Your Code is Dying... Are You Ready to Lose Your Performance?

In the world of C++, performance is everything. Every second your program wastes is a second of your user’s patience lost. But what if the reason for your program’s sluggishness isn’t the logic but something as simple as a keyword you’re not using? Enter constexpr, a powerful tool that allows you to do computations at compile time—not runtime. But here’s the catch: not every type or function can be constexpr. In this article, we’ll explore how constexpr can save you from performance nightmares, and why ignoring it might mean the slow death of your code.

What is constexpr?

constexpr is a keyword in C++ that allows functions and variables to be evaluated at compile time instead of runtime. This can drastically reduce runtime overhead, especially when dealing with repeated computations, constants, or even recursive functions like Fibonacci calculations. However, not all types and functions can be constexpr.

To use constexpr, your function must:

• Be deterministic, meaning it must return the same result for the same input.
• Only use other constexpr functions and literal types (e.g., integers, floats, pointers, and other constexpr-friendly types).

That said, it’s a tool with immense potential. Let’s see how it works in action.

Development Time, Compile Time, and Run Time: The Three Phases of Your Code

Before diving into the examples, let’s define the key phases of code execution:

• Development Time: This is when you’re writing and testing your code. You’re figuring out the logic, creating functions, and organizing your program.
• Compile Time: After you’ve written your code, it’s compiled into an executable. constexpr allows computations to be done during this phase, before the program even starts running.
• Run Time: This is when your program is actually running. It’s the phase where non-constexpr computations happen—everything from data processing to recursive calls and I/O operations.

By understanding these phases, it becomes clear how constexpr can drastically improve your program’s efficiency.

Without constexpr (Runtime Calculation)

Let’s start by calculating Fibonacci numbers at runtime using a normal recursive function. This is how you might write it traditionally:

#include <iostream>

int fibonacci(int n) {
    if (n <= 1) return n;
    return fibonacci(n - 1) + fibonacci(n - 2);
}

int main() {
    int result = fibonacci(30);  // Computed at runtime
    std::cout << result << std::endl;
    return 0;
}

In this code, the Fibonacci number for n = 30 is computed during execution using a recursive function. This means that each time the program runs, it will have to go through the recursion and re-calculate the value.

Compiled Assembly for the Runtime Version

Let’s examine the assembly code generated by this version. When compiled for an x86 system, you’ll see recursion happening at runtime. The function calls itself repeatedly, causing the program to repeatedly calculate the same Fibonacci numbers.

fibonacci:
    ; Entry into fibonacci function
    cmp     rdi, 1           ; Compare n with 1
    jle     .L1              ; If n <= 1, jump to the base case
    sub     rdi, 1           ; Decrement n by 1 (first recursive call)
    call    fibonacci        ; Recursive call: fibonacci(n - 1)
    mov     rbx, rdi         ; Save the result in rbx
    sub     rdi, 2           ; Decrement n by 2 (second recursive call)
    call    fibonacci        ; Recursive call: fibonacci(n - 2)
    add     rdi, rbx         ; Add the two results together
.L1:
    ret                      ; Return the result

• Recursive Calls: This code shows multiple recursive calls to fibonacci(), and each call adds more overhead during execution.
• Run-Time Overhead: The program repeatedly calls fibonacci and recalculates values, which is inefficient for larger input values.

With constexpr (Compile-Time Calculation)

Now, let’s rewrite the Fibonacci function to calculate the value at compile time using constexpr. This version will have the Fibonacci number precomputed by the compiler:

#include <iostream>

constexpr int fibonacci(int n) {
    if (n <= 1) return n;
    return fibonacci(n - 1) + fibonacci(n - 2);
}

int main() {
    constexpr int result = fibonacci(30);  // Computed at compile time
    std::cout << result << std::endl;
    return 0;
}

In this version, fibonacci(30) will be calculated at compile time, and the result will be stored directly in the executable. This eliminates the need for recursion at runtime.

Compiled Assembly for the constexpr Version

Let’s look at the assembly code for the constexpr version. Here’s what you get when the Fibonacci value is precomputed during compilation:

mov eax, 832040  ; The result of fibonacci(30) is precomputed at compile time

• No Recursion: The constexpr version does not involve any function calls or recursion. Instead, the result (832040 for fibonacci(30)) is directly inserted into the program during compile time.
• Optimized Execution: The program just loads the precomputed Fibonacci number into a register (eax in this case) and outputs it, making it faster and more efficient at runtime.

Why This Matters: Performance Benefits

The main takeaway here is the performance difference between runtime and compile-time computation. Here’s why using constexpr is important:

Reduced Run-Time Overhead

The Fibonacci calculation done at runtime involves recursion, which leads to redundant work and increased run-time overhead. On the other hand, with constexpr, the computation is done at compile time, and the result is directly inserted into the program’s final executable.

Smaller Executables and Faster Execution

Since constexpr computes values at compile time, the resulting program is typically smaller and faster. It doesn’t need to spend time making recursive calls or performing repetitive calculations. The compiler does the heavy lifting for you.

Compile-Time Safety

constexpr allows you to ensure that certain values are computed before runtime, leading to compile-time errors if something goes wrong. This makes it easier to catch mistakes early.

Limitations of constexpr

It’s important to note that not every type or function can be constexpr. Here’s what you need to consider:

• Only constexpr-friendly types can be used: Your function can only use literal types (e.g., integers, floats, pointers), other constexpr functions, and types that are known at compile time.
• No dynamic memory allocation: constexpr functions cannot dynamically allocate memory (like using new or malloc).
• No runtime I/O: Functions that depend on runtime data (like file reading or user input) cannot be constexpr.

This is why not all functions can be marked as constexpr, but where possible, it’s a game-changer for performance.

Conclusion: The Importance of constexpr

If you’re not using constexpr when you can, you’re letting your program suffer. It’s like choosing to repeatedly re-calculate the same values at runtime when you could be precomputing them at compile time. Without constexpr, your program wastes time, memory, and CPU resources during execution. But with constexpr, you’re ensuring that your program runs faster, more efficiently, and with less overhead.

Here’s the bottom line:

• Without constexpr, your program calculates values at runtime, which leads to slower execution and higher resource usage.
• With constexpr, your program computes values at compile time, resulting in faster and more efficient code that runs with minimal runtime overhead.

Don’t let your code die a slow, painful death—embrace constexpr and make your code run at lightning speed from the moment it’s compiled. The choice is yours, but beware: ignoring constexpr might just be the reason your code gets left behind.