| 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 |
| } |
| } |
