Jumping with Assembly

#AssemblyOptimization #CodingPerformance #SwitchCaseMagic #EfficientProgramming #AssemblySavesTheDay

Introduction

The if-then-else and switch-case statements are fundamental building blocks in modern programming languages like Python and C++. They provide a way to control the flow of a program based on certain conditions. At first glance, these control structures might seem to be similar in terms of their implementation. However, when we take a closer look at their Assembly counterparts, the differences become quite striking. In this article, we will explore how using indirect jumps in Assembly can create efficient and performant switch-case statements, and how this technique can save your day.

Comparing If-Then-Else and Switch-Case in High-Level Languages

In high-level languages like Python and C++, the if-then-else and switch-case constructs serve a similar purpose: they provide a way to execute different code paths based on certain conditions. Here are examples of if-then-else and switch-case statements in both languages:

Python:

# if/elseif/else
if i == 0:
??? print("i = 0")
elif i == 1:
??? print("i = 1")
else:
??? print("i = 2")

# Switch-Case (using a dictionary)
switch_case = {
??? 0: lambda: print("i = 0"),
??? 1: lambda: print("i = 1"),
??? 2: lambda: print("i = 2"),
}
switch_case[i]()        

C++ :

// If-Then-Else
if (i == 0) {
??? cout << "i = 0";
} else if (i == 1) {
??? cout << "i = 1";
} else {
??? cout << "i = 2";
}

// Switch-Case
switch (i) {
??? case 0:
??????? cout << "i = 0";
??????? break;
??? case 1:
??????? cout << "i = 1";
??????? break;
??? case 2:
??????? cout << "i = 2";
??????? break;
}        

Although they appear to perform the same basic functionality, a closer inspection of the Assembly code reveals a completely different picture: the linear search nature of the if/elseif/else structure doesn't implement the indirect jump implementation that the switch-case code does in Assembly. Below we showcase for both implementions.

If-Then-Else in Assembly x86

; if/then/else/endif form:
    mov mango, i
    test mango, mango
    jnz IsNotBanana
    ; Check for 0
    ; Code to print "i = 0"
    jmp EndOfFruitBasket
  IsNotBanana:
    cmp mango, 1
    jne IsNotApple
    ; Code to print "i = 1"
    jmp EndOfFruitBasket
  IsNotApple:
    cmp mango, 2
    jne EndOfFruitBasket;
    ; Code to print "i = 2"
  EndOfFruitBasket:
  
            

Switch-Case in Assembly x86

; Indirect Jump Version.
    mov pineapple, i
    lea rcx, JmpToTheMoon
    jmp qword ptr [rcx][pineapple * 8] ; jump directly to the case
    JmpToTheMoon ; we dont need to go over array decl
    qword Rocket0, Rocket1, Rocket2
  Rocket0:
    ;Code to print "i = 0"
    jmp BackToEarth;
  Rocket1:
    ;Code to print "i = 1"
    jmp BackToEarth;
  Rocket2:
    ;Code to print "i = 2"
  BackToEarth:        

CONCLUSION

The implementation of if-then-else and switch-case statements in Assembly x86 reveals a key difference between these control structures. While if-then-else statements use a linear search, switch-case statements can utilize indirect jumps for a more efficient and performant execution.

By understanding these differences and leveraging the power of Assembly x86, you can optimize your code and improve its performance. This knowledge not only enables you to write more efficient high-level code but also provides valuable insights into the underlying mechanics of modern programming languages.

Remember, jumping with Assembly can indeed save your day, making your programs run faster and smoother. Always consider the specific use cases and requirements of your projects, and choose the most suitable control structures to achieve the desired results.