I wrote these two solutions for Project Euler Q14, in assembly and in C++. They implement identical brute force approach for testing the Collatz conjecture. The assembly solution was assembled with:

nasm -felf64 p14.asm && gcc p14.o -o p14

The C++ was compiled with:

g++ p14.cpp -o p14

Assembly, p14.asm:

section .data
    fmt db "%d", 10, 0

global main
extern printf

section .text

main:
    mov rcx, 1000000
    xor rdi, rdi        ; max i
    xor rsi, rsi        ; i

l1:
    dec rcx
    xor r10, r10        ; count
    mov rax, rcx

l2:
    test rax, 1
    jpe even

    mov rbx, 3
    mul rbx
    inc rax
    jmp c1

even:
    mov rbx, 2
    xor rdx, rdx
    div rbx

c1:
    inc r10
    cmp rax, 1
    jne l2

    cmp rdi, r10
    cmovl rdi, r10
    cmovl rsi, rcx

    cmp rcx, 2
    jne l1

    mov rdi, fmt
    xor rax, rax
    call printf
    ret

C++, p14.cpp:

#include <iostream>

int sequence(long n) {
    int count = 1;
    while (n != 1) {
        if (n % 2 == 0)
            n /= 2;
        else
            n = 3*n + 1;
        ++count;
    }
    return count;
}

int main() {
    int max = 0, maxi;
    for (int i = 999999; i > 0; --i) {
        int s = sequence(i);
        if (s > max) {
            max = s;
            maxi = i;
        }
    }
    std::cout << maxi << std::endl;
}

I know about the compiler optimizations to improve speed and everything, but I don’t see many ways to further optimize my assembly solution (speaking programmatically, not mathematically).

The C++ code uses modulus every term and division every other term, while the assembly code only uses a single division every other term.

But the assembly is taking on average 1 second longer than the C++ solution. Why is this? I am asking mainly out of curiosity.

Execution times

My system: 64-bit Linux on 1.4 GHz Intel Celeron 2955U (Haswell microarchitecture).

• g++ (unoptimized): avg 1272 ms.
• g++ -O3: avg 578 ms.
• asm (div) (original): avg 2650 ms.
• asm (shr): avg 679 ms.
• @johnfound asm (assembled with NASM): avg 501 ms.
• @hidefromkgb asm: avg 200 ms.
• @hidefromkgb asm, optimized by @Peter Cordes: avg 145 ms.
• @Veedrac C++: avg 81 ms with -O3, 305 ms with -O0.