From 8266a1343b3ee164da223f9f034562ba1aa44d45 Mon Sep 17 00:00:00 2001 From: "Felix S. Klock II" Date: Tue, 25 Oct 2022 11:25:14 -0400 Subject: [PATCH] Document origins of the multiplication method being used here. --- library/compiler-builtins/src/riscv.rs | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-) diff --git a/library/compiler-builtins/src/riscv.rs b/library/compiler-builtins/src/riscv.rs index ee78b9dbacf4..eff249692ce3 100644 --- a/library/compiler-builtins/src/riscv.rs +++ b/library/compiler-builtins/src/riscv.rs @@ -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);