relibc_compiler-builtins/src/riscv32.rs

intrinsics! {
    // Implementation from gcc
    // https://raw.githubusercontent.com/gcc-mirror/gcc/master/libgcc/config/epiphany/mulsi3.c
    pub extern "C" fn __mulsi3(mut a: u32, mut b: u32) -> u32 {
        let mut r: usize = 0;

        while a > 0 {
            if a & 1 > 0 {
                r += b;
            }
            a >>= 1;
            b <<= 1;
        }

        r
    }
}