It doesn't actually use recursion; it is tail recursion and LLVM optimises it to a loop: .LBB0_77: movq %rax, %rbx movq %rcx, %rax addq %rbx, %rax

It doesn't actually use recursion; it is tail recursion and LLVM optimises it to a loop: .LBB0_77: movq %rax, %rbx movq %rcx, %rax addq %rbx, %rax incq %rdx movq %rbx, %rcx jne .LBB0_77 The difference on x64-64 is the Rust is using 64-bit integers (`int` is pointer sized), while the C is only using 32-bit ones; changing them to the 32-bit `i32` to match the C makes Rust super-fast: 75699 LANGUAGE Rust 2167 75699 LANGUAGE C 2220 # gcc 75699 LANGUAGE C 2271 # clang I opened https://github.com/logicchains/armfib/pull/1 https://github.com/logicchains/armfib/pull/1 .