DmitrySoshnikov

Addressing modes in mov, and their C version

Below are addressing modes (immediate, register, memory) on the examples of the mov assembly instruction. Here we use movl ("move long") for a 32-bit word.

Note: cannot move memory-to-memory, need two instructions.

Src

movl

DmitrySoshnikov

/**
 * S-expression parser
 *
 * Recursive descent parser of a simplified sub-set of s-expressions.
 * 
 * NOTE: the format of the programs is used in the "Essentials of interpretation"
 * course: https://github.com/DmitrySoshnikov/Essentials-of-interpretation
 *
 * Grammar:
 *

DmitrySoshnikov

/**
 * Contiguous and non-contiguous C-arrays allocation in memory.
 * by Dmitry Soshnikov <[email protected]>
 */

#include <iostream>

int main()
{
    // -----------------------------

DmitrySoshnikov

x86: More code - less code

Sometimes more code in a source language, can mean less code in a generated language.

NOTE: below we used gcc 5.3 with -std=c++14 -O3 flags.

On the example of C programming language, and its translation to x64 assembly, let's take a look at the following example:

// 7 times `x`.

DmitrySoshnikov

(*
 * "Essentials of interpretation. Semantics of programming languages"
 *
 * Lesson 1: Arithmetic expressions evaluator.
 *
 * BNF grammar:
 *
 *   NUMBER -> \d+
 *
 *   Exp -> NUMBER

DmitrySoshnikov

/**
 * Stack buffer overflow.
 *
 * TL;DR: In older OSes, a buffer overflow of the data on the stack
 * allowed overriding a return address, and execute an exploit code,
 * instead of returning to a caller.
 *
 * Docs: https://en.wikipedia.org/wiki/Stack_buffer_overflow
 *
 * by Dmitry Soshnikov <[email protected]>

DmitrySoshnikov

"Everything is an expression"... since ES1?

Many languages support "expression-only" semantics, allowing to use any construct in an expression position.

NOTE: the difference between expressions and statements is that the former produce a value, while the later don't.

For example, Ruby's if-expression:

x = 10

DmitrySoshnikov

/**
 * Reverse indices of Lists/Arrays in ES6.
 *
 * by Dmitry Soshnikov <[email protected]>
 * MIT Style License
 */

'use strict';

class List extends Array {

DmitrySoshnikov

/**
 * JS scoping, and this property resolutions rules in ES7/ES8.
 *
 * Quiz: x is gone, and x is everywhere!
 *
 * Help find Xs! What's the output?
 */ 

let x = 1;

DmitrySoshnikov

class Foo {
  get x() {}
  set x(v) {}
}

class Bar extends Foo {
  x = 42;
}

console.log(new Bar().x); // undefined, should be 42