[ec]: batchAffine does not use alloca anymore

[mratsim]

Fix #578, should help with status-im/nimbus-eth1#3389

Though looking at the CI, nimbus-eth1 should probably not use LTO for constantine as well.

cc @advaita-saha, @arnetheduck

[gemini-code-assist]

Summary of Changes

Hello @mratsim, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request refactors the batchAffine function within the elliptic curve cryptography module to optimize memory usage and enhance its robustness. By eliminating the use of alloca and repurposing existing data structures, it addresses potential stack-related issues and ensures correct handling of edge cases involving infinite points, contributing to a more stable and efficient cryptographic library.

Highlights

• Removal of alloca: The batchAffine function no longer uses alloca for temporary SecretBool arrays, addressing potential stack overflow issues and improving memory safety.
• Memory Optimization: Instead of allocating a separate array, the affs[i].y field is now cleverly repurposed to store the isZero flag, reducing memory footprint and avoiding unnecessary allocations.
• Introduction of zero template: A new zero template provides a clean and type-safe abstraction for accessing the isZero flag stored within affs[i].y, improving code readability and maintainability.
• Enhanced Test Coverage: A new test case has been added to t_ec_template.nim to specifically validate the batchAffine function's behavior when handling elliptic curve points at infinity, improving robustness and correctness for edge cases.

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[gemini-code-assist]

Code Review

This pull request refactors the batchAffine functions to eliminate stack allocations (alloca) by cleverly reusing a portion of the output array for temporary storage. This is a good performance optimization. The logic is correctly adjusted to handle the memory aliasing introduced by this change. A new test case is also added to validate the correct handling of points at infinity, which is crucial for this modification. The changes are solid, and I've only found a minor opportunity to improve a comment for better clarity and consistency.

[gemini-code-assist]

The comment is slightly inaccurate. affs[i].x is used for partial accumulations of products, while a part of affs[i].y is repurposed to store the zero flag. The comment for the batchAffine implementation for Jacobian coordinates on line 109 is more precise. For consistency and clarity, I suggest updating this comment to be more descriptive of the implementation.

  # in affs[i].x. A part of `affs[i].y` is used as scratch space to store if the input was at infinity.