feat: make bitReverse generic

[gbotrel]

Description

BitReverse is now generic. With tweaks, benchmarks are actually slightly faster (tested on c7a.8xlarge for koalabear and bls12377),, and faster for most sizes on koalabear E4:

BitReverse/name=bitReverseNaive/size=262144-32            1.121m ±  1%   1.015m ± 1%   -9.46% (p=0.000 n=10)
BitReverse/name=BitReverse/size=262144-32                 1.134m ±  1%   1.032m ± 1%   -8.97% (p=0.000 n=10)
BitReverse/name=bitReverseCobraInPlace/size=262144-32     3.489m ±  1%   3.228m ± 1%   -7.47% (p=0.000 n=10)
BitReverse/name=bitReverseNaive/size=524288-32            3.146m ±  3%   2.796m ± 1%  -11.13% (p=0.000 n=10)
BitReverse/name=BitReverse/size=524288-32                 3.145m ±  2%   2.747m ± 1%  -12.68% (p=0.000 n=10)
BitReverse/name=bitReverseCobraInPlace/size=524288-32     8.012m ± 12%   7.128m ± 2%  -11.04% (p=0.000 n=10)
BitReverse/name=bitReverseNaive/size=1048576-32           13.46m ±  2%   11.96m ± 1%  -11.16% (p=0.000 n=10)
BitReverse/name=BitReverse/size=1048576-32                13.28m ±  2%   11.74m ± 2%  -11.61% (p=0.000 n=10)
BitReverse/name=bitReverseCobraInPlace/size=1048576-32    17.15m ±  6%   14.96m ± 9%  -12.79% (p=0.000 n=10)
BitReverse/name=bitReverseNaive/size=2097152-32           46.55m ±  3%   43.40m ± 2%   -6.77% (p=0.000 n=10)
BitReverse/name=BitReverse/size=2097152-32                28.14m ±  5%   26.57m ± 6%   -5.57% (p=0.043 n=10)
BitReverse/name=bitReverseCobraInPlace/size=2097152-32    31.77m ±  4%   31.31m ± 3%   -1.46% (p=0.029 n=10)
BitReverse/name=bitReverseNaive/size=4194304-32          104.83m ±  1%   96.82m ± 3%   -7.64% (p=0.000 n=10)
BitReverse/name=BitReverse/size=4194304-32                57.14m ±  2%   55.12m ± 4%   -3.54% (p=0.004 n=10)
BitReverse/name=bitReverseCobraInPlace/size=4194304-32    66.32m ±  2%   61.54m ± 3%   -7.21% (p=0.000 n=10)
BitReverse/name=bitReverseNaive/size=8388608-32           221.3m ±  2%   188.0m ± 2%  -15.06% (p=0.000 n=10)
BitReverse/name=BitReverse/size=8388608-32                115.6m ±  3%   108.9m ± 2%   -5.79% (p=0.000 n=10)
BitReverse/name=bitReverseCobraInPlace/size=8388608-32    139.1m ±  2%   126.3m ± 2%   -9.24% (p=0.000 n=10)

[Copilot]

Pull Request Overview

This PR makes the BitReverse function generic by moving it from field-specific packages to a centralized utils package, allowing it to work with any type while maintaining performance optimizations. The function now accepts func BitReverse[T any](v []T) instead of being tied to specific field element types.

• Move BitReverse implementation from field-specific fft packages to utils/bitreverse.go as a generic function
• Update all references throughout the codebase to use utils.BitReverse instead of fft.BitReverse
• Remove field-specific bitreverse.go and bitreverse_test.go files that are no longer needed

Reviewed Changes

Copilot reviewed 125 out of 125 changed files in this pull request and generated 4 comments.

File	Description
utils/bitreverse.go	New generic BitReverse implementation with performance optimizations
utils/bitreverse_test.go	Comprehensive tests for the generic BitReverse function
Multiple fft packages	Removal of field-specific bitreverse implementations
Multiple files	Updated import statements and function calls to use utils.BitReverse

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

The base branch was changed.

[cursor]

Bug: BitReverse Function Fallback Issue

The bitReverseCobra function's default case no longer falls back to bitReverseNaive for small input sizes. This makes the len(v) < (1<<21) check in the top-level BitReverse function essential. Without it, small inputs would incorrectly use bitReverseCobraInPlace, which may lead to performance regressions.

Additional Locations (1)

• utils/bitreverse.go#L19-L20