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Introduce --cfg zerocopy_inline_always#3139

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Introduce --cfg zerocopy_inline_always#3139
joshlf merged 1 commit into
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G750518420a745aec19276f7a24bcd0f66584bb2c

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@jswrenn jswrenn commented Mar 19, 2026 •
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Enables customers to tune inlining aggressiveness of many of zerocopy's
public functions. At higher codegen-units, the Rust compiler may fail
to inline invocations of conversion functions. Applying the
#[inline(always)] attribute (as in #3137) forces inlining, but that
might not be desirable for all monomorphizations or invocations.
Dynamically padded types, for instance, require much more complex
codegen than statically sized for unpadded dynamically sized types.

While reducing codegen-units for a build can improve codegen, it
carries a large penalty in build times. The --cfg zerocopy_inline_always
flag provides a narrowly-scoped lever for achieving some of the
benefits of codegen-units=1 without the broad build time penalties.

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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 introduces a new configuration flag, --cfg zerocopy_inline_always, designed to give users finer control over compiler inlining behavior for core zerocopy functions. This allows for targeted performance improvements in critical code paths by forcing aggressive inlining, offering a more granular approach than global compiler settings and avoiding their associated build time overheads. The change also includes updates to benchmark results, likely reflecting the impact of these inlining considerations.

Highlights

  • New Build Tuning Flag: Introduced a new --cfg zerocopy_inline_always flag to allow users to explicitly force #[inline(always)] on many of zerocopy's public functions.
  • Performance Optimization Alternative: This flag provides a narrowly-scoped alternative for improving the optimization of hot paths without incurring the broad build time penalties associated with codegen-units=1.
  • Conditional Inlining: Public methods within the TryFromBytes, FromBytes, and IntoBytes traits now conditionally apply #[inline(always)] when the zerocopy_inline_always cfg is enabled, otherwise defaulting to #[inline].
  • Benchmark Updates: Updated assembly and performance metrics for dynamic padding and dynamic size benchmarks, reflecting potential changes in code generation.

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@jswrenn jswrenn force-pushed the G750518420a745aec19276f7a24bcd0f66584bb2c branch from e77105c to 4b8fff9 Compare March 19, 2026 16:44
@jswrenn

jswrenn commented Mar 19, 2026

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@korran, this is a more general solution to the problem you're addressing in #3137. Curious to hear if you think it'd be useful to you!

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@jswrenn jswrenn force-pushed the G750518420a745aec19276f7a24bcd0f66584bb2c branch from 4b8fff9 to c9a374e Compare March 19, 2026 16:47
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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.85%. Comparing base (0406c5c) to head (b19e71c).

Additional details and impacted files 
@@           Coverage Diff           @@
##             main    #3139   +/-   ##
=======================================
  Coverage   91.85%   91.85%           
=======================================
  Files          20       20           
  Lines        6067     6067           
=======================================
  Hits         5573     5573           
  Misses        494      494

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

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jswrenn
jswrenn commented Mar 19, 2026
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Comment thread README.md
@jswrenn jswrenn force-pushed the G750518420a745aec19276f7a24bcd0f66584bb2c branch 2 times, most recently from 165c805 to d8bb9f9 Compare March 20, 2026 17:30
@jswrenn
Introduce --cfg zerocopy_inline_always
b19e71c 
Enables customers to tune inlining aggressiveness of many of zerocopy's
public functions. At higher `codegen-units`, the Rust compiler may fail
to inline invocations of conversion functions. Applying the
`#[inline(always)]` attribute (as in #3137) forces inlining, but that
might not be desirable for all monomorphizations or invocations.
Dynamically padded types, for instance, require much more complex
codegen than statically sized for unpadded dynamically sized types.

While reducing `codegen-units` for a build can improve codegen, it
carries a large penalty in build times. The `--cfg zerocopy_inline_always`
flag provides a narrowly-scoped lever for achieving some of the
benefits of `codegen-units=1` without the broad build time penalties.

gherrit-pr-id: G750518420a745aec19276f7a24bcd0f66584bb2c
@jswrenn jswrenn force-pushed the G750518420a745aec19276f7a24bcd0f66584bb2c branch from d8bb9f9 to b19e71c Compare March 20, 2026 17:37
@joshlf joshlf added this pull request to the merge queue Mar 20, 2026
Merged via the queue into main with commit a5f7fa1 Mar 20, 2026
108 checks passed
@joshlf joshlf deleted the G750518420a745aec19276f7a24bcd0f66584bb2c branch March 20, 2026 18:24
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