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Shift the runtime in bench:runtime layouts

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The --layouts padding object was linked directly before the program
object. The runtime is linked before the program object, so every layout
moved only the program's code and left each runtime function at the same
address. A change to the runtime's code size then moved its functions in
all layouts at once, and the verdicts could not tell that placement
change from a real one.

The padding object now goes before every other input on the link line,
so each layout shifts the runtime and the program code.
This commit is contained in:
Chris Tate
2026-10-17 21:34:52 -05:00
parent f1f7302179
commit de4e10d542
1 changed files with 11 additions and 10 deletions
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scripts/bench-runtime.mjs
+11 -10
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@@ -56,9 +56,10 @@ Each checkout must have a built CLI (packages/cli/dist/bootstrap.js) and native
artifacts for this host. Without --baseline, reports absolute times only. Node is
timed in the same interleaved loop as the reference to beat (--no-node skips it).
--layouts=N (macOS/Linux) links every executable N times, shifting the program
object by 0, 16, 32, ... bytes of padding, and spreads the timed runs evenly over
those layouts, so verdicts cover code-placement luck instead of one placement.
--layouts=N (macOS/Linux) links every executable N times, shifting the runtime
and program code by 0, 16, 32, ... bytes of padding, and spreads the timed runs
evenly over those layouts, so verdicts cover code-placement luck instead of one
placement.
Use it for small codegen changes (e.g. --layouts=4 --runs=16).
Workload names are listed in benchmarks/runtime/workloads.json; for example
@@ -103,10 +104,11 @@ const work = mkdtempSync(
);
/* ── code-layout perturbation ──────────────────────────────────────────── */
// Layout k > 0 links a retained k*16-byte padding object directly before the
// program object, through a SCRIPTC_LINKER wrapper that forwards to clang (the
// default linker driver on macOS and Linux). Code after that point moves as if
// earlier code had grown; any checkout's compiler can be perturbed this way.
// Layout k > 0 links a retained k*16-byte padding object before every other
// input, through a SCRIPTC_LINKER wrapper that forwards to clang (the default
// linker driver on macOS and Linux). The runtime is linked before the program
// object, so both move as if earlier code had grown; any checkout's compiler can
// be perturbed this way.
function layoutTooling() {
if (layouts === 1) return null;
const root = join(work, "layouts");
@@ -137,12 +139,11 @@ const args = process.argv.slice(2);
const pad = process.env.SCRIPTC_BENCH_LAYOUT_PAD;
let out = args;
if (pad && !args.some((a) => a.startsWith("-print-prog-name"))) {
const at = args.findIndex((a) => basename(a).startsWith(".scriptc-native-program-object-"));
if (at < 0) {
if (!args.some((a) => basename(a).startsWith(".scriptc-native-program-object-"))) {
process.stderr.write("bench layout wrapper: no program object on the link line\\n");
process.exit(2);
}
out = ["-Wl,-u,${symbol}", ...args.slice(0, at), pad, ...args.slice(at)];
out = ["-Wl,-u,${symbol}", pad, ...args];
}
const result = spawnSync("clang", out, { stdio: "inherit" });
process.exit(result.status ?? 1);