74 lines
5.0 KiB
Markdown
74 lines
5.0 KiB
Markdown
# Handoff — what the x86 backend costs, and keeping it from rotting
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Branch: `dev-loop`. This lane is item 7 of `HANDOFF-x86-rt.md`'s "What remains" — code size and speed, measured
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rather than guessed — plus the closing note under it, which asks for the survey to run on its own so a refusal
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cannot sit unnoticed for a month again.
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Nothing in `lib/` is touched. Another lane is rewriting `lib/x86.ml` at the same time, so everything here lives in
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`spike/x86` and in `test/dune`, and anything this lane finds that would want a compiler change is written down at
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the bottom rather than made.
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## 1. The `@x86` alias — done, and it fails when it should
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`test/dune` has a fourth alias beside `@sanitize` and `@valgrind`, shaped the same way and opt-in for the same
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reason: it builds every corpus program twice and runs both, which is minutes where `dune test` is forty seconds.
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dune build --root . @x86
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It is a `(rule ...)` with no `(executable ...)` beside it, which is where it differs from its two neighbours. The
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check already exists — `spike/x86/survey.sh` is what every handoff quotes its counts from — and a second
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implementation of it in OCaml would be a second thing to drift, which is exactly the failure this alias is meant
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to prevent. So the rule runs the script, and the script grew three environment variables to make that possible:
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- `SURVEY_STRICT=1` turns the report into an exit status. A DIFFER is a wrong answer and a refusal by name is a
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node the backend has stopped lowering; either fails the build. NOX86 and SKIP do not — the first is usually a
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toolchain that is not installed on the machine, the second is the frontend refusing the program on both sides.
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- `FLAN=<path>` hands it a compiler somebody else built. Without it the script runs `dune build` on `bin/main.exe`,
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and a dune inside a dune action waits on a lock it will never get. The rule has `bin/main.exe` in its deps
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instead, exactly as the `(tests ...)` stanza already does.
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- `SURVEY_CORPUS` for where the programs are read from, which under dune is the build tree.
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Standalone, with nothing set, the script is byte-for-byte the measurement it always was.
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**Verified both directions.** `dune build --root . @x86` passes on a clean tree and prints the same
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**97 MATCH / 0 DIFFER / 0 REFUSED / 0 NOX86 / 36 SKIP** the script prints standalone — the build tree and the
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source tree agree about the corpus, which was not obvious in advance. Broken deliberately (a fake DIFFER injected
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into the script, the rule narrowed to one program so the round trip was thirty seconds) it fails the dune build
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with `x86 survey FAILED: 1 differ, 0 refused` and a nonzero status. Both edits reverted.
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`dune test --root .` is still green and still about forty seconds. The new stanza is a rule on its own alias and
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touches no `(modules ...)` list, so it cannot be pulled into `@runtest` by accident. The two
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`clang: error: linker command failed` lines it prints are the `dev-robust` fixture doing its job and are called
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out in `HANDOFF-x86-rt.md` §4 already.
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## 2. The measurement — `spike/x86/cost.sh`, `spike/x86/bench.sh`, `spike/x86/COST.md`
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`COST.md` is the deliverable and it carries the numbers and the reading of them. The two scripts are how it was
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produced:
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- `cost.sh` sweeps the corpus and prints a TSV: file size, `.text`, and **the sum of the `flan.*` defined symbols**,
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which is the program's own code with the runtime excluded. `COST_FLAGS=--dev` is the `SURVEY_FLAGS` precedent.
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- `bench.sh` runs four programs in `spike/x86/bench/`, each written so that one suspected cost is most of what the
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program does, because every program in `test/programs` runs in about two and a half milliseconds of which nearly
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all is `execve`.
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They live in `spike/x86/bench/`, a subdirectory, *deliberately*: `survey.sh` globs `spike/x86/*.flan`, and a
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benchmark landing in the survey would move the 97 that three handoffs quote.
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## 3. One trap, paid for once
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Two copies of `cost.sh` sharing a scratch directory silently corrupt each other's numbers. The first release sweep
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was run, killed, and restarted; `pkill` missed a child, and the survivor kept writing the same `l` and `x` files
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the new run was building and timing. The result was a row for `math` carrying `dev-locals`' binaries — which is
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visible only because those two happened to collide *exactly*, and a collision between two programs of merely
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similar size would have produced a plausible row nobody would question.
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`cost.sh` now makes a `run.$$` subdirectory with a plain `mkdir`, so a second copy cannot land in the first one's.
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The sweep behind `COST.md` was re-run clean afterwards. If you extend these scripts, keep that property.
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## 4. Nothing was changed in `lib/`, and nothing needed to be
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No finding here wants a compiler change that this lane was not allowed to make. The costs are all in
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`lib/x86.ml`'s lowering and the next lane rewriting that file will meet them; `COST.md`'s last section says which
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ones are worth its attention and which are not, which was the point of measuring rather than guessing.
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