[LLVMdev] Large integers as first-class values

[Eli Friedman]

On Sun, Feb 28, 2010 at 11:54 AM, Russell Wallace
<russell.wallace at gmail.com> wrote:
> So my question is, is it okay to go ahead and do this, or are there
> any caveats in terms of efficiency or correctness? In particular, I
> remember reading something about there being problems with returning
> integers larger than two machine words, but I can't find it again; is
> there currently any such problem, or is it the case that if there was,
> it's fixed now?

In terms of correctness, it should work except for the fact that the
LLVM code generators don't implement more complicated operations on
such integers, like multiplication, division, and variable-width
shifts.  The issues with returning large integers are fixed, at least
on x86.

In terms of efficiency, the generated code is likely to be less than
ideal; juggling 512-bit numbers takes a lot of registers, and
everything will be unrolled.  This might be okay for a 512-bit number,
but it would be a complete mess for a 2048-bit number.

Overall, for arbitrary uses, you're probably better off using a more
conventional bignum library.

-Eli