feature(sierra): Added a split fn from u128 to u32 guarantees.

[orizi]

Summary

Adds a new u128_to_u32_guarantees libfunc that splits a u128 value into four u32 guarantee values using iterative DivMod hints and verification.

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Type of change

Please check one:

• Bug fix (fixes incorrect behavior)
• New feature
• Performance improvement
• Documentation change with concrete technical impact
• Style, wording, formatting, or typo-only change

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Why is this change needed?

This libfunc provides an efficient way to decompose u128 values into their constituent u32 parts with mathematical guarantees, which is useful for operations that need to work with smaller integer components or for cryptographic applications that require bounded integer guarantees.

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What was the behavior or documentation before?

There was no built-in way to split a u128 into guaranteed u32 components in the Sierra instruction set.

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What is the behavior or documentation after?

The new u128_to_u32_guarantees libfunc takes a u128 input and returns four BoundedIntGuarantee<0, 4294967295> values representing the u32 components. The implementation uses DivMod hints to perform the splitting and verifies the reconstruction using Horner's method. The libfunc has an AP change cost of 9 and a gas cost of 7 steps.

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Related issue or discussion (if any)

None specified.

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Additional context

The implementation includes comprehensive test coverage and the libfunc has been added to the allowed libfuncs list for Starknet classes. The verification process ensures mathematical correctness by reconstructing the original u128 value from the four u32 components.

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Note

Medium Risk
Introduces a new Sierra libfunc with custom CASM generation and new AP/gas accounting, which could affect compilation determinism and cost metering if the implementation or constants are incorrect.

Overview
Adds a new Sierra libfunc, u128_to_u32_guarantees, that decomposes a u128 into four BoundedIntGuarantee<0, 2^32-1> outputs.

Implements CASM lowering using iterative DivMod hints plus reconstruction asserts, wires the new variant through bounded-int libfunc specialization, and assigns fixed metering (ApChange::Known(9) and ConstCost::steps(7)). The libfunc is added to Starknet’s allowed_libfuncs list and covered by a new bounded-int e2e test vector.

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[reviewable-StarkWare]

This change is 

[orizi]

• Added e2e tests of blake improvement. #9780
• feature(sierra): Added guarantee version for blake libfuncs. #9779
• feature(sierra): Added a split fn from u128 to u32 guarantees. #9778 👈 (View in Graphite)
• feature(sierra): Added BoundedIntGuarantee and verify libfunc. #9777
• main

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[orizi]

@orizi made 1 comment.
Reviewable status: 0 of 6 files reviewed, 2 unresolved discussions (waiting on ilyalesokhin-starkware).

[ilyalesokhin-starkware]

@ilyalesokhin-starkware reviewed 5 files and all commit messages.
Reviewable status: 5 of 6 files reviewed, 2 unresolved discussions.

[orizi]

@orizi made 1 comment and resolved 1 discussion.
Reviewable status: 5 of 6 files reviewed, 1 unresolved discussion (waiting on ilyalesokhin-starkware).

[ilyalesokhin-starkware]

@ilyalesokhin-starkware reviewed 5 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on orizi).

[orizi]

@orizi reviewed 2 files and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on orizi).