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Document origins of the multiplication method being used here.

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This commit is contained in:
Felix S. Klock II 2022-10-25 11:25:14 -04:00
parent 6f53a4f074
commit 8266a1343b
1 changed files with 14 additions and 1 deletions
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15
library/compiler-builtins/src/riscv.rs
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@ -1,5 +1,18 @@
intrinsics! {
// Implementation from gcc
// 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
pub extern "C" fn __mulsi3(a: u32, b: u32) -> u32 {
let (mut a, mut b) = (a, b);