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…position decodes one chapter JavaScriptCore finds the source position of a call site in the expression info of its function, which it decodes from the start of a chapter. A chapter was 10,000 words, so the first lookup at each of N call sites in one function cost O(N). The engine of oven-sh/WebKit#995 starts a chapter every 256 words. WEBKIT_VERSION is the preview build of that pull request: the engine that main pins (4d45755fed) plus the two commits. Move it to a commit of oven-sh/WebKit main with that change before this lands. jscInternals.expressionInfoStats() (bun:internal-for-testing) returns the words and the chapter starts of the expression info of the code that calls it. bun-jsc.test.ts checks the line of 400 call sites in a function, a generator and an async function, and that each of the three has chapters of 256 words. bundler_bytecode_portable.test.ts: 17 hashes and 17 sizes move, each output 0.02% to 0.11% larger. A chapter is 16 bytes.
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Status. Reproduced with The branch has main at e57d89b. The pin is the preview build of oven-sh/WebKit#995 at CI on f3d2005, build 137748: 129 of 188 jobs passed. The two test files that this pull request changes fail on no lane. 28 test files are red, and this change touches none of them:
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Problem
bytecode/ExpressionInfo.cpp:901). A chapter is 10,000 words, so N call sites in one function cost O(N) each.bun test --reporter=junitasks for the line of eachdescribe()call: +171,969 instructions each in a file of 2,000. No issue reports it. It is not a regression.Fix
WEBKIT_VERSIONis the preview build of [JSC] ExpressionInfo: start a chapter every 256 words. BytecodeRewriter: keep running sums WebKit#995:main's engine plus that change. Move it to a commit of oven-sh/WebKitmainwith that change before this lands.describe()calls take 13.1 ms, was 23.9 ms.bun-jsc.test.tschecks the lines of 400 call sites in a function, a generator and an async function, and their 256-word chapters.bun bd test test/js/bun/jsc/bun-jsc.test.ts(54 pass, 3 new tests fail on main).bundler_bytecode_portable.test.ts: 17 hashes and sizes re-blessed.Background
WEBKIT_VERSION(scripts/build/deps/webkit.ts) names the oven-sh/WebKit release the build links.autobuild-preview-pr-<n>-<sha>is the build of a pull request there.Downsides
bun build --bytecodeoutput grows by 16 bytes per chapter: +1,672 B (+0.02%) for TypeScript's_tsc.js.src/alone back on main, the new tests fail for the missing helper. The engine's proof is in [JSC] ExpressionInfo: start a chapter every 256 words. BytecodeRewriter: keep running sums WebKit#995.Notes
What is in the diff
scripts/build/deps/webkit.ts: the pin, the preview build of [JSC] ExpressionInfo: start a chapter every 256 words. BytecodeRewriter: keep running sums WebKit#995 at 0f1ec592ee. The old value appears nowhere else in the repo.src/jsc/bindings/JSCTestingHelpers.cpp:jscInternals.expressionInfoStats()forbun:internal-for-testing. It reads the code block of its caller, because a generator or an async function keeps its expression info in the code block of its body, which no function object gives.test/js/bun/jsc/bun-jsc.test.ts: three tests, 120 to 210 ms each on a debug ASan build.test/bundler/bundler_bytecode_portable.test.ts: the snapshot. 17jschashes and 17 sizes move and nojshash. The sizes grow by 48 to 4,488 bytes, 12,120 bytes over 44.0 MB. I ranbun installintest/first: with a staletest/node_modulesthelibraries.jsrow measures other package versions. Onmainthe old snapshot passes with the fresh install (22 pass).The snapshot was written by a release build with JavaScriptCore built from the pull request's head, and it passes on
bun bdwith the preview build that CI made (22 pass with--timeout 300000: two of its tests take 7 to 16 seconds on a debug ASan build on this machine, over the default 5 seconds).Numbers
All from oven-sh/WebKit#995, where the method is. Two release builds of Bun made the same way,
mainwith the engine at 4d45755fed and with the pull request.bun test --reporter=juniton a file of N linesdescribe("d<i>", () => {});, extra instructions perdescribe()over the run without the reporter, and the wall time of the run:The same file with released builds and with Node (
node --test --test-reporter=junit), median of 20 runs:Real test files: of the 3,543 test files in this repo, 11 have 500 or more
describe,testoritcalls.test/js/bun/xml/xml-test-suite.test.tshas the most (2,007). Under the junit reporter it goes from 99.4 ms to 97.8 ms (89.7M to 37.3M instructions over the run without the reporter).A debugger with a breakpoint: a module of N lines
x += <i>;underbun --inspect-wait, with a breakpoint by URL at the middle line.Debugger::pauseIfNeededthen asks for the line and column of each statement that runs. Instructions of the process: 1,590.8M to 155.9M at N = 5,000, 7,474.6M to 556.9M at N = 20,000 (279 ms to 69 ms with the client). A debugger that is only attached asks for no position: 291.5M before and after at N = 20,000. Without a debugger: 91.3M before and after.Binary: text of a release
bungrows by 1,344 bytes (136,784,361 to 136,785,705), the helper included. A run that asks for no position executes the same instructions:bun testof the file of 2,000 without the reporter runs 79.22M before and 79.15M after.Not in this PR
InternalSourceMap::find,src/sourcemap/InternalSourceMap.rs), which walks up to 63 mappings.bun test --aotreads call-site positions from its own table. This does not change it.Self-review
The engine change was reviewed twice in oven-sh/WebKit#995: 45 concerns raised, 41 addressed. For this half the reviews asked for the fresh install before the re-bless, for an async function and a generator in the test, and for the pin of the amended head. The snapshot did not move with that head.
no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/jsc/bun-jsc.test.ts, test/bundler/bundler_bytecode_portable.test.ts