diff options
Diffstat (limited to 'vendor/compiler_builtins/src/riscv.rs')
| -rw-r--r-- | vendor/compiler_builtins/src/riscv.rs | 18 |
1 files changed, 17 insertions, 1 deletions
diff --git a/vendor/compiler_builtins/src/riscv.rs b/vendor/compiler_builtins/src/riscv.rs index ee78b9dba..ae361b33a 100644 --- a/vendor/compiler_builtins/src/riscv.rs +++ b/vendor/compiler_builtins/src/riscv.rs @@ -1,6 +1,22 @@ 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 + // + // 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; |