The default godbolt page runs the compiler with no flags, which means without any optimizations. This explains why the code unnecessarily shuffles stuff to the

The default godbolt page runs the compiler with no flags, which means without any optimizations. This explains why the code unnecessarily shuffles stuff to the stack and back. Unoptimized clang/llvm output spills everything to the stack, and register allocation is an optimization. With -O3, the code is: imul edi, edi mov eax, edi ret Yep, the calling convention for x86-64 on Linux and macOS passes the first six integer arguments in rdi, rsi, rdx, rcx, r8, and r9, and then spills to stack.