Report code churn across the codebase

[typeless]

A per-PR rework number answers "is this change rewriting recent work" — a property of the change. The question worth a standing metric is a property of the tree: how much of what we wrote recently is already gone?

The metric

Over a window ending at the analyzed commit: of the lines written in that window, how many are no longer present.

One git log --numstat --since pass gives what was written per file; one blame pass over those files counts what survived; the difference is work written and then discarded or rewritten inside the same window. Dated against the analyzed commit rather than wall-clock now, so a re-run agrees.

On a pull request it analyzes the PR head, so the reading is the state of the tree if this lands — not a score for the PR.

### Code churn (whole codebase, last 30d)

| Files | Lines written | Still present | Churned | Churn rate |
|---|---|---|---|---|
| 145 | 9916 | 8695 | 1221 | 12.3% |

Where the churn is

File	Lines written then discarded
src/cli/cmd_build.cpp	249
.github/scripts/compose_metrics.py	143
.github/scripts/bench_metrics.sh	92
src/core/layout.cpp	69
src/core/new_delete.cpp	63
…

Top of the list is src/cli/cmd_build.cpp at 249 lines written then discarded — where the identity and join work has been concentrated. The metric finds the area we already knew had been rewritten repeatedly, which is the check that it measures something real.

Stability

window	written	churned	rate
7d	3230	471	14.6%
14d	9335	1151	12.3%
30d	9916	1221	12.3%
90d	10399	1296	12.5%

Stable across windows, and highest over the shortest one — what you would expect if fresh code is what gets revised. The number is not an artifact of the window choice.

Exclusions, and how one of them was found

Tests, third_party/, examples/, and the generated bootstrap scripts.

• Tests are rewritten whenever the code they cover is, so counting them measures the same production change twice. test/ is 408 of 694 tracked files.
• third_party/ is vendored — its history is imports, not our work.
• examples/ is build recipes for other people's source.
• bootstrap-linux.sh / bootstrap-macos.sh earned their exclusion by ranking 2nd and 3rd on the first run. make bootstrap regenerates them from the Tupfiles, so their churn is a mechanical echo — at roughly ten times the volume — of a change already counted at its source.

That last one is the argument for looking at the output before trusting it: it was 242 and 241 lines of pure noise sitting directly under the top result.

Failure behaviour

Every git call degrades to empty output rather than raising, so a missing history would report zero churn instead of failing — silence and "nothing churned" look identical. A shallow clone, an unknown revision, and a window containing no production changes each refuse with a message. Verified against a real --depth 1 clone.

Cost

2.1s, inside the existing pr-metrics-comment job — which already checks out and only runs on pull requests, so no new job and no artifact round-trip. It needs fetch-depth: 0, since neither blame nor a 30-day log works in a shallow clone.

🤖 Generated with Claude Code

https://claude.ai/code/session_01TLob4Ef3qyMd3Dnp9q3XmA

[github-actions]

PR metrics

Performance (gcc example, Linux)

Workload	Instructions	CPU time	Page faults	D1 miss	LL miss	Wall	Peak RSS
parse	1076 M	0.51 s	14.2 k	0.8%	0.1%	0.524 s	31.2 MB
dry-run	1881 M	0.62 s	16.6 k	0.9%	0.1%	0.635 s	40 MB (-0.2MB)

Deterministic signals: instructions (cachegrind-simulated instruction reads — exact across runs, no PMU needed), page faults, peak RSS, and the cachegrind D1/LL miss rates. CPU time is user+sys from time(1).

Internal statistics (gcc example, up-to-date dry run)

Metric	Value
Tupfiles parsed	24
Commands	3545
Commands scheduled	0
Files checked	5818
Files changed	0
Files in index	6087
Graph edges	367913
Index size (bytes)	7378282
Implicit deps	330624
Hash computations	0
Hashes skipped (stat cache)	5818
Stat calls	5869
Parse time (ms)	503.6 (+0.4%)
Total time (ms)	613.8 (+0.7%)
Runner CPU	AMD EPYC 7763 64-Core Processor

Counters from putup -n --stat on the fully-built gcc example (up-to-date dry run): deterministic work measures — a jump in commands scheduled, hash computations, or stat calls is a real behavior change, not noise. Timings are the minimum over repeated runs, compared only against a baseline from the same CPU model; the counters are the regression signal.

Binary size (Linux)

Binary	.text	.data	.bss	File
putup	517.6 KB	1.6 KB	98.7 KB	615.2 KB

Code churn (whole codebase, last 30d)

Files	Lines written	Still present	Churned	Churn rate
119	7492	6728	764	10.2%

Of the lines written across the codebase in the last 30 days, how many are already gone — work that was written and then discarded or rewritten inside the same window. This is the state of the tree including this PR, not a measure of the PR itself. Only code we write is counted: tests, examples, vendored and generated files, CI plumbing and prose are excluded. 3562 lines were deleted in the window in total, most of them older than it.

Where the churn is

File	Lines written then discarded
src/cli/cmd_build.cpp	247
src/core/layout.cpp	69
src/core/new_delete.cpp	63
src/cli/context.cpp	55
src/graph/builder.cpp	35
src/core/string_pool.cpp	34
src/core/heap_buf.cpp	33
src/platform/vm-posix.cpp	22
src/cli/options.cpp	21
include/pup/core/layout.hpp	15

Test coverage (lines)

Overall	Median file	Min file	Max file
86.7%	95%	0.0% src/platform/path-posix.cpp	100.0% include/pup/core/arena.hpp

103 files · 15163/17494 lines covered

Deltas vs main@fc14f89de.

Updated for 6d0e89b