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