intrinsics! { // Ancient Egyptian/Ethiopian/Russian multiplication method // see https://en.wikipedia.org/wiki/Ancient_Egyptian_multiplication // // This is a long-available stock algorithm; e.g. it is documented in // Knuth's "The Art of Computer Programming" volume 2 (under the section // "Evaluation of Powers") since at least the 2nd edition (1981). // // The main attraction of this method is that it implements (software) // multiplication atop four simple operations: doubling, halving, checking // if a value is even/odd, and addition. This is *not* considered to be the // fastest multiplication method, but it may be amongst the simplest (and // smallest with respect to code size). // // for reference, see also implementation from gcc // https://raw.githubusercontent.com/gcc-mirror/gcc/master/libgcc/config/epiphany/mulsi3.c // // and from LLVM (in relatively readable RISC-V assembly): // https://github.com/llvm/llvm-project/blob/main/compiler-rt/lib/builtins/riscv/int_mul_impl.inc pub extern "C" fn __mulsi3(a: u32, b: u32) -> u32 { let (mut a, mut b) = (a, b); let mut r: u32 = 0; while a > 0 { if a & 1 > 0 { r = r.wrapping_add(b); } a >>= 1; b <<= 1; } r } #[cfg(not(target_feature = "m"))] pub extern "C" fn __muldi3(a: u64, b: u64) -> u64 { let (mut a, mut b) = (a, b); let mut r: u64 = 0; while a > 0 { if a & 1 > 0 { r = r.wrapping_add(b); } a >>= 1; b <<= 1; } r } }