I just tested this on my machine (gcc 5.4.0). At -O2, gcc produced normal looking assembly code. At -O3, gcc produced a monstrosity [0] that I don't feel like f

I just tested this on my machine (gcc 5.4.0). At -O2, gcc produced normal looking assembly code. At -O3, gcc produced a monstrosity [0] that I don't feel like fully deciphering. However, from a brief glance, it does not appear to have created a closed form solution. Instead, it contains a single loop: .L4: addl $1, %edx paddd %xmm1, %xmm0 paddd %xmm2, %xmm1 cmpl %edx, %eax ja .L4 which seems to be using a SIMD instruction (paddd[1]) that adds does 4 32-bit integer additions in parallel. After this loop, it does some "housekeeping" (read, something I don't understand) before proceeding to an unwound version of the last iterations of the loop: leal 4(%rdx), %ecx addl %edx, %eax cmpl %ecx, %edi jl .L2 addl %ecx, %eax leal 8(%rdx), %ecx cmpl %ecx, %edi ... jl .L2 addl %ecx, %eax leal 28(%rdx), %ecx cmpl %ecx, %edi jl .L2 addl %ecx, %eax addl $32, %edx leal (%rax,%rdx), %ecx cmpl %edx, %edi cmovge %ecx, %eax ret Where .L2 is just: .L2: rep ret I assume that this is just some form of return, but the documentation I could find [2] seems to suggest that rep is a prefix for string operations, which doesn't make sense. [0]https://pastebin.com/raw/Y55gQG7p https://pastebin.com/raw/Y55gQG7p [1] http://x86.renejeschke.de/html/file_module_x86_id_226.html http://x86.renejeschke.de/html/file_module_x86_id_226.html [2] https://c9x.me/x86/html/file_module_x86_id_279.html https://c9x.me/x86/html/file_module_x86_id_279.html