Hello, JIT World from D

Posted on 2013-01-15
Tagged with just-in-time, d-programming-language, beginner

This post is inspired by Hello, JIT World: The Joy of Simple JITs, especially the first short program that he used to demonstrate that JIT programming can be simple. In this post, I will convert the program he wrote in C to [D][wiki] to show some capabilities and convenience of D.

What follows is the whole source of the program in D. You can view the original source in C in the blog post linked above.

import std.c.string;
import std.c.linux.linux;
import std.conv;
import std.stdio;

int main(string[] args)
{
    // Machine code for:
    //   mov eax, 0
    //   ret
    ubyte code[] = [0xb8, 0x00, 0x00, 0x00, 0x00, 0xc3];

    if (args.length < 2)
    {
        stderr.writeln("Usage: jit1 <integer>");
        return 1;
    }

    // Overwrite immediate value "0" in the instruction
    // with the user's value.  This will make our code:
    //   mov eax, <user's value>
    //   ret
    int num = to!int(args[1]);
    memcpy(&code[1], &num, 4);

    // Allocate writable/executable memory.
    // Note: real programs should not map memory both writable
    // and executable because it is a security risk.
    void* mem = mmap(null, code.sizeof, PROT_WRITE | PROT_EXEC,
            MAP_ANON | MAP_PRIVATE, -1, 0);
    memcpy(mem, code, code.sizeof);

    // The function will return the user's value.
    int function() func = cast(int function())mem;
    return func();
}

Let’s go through some first impressions.

D programming language is in the same family of languages that have the C flavor, so you might already find the code familiar and quite easy to read. Some familiarities are

// a line comment which extends only to the end of the line
{
    // block of code
}
statement; // assuming you have statement defined
int a;
long b;
int main();
int main(string[]);
void main();
void main(string[]);
int[2] a;
a[1] = 1; // use this to index into the second element of a
int a;
int* b = &a;
void* a;
char* b = cast(char*)a;
return expression; // where expression is any expression, e.g. 1 + 1

So D is quite similar to C/C++. But the differences that set D apart is even more interesting.

int[200] a;
writeln(a.length); // assuming you have `import std.stdio;` before this.
// Prints out 200.
// fp is a pointer to a function that takes a function taking a char
// returning an int and returns a typeless pointer
// (try saying that five times fast)
void* function(int function(char)) fp;
// the equivalent for that in C is
// void* (*fp)(int (*)(char));

// let's look at something more interesting
// signal is a function that takes an int and a pointer to a function
// taking an int returning nothing and returns a pointer to a function
// taking an int returning nothing
void function(int) signal(int sig, void function(int) func));
// the equivalent for that in C is
// void (*signal (int sig, void (*func)(int)) )(int);
// try reading that five times fast :)

Those are the differences that are present in this code snippet, you can check out more about D at the D home page or a summary at the wikipedia page.

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