On Fri, 3 Apr 2026 at 15:59, Matt Borland <matt@mattborland.com> wrote:

> Another thought I had when it comes to std::big_int procedurally is that we may want to walk before we run.
>

> Tiago's P3161R4 has been in the pipeline for a long time, and adds prerequisite functions such as std::mul_wide. Widening multiplication is necessary to implement multi-precision multiplication in std::big_int. Similarly, Daniel Towner provides funnel-shifting in P4010R0, which is necessary to implement multi-precision bit-shifting.
>

I'm not sure operations on std::big_int would be in scope. In the std::mul_wide case the function would need some manner of taking two arrays of limbs (since std::big_int doesn't exist), and then applying a variety of algorithms depending on the bit counts (e.g. Karatsuba, Toom-Cook, etc). P3161R4 reads to me that the goal is to provide thin wrappers around intrinsics rather than implement true arbitrary precision. Having std::add_carry would be convenient, but certainly not show stopping.

Yeah, P3161 provides the low-level operations that are used to implement those higher-level algorithms such as Karatsuba. Regardless of which high-level algorithm you choose, you reduce the instruction count significantly if you can compute the high bits of a multiplication at the same time as the low bits. If you didn't have the standard operation such as std::mul_wide, you would implement that yourself in terms of __int128 multiplication or something else, so there's no way around it. It's somewhere in the implementation of std::big_int.