Single-byte counter support in coverage instrumentation

[woodruffw]

Proposal

I propose extending rustc's -C instrument-coverage to allow support for LLVM's single-byte counters. No defaults would be changed, but -C instrument-coverage=counter would become the "canonical" variant of the current default.

Specifically, I propose -C instrument-coverage=presence-only as an opt-in mechanism for switching to single-byte counters. This option would be guarded behind -Z unstable-options.

(The name presence-only is picked as a happy medium -- "counters" is confusing in a context where the coverage is binary, and "presence" alone would also be confusing because all coverage is presence.)

Background and motivation

Per llvm/llvm-project#75425, single-byte counters are available in LLVM 19.1.0 and later. The equivalent clang frontend specific option for this is currently tucked behind an -mllvm passthrough, e.g. -mllvm -enable-single-byte-coverage.

The main advantage to single-byte counters is performance: LLVM's own (older) benchmarks show a 3-5% runtime improvement on x86(-64) platforms, and a much more dramatic 40-60% runtime improvement on ARM(-64) platforms. Single-byte counters also significant improve code size, since the instrumentation only needs to inject 8-bit counters rather an 64-bit ones.

The main disadvantage to single-byte counters is a decrease in fidelity: single-byte counters no longer track actual block hit counts, and instead only offer a binary "covered or not." This makes them unsuitable for use cases where the "heat" of covered regions is important, but perfectly suitable for use cases where line/block coverage is the primary concern.

Mentors or Reviewers

None yet. This is my first non-trivial compiler-side contribution to Rust, so I would appreciate a reviewer who is familiar with this part of the compiler.

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
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  • Refer to Proposals, Approvals and Stabilization docs for when a second is sufficient, or when a full team FCP is required.
• Once an MCP is seconded, the Final Comment Period begins.
  • Final Comment Period lasts for 10 days after all outstanding concerns are solved.
  • Outstanding concerns will block the Final Comment Period from finishing. Once all concerns are resolved, the 10 day countdown is restarted.
  • If no concerns are raised after 10 days since the resolution of the last outstanding concern, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Caution

Concerns (2 active)

• question-boolean-valued-counters
• stabilization-pressure resolved in this comment
• state-of-the-impl

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[rustbot]

Important

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cc @rust-lang/compiler

[jackh726]

@rustbot second

[rustbot]

The final comment period is now complete, this major change is now accepted. 🎉

As the automated representative, I want to express gratitude to the author for their work and everyone else who contributed to this major change proposal.

If you think this major change shouldn't have been accepted, feel free to remove the major-change-accepted label and reopen this issue.

Note

Further progress is now tracked over at rust-lang/rust#158408.

[apiraino]

@rustbot concern question-boolean-valued-counters

From Zulip:

One concrete concern I have is the question of how boolean-valued counters interact with counter expressions.
IIRC, counter expressions can only perform addition and subtraction, which give meaningless results when used on counters that saturate to 1. And our current counter-expression handling assumes that addition and subtraction can be used
(I suspect this is something that LLVM will not handle automatically just by switching to different intrinsics.)

cc @Zalathar