[EC] Fix mutable assignment in batch affine - followup #579

[mratsim]

Currently we have a failure constantine/constantine/math/elliptic/ec_shortweierstrass_batch_ops.nim(52, 3) Error: 'SecretBool(affs[0].y.mres.limbs[0])' cannot be assigned to

This is because after #579, the SecretWord myExpression creates an immutable expression

constantine/constantine/math/elliptic/ec_shortweierstrass_batch_ops.nim

Lines 45 to 59 in cedcf0d

	template zero(i: int): SecretBool =
	when F is Fp:
	SecretBool affs[i].y.mres.limbs[0]
	else:
	SecretBool affs[i].y.coords[0].mres.limbs[0]
	affs[0].x = projs[0].z
	zero(0) = affs[0].x.isZero()
	affs[0].x.csetOne(zero(0))
	for i in 1 ..< N:
	# Skip zero z-coordinates (infinity points)
	var z = projs[i].z
	zero(i) = z.isZero()
	z.csetOne(zero(i))

The strange thing is that, when compiling through nimble test or nimble test_parallel that do a simple exec, compilation works and test is run:

but compiling the code directly triggers a compiler error.

Summary by CodeRabbit

• Refactor
  • Enhanced internal consistency and performance in cryptographic batch operations through improved type handling in elliptic curve computations.

[gemini-code-assist]

Summary of Changes

Hello, 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 addresses a compilation error in the batch affine operations by refining the type handling for SecretBool and SecretWord. The changes ensure that expressions are correctly typed and cast when assigned or passed to functions, resolving an issue where an immutable SecretWord expression was implicitly assigned to a SecretBool type, leading to a compiler error.

Highlights

• Type Correction for zero Template: The zero template's return type was changed from SecretBool to SecretWord to correctly handle the type of the expression isZero() which returns a SecretWord.
• Explicit Type Casting: Introduced explicit SecretBool casts when passing the result of the zero template to functions like csetOne and csetZero, which expect a SecretBool argument.
• Resolution of Compilation Error: This change resolves a compilation error related to immutable assignment, specifically 'SecretBool(...) cannot be assigned to', which occurred due to a type mismatch after a previous update.

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Changelog

• constantine/math/elliptic/ec_shortweierstrass_batch_ops.nim
  • Updated the zero template's return type from SecretBool to SecretWord in both batchAffine function overloads.
  • Modified assignments to zero(i) to explicitly cast the isZero() result to SecretWord.
  • Applied explicit SecretBool casts to the zero(i) and zero(0) template calls when used as arguments for csetOne and csetZero functions.

Activity

• Identified a compilation failure: 'SecretBool(...) cannot be assigned to' in ec_shortweierstrass_batch_ops.nim.
• Traced the error to an immutable expression created after changes introduced in [ec]: batchAffine does not use alloca anymore #579.
• Noted that the error only occurred during direct compilation, not via nimble test or nimble test_parallel.

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 81867b9a-239e-40fc-8c16-c31e0561ad91

📥 Commits

Reviewing files that changed from the base of the PR and between cedcf0d and b905691.

📒 Files selected for processing (1)

• constantine/math/elliptic/ec_shortweierstrass_batch_ops.nim

---

📝 Walkthrough

Walkthrough

Type system refactoring in elliptic curve batch operations module replacing SecretBool with SecretWord for zero-tracking mechanism. The template zero(i: int) return type changed accordingly, with cascading updates to isZero checks, set operations, and batch inversion paths.

Changes

Cohort / File(s)	Summary
Zero-tracking type refactoring constantine/math/elliptic/ec_shortweierstrass_batch_ops.nim	Updated batch inversion logic to use SecretWord instead of SecretBool for zero-tracking; changed template zero(i: int) return type, adjusted isZero assignments, modified csetZero/csetOne calls with appropriate type conversions, and added explicit non-lazy path handling in loop branches.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Poem

🐰 ✨ A Secret Word now tracks the zero,
Where once a Bool did dance so fair,
Through batch inversions we shall go,
With types aligned with crypto care!
Type safety blooms, the refactor's true. 🌟

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly references a specific fix for mutable assignment in batch affine operations and directly links to a follow-up issue, accurately summarizing the main change.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch batch-affine-mut

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[gemini-code-assist]

Code Review

This pull request addresses a compilation error related to assigning to an immutable expression, which was introduced in a previous change. The fix involves modifying a template to return a mutable l-value instead of a type-casted r-value, and then adding explicit type casts at the call sites to match the expected types. The changes are correct, consistent across the file, and effectively resolve the issue.

[mratsim]

@greptile