This is intended to be used in the new exact mesh boolean algorithm by @howardt.
The new `BLI_fixed_width_int.hh` header provides types like `Int256` and
`UInt256` which are like e.g. `uint64_t` but with higher precision. The code
supports many different integer sizes.
The following operations are supported:
* Addition
* Subtraction
* Multiplication
* Comparisons
* Negation
* Conversion to and from other number types
* Conversion to and from string (based on `GMP`)
Division is not implemented. It could be implemented, but it's more complex and
is not required for the new mesh boolean algorithm.
Some alternatives to having a custom implementation have been discussed in
https://devtalk.blender.org/t/fixed-length-multiprecision-arithmetic/29189/.
Generally, the implementation is fairly straight forward. The main complexity is
the addition/multiplication algorithm which isn't too complicated. It's nice to
have control over this part as it allows us to optimize the code more if
necessary. Also, from what I understand, we might be able to benefit from some
special cases like multiplying a large integer with a smaller one.
I tried some different ways to optimize this already, but so far the normal
compiler optimization turned out to work best. Not sure if the same is true on
windows though, as it doesn't have native support for an `int128` which helps
the compiler understand what I'm doing. Alternatives I tried so far are using
intrinsics directly (mainly `_addcarry_u64` and similar), writing inline
assembly manually and copying the assembly output from the compiler. I assume
the assembly implementation didn't help for me because it prohibited other
compiler optimizations.
Pull Request: https://projects.blender.org/blender/blender/pulls/119528
7314c86869