionlist);
}

void *jitcode(dasm_State **state)
{
    size_t size;
    int dasm_status = dasm_link(state, &size);
    assert(dasm_status == DASM_S_OK);

    // Allocate memory readable and writable so we can
    // write the encoded instructions there.
    char *mem = mmap(NULL, size + sizeof(size_t),
                     PROT_READ | PROT_WRITE,
                     MAP_ANON | MAP_PRIVATE, -1, 0);
    assert(mem != MAP_FAILED);

    // Store length at the beginning of the region, so we
    // can free it without additional context.
    *(size_t *) mem = size;
    void *ret = mem + sizeof(size_t);

    dasm_encode(state, ret);
    dasm_free(state);

    // Adjust the memory permissions so it is executable
    // but no longer writable.
    int success = mprotect(mem, size, PROT_EXEC | PROT_READ);
    assert(success == 0);

#ifndef NDEBUG
    // Write generated machine code to a temporary file.
    // View with: (x86-64)
    //  objdump -D -b binary -mi386 -Mx86-64 /tmp/jitcode
    // Or: (arm)
    //  arm-linux-gnueabihf-objdump -D -b binary -marm /tmp/jitcode
    FILE *f = fopen("/tmp/jitcode", "wb");
    fwrite(ret, size, 1, f);
    fclose(f);
#endif

    return ret;
}

void free_jitcode(void *code)
{
    void *mem = (char *) code - sizeof(size_t);
    int status = munmap(mem, *(size_t *) mem);
    assert(status == 0);
}