my rust fork
https://www.rust-lang.org
b53c72ffaa
Partial-stabilize the basics from `bigint_helper_methods`
Direct link to p-FCP comment: https://github.com/rust-lang/rust/pull/144494#issuecomment-3133172161
After libs-api discussion, this is now the following methods:
- [`uN::carrying_add`](https://doc.rust-lang.org/nightly/std/primitive.u64.html#method.carrying_add): uN + uN + bool -> (uN, bool)
- [`uN::borrowing_sub`](https://doc.rust-lang.org/nightly/std/primitive.u64.html#method.borrowing_sub): uN + uN + bool -> (uN, bool)
- [`uN::carrying_mul`](https://doc.rust-lang.org/nightly/std/primitive.u64.html#method.carrying_mul): uN * uN + uN -> (uN, uN)
- [`uN::carrying_mul_add`](https://doc.rust-lang.org/nightly/std/primitive.u64.html#method.carrying_mul_add): uN * uN + uN + uN -> (uN, uN)
Specifically, these are the ones that are specifically about working with `uN` as a "digit" (or "limb") where the output, despite being larger than can fit in a single digit, wants to be phrased in terms of those *digits*, not in terms of a wider type.
(This leaves open the possibility of things like `widening_mul: u32 * u32 -> u64` for places where one wants to only think in terms of the *number*s, rather than as carries between multiple digits. Though of course discussions about how best to phrase such a thing are best for the tracking issue, not for this PR.)
---
**Original PR description**:
A [conversation on IRLO](https://internals.rust-lang.org/t/methods-for-splitting-integers-into-their-halves/23210/7?u=scottmcm) the other day pushed me to write this up 🙂
This PR proposes a partial stabilization of `bigint_helper_methods` (rust-lang/rust#85532), focusing on a basic set that hopefully can be non-controversial. Specifically:
- [`uN::carrying_add`](https://doc.rust-lang.org/nightly/std/primitive.u64.html#method.carrying_add): uN + uN + bool -> (uN, bool)
- [`uN::widening_mul`](https://doc.rust-lang.org/nightly/std/primitive.u64.html#method.widening_mul): uN * uN -> (uN, uN)
- [`uN::carrying_mul_add`](https://doc.rust-lang.org/nightly/std/primitive.u64.html#method.carrying_mul_add): uN * uN + uN + uN -> (uN, uN)
Why these?
- We should let people write Rust without needing to be backend experts to know what the magic incantation is to do this. Even `carrying_add`, which doesn't seem that complicated, actually broke in 1.82 (see rust-lang/rust#133674) so we should just offer something fit-for-purpose rather than making people keep up with whatever the secret sauce is today. We also get to do things that users cannot, like have the LLVM version emit operations on `i256` in the implementation of `u128::carrying_mul_add` (https://rust.godbolt.org/z/cjG7eKcxd).
- Unsigned only because the behaviour is much clearer than when signed is involved, as everything is just unsigned (vs questions like whether `iN * iN` should give `(uN, iN)`) and carries can only happen in one direction (vs questions about whether the carry from `-128_u8 + -128_u8` should be considered `-1`).
- `carrying_add` is the core [full adder](https://en.wikipedia.org/wiki/Adder_(electronics)#Full_adder) primitive for implementing addition.
- `carrying_mul_add` is the core primitive for [grade school](https://en.wikipedia.org/wiki/Multiplication_algorithm#Long_multiplication) multiplication (see the example in its docs for why both carries are needed).
- `widening_mul` even though it's not strictly needed (its implementation is just `carrying_mul_add(a, b, 0, 0)` right now) as the simplest way for users to get to [cranelift's `umulhi`](https://docs.rs/cranelift/latest/cranelift/prelude/trait.InstBuilder.html#method.umulhi), RISC-V's `MULHU`, Arm's `UMULL`, etc. (For example, I added an ISLE pattern
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