Adopt and document a preference for aligned loads in validation

[jswrenn]

This commit parameterizes TryFromBytes::is_bit_valid and
Ptr::read over alignment, ensuring that well-aligned loads are
used at the leaves of validation whenever possible. This carries
a performance advantage over unaligned loads on many platforms.
This manifests in our API as an optimization for try_transmute!
and TryFromBytes::try_read_from_* for destination types with
alignments greater than 1. For trivially-aligned destination types,
this commit introduces an optimization FIXME noting that validation
could (and should) be performed in-place.

Makes progress towards #2981.

---

• 👉 Adopt and document a preference for aligned loads in validation #2985

Latest Update: v5 — Compare vs v4

📚 Full Patch History

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[gemini-code-assist]

Summary of Changes

Hello @jswrenn, 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 introduces a significant performance enhancement to the zerocopy library by making its validation mechanisms alignment-aware. By parameterizing key validation functions with alignment information, the library can now utilize optimized aligned memory loads, leading to faster data validation. This change also includes updates to the API and documentation to reflect these new capabilities and sets the stage for further optimizations for trivially-aligned types.

Highlights

• Performance Optimization for Validation: The core validation functions, TryFromBytes::is_bit_valid and Ptr::read, have been parameterized over alignment. This change ensures that well-aligned memory loads are used during validation whenever possible, which can significantly improve performance on platforms where unaligned loads are slower.
• API Changes for Alignment Awareness: The is_bit_valid trait method now accepts a generic Alignment parameter, allowing for more precise control over memory access patterns during validation. The Ptr::read_unaligned method has been renamed to Ptr::read and its implementation now leverages the new alignment-aware Alignment::read method.
• Improved Documentation and Future Work: New documentation has been added to try_read_from_bytes, try_read_from_prefix, and try_read_from_suffix methods, explaining the performance benefits of using aligned stack allocations for validation. A FIXME note has also been added, indicating a future optimization to perform in-place validation for trivially-aligned types (alignment of 1).
• Dependency Update: The zerocopy and zerocopy-derive crates have been updated to version 0.8.37 in Cargo.lock.

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Changelog

• Cargo.lock
  • Updated zerocopy and zerocopy-derive dependencies to version 0.8.37.
• src/impls.rs
  • Replaced read_unaligned() with the new read() method in TryFromBytes implementations for char and nonzero types.
  • Modified is_bit_valid for UnsafeCell<T> to accept a generic Alignment parameter.
• src/lib.rs
  • Updated the TryFromBytes::is_bit_valid trait method signature to include a generic Alignment parameter.
  • Added detailed performance considerations to the documentation of try_read_from_bytes, try_read_from_prefix, and try_read_from_suffix, explaining the use of aligned stack allocations and noting a FIXME for in-place validation of trivially-aligned types.
• src/macros.rs
  • Adjusted is_bit_valid macro implementations to correctly handle the new generic Alignment parameter.
• src/pointer/invariant.rs
  • Introduced a read method to the Alignment trait.
  • Implemented the read method for Unaligned and Aligned enums, providing specific logic for each alignment type.
• src/pointer/mod.rs
  • Refined the Maybe type alias by simplifying its generic parameters, specifically removing the explicit Aliasing parameter and defaulting Alignment to invariant::Unaligned.
• src/pointer/ptr.rs
  • Renamed the read_unaligned method to read and updated its internal implementation to leverage the new Alignment::read method for alignment-aware data access.
• src/util/macros.rs
  • Modified is_bit_valid macro implementations to support the new generic Alignment parameter.
• zerocopy-derive/src/derive/try_from_bytes.rs
  • Updated derive_is_bit_valid functions for enums, structs, and unions to accept a generic _ZcAlignment parameter.
  • Replaced read_unaligned() with read() in the derived is_bit_valid implementation for enums.
• zerocopy-derive/src/output_tests/expected/*.rs
  • Updated various expected output files for TryFromBytes derivations to reflect the changes in the is_bit_valid signature, including the addition of the _ZcAlignment generic parameter and #[inline(always)] attribute.

Activity

• This pull request makes progress towards resolving issue TryFromBytes::read_read_from_* should validate in-place before reading #2981.
• It is related to pull request Adopt and document a preference for aligned loads in validation #2985.
• The changes are introduced as a single commit, focusing on parameterizing validation over alignment.

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[gemini-code-assist]

Code Review

This pull request introduces a well-thought-out refactoring to make validation aware of memory alignment, which can lead to significant performance improvements. The core change is parameterizing TryFromBytes::is_bit_valid and Ptr::read over a new Alignment trait, allowing the system to use more efficient aligned loads when possible. The implementation is clean, consistent, and applied thoroughly across the library, including the derive macros. The documentation updates and FIXME comments regarding future in-place validation optimizations are also valuable additions. I found no issues with the changes.

[joshlf]

🔥

[joshlf]

Please address existing comments, but otherwise approved.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 67.50000% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.87%. Comparing base (6b81374) to head (8923b44).

Files with missing lines	Patch %	Lines
src/pointer/invariant.rs	50.00%	7 Missing ⚠️
src/macros.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2985      +/-   ##
==========================================
- Coverage   92.02%   91.87%   -0.15%     
==========================================
  Files          19       20       +1     
  Lines        6029     6057      +28     
==========================================
+ Hits         5548     5565      +17     
- Misses        481      492      +11     

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