Add a --symbolicate-wasm arg to profiler-edit. by mstange · Pull Request #6008 · firefox-devtools/profiler

mstange · 2026-05-07T19:29:06Z

This allows applying wasm symbols to existing profiles that were captured with a stripped wasm bundle.

The script looks for functions with names of the shape wasm-function[123], which is what Firefox uses when the wasm file doesn't have a names section.

Usage:

yarn build-node-tools && \
node node-tools-dist/profiler-edit.js -i input.json.gz \
--symbolicate-wasm http://host/a.wasm=./a-unstripped.wasm \
--symbolicate-wasm http://host/b.wasm=./b-unstripped.wasm \
-o out.json.gz

I've successfully used it to turn https://share.firefox.dev/4f9O7oh into https://share.firefox.dev/4nd6GKk .

codecov · 2026-05-07T19:33:41Z

Codecov Report

❌ Patch coverage is 74.45255% with 35 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.78%. Comparing base (5783a76) to head (a8608c4).
⚠️ Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
src/profile-logic/wasm-symbolication.ts	84.21%	17 Missing and 1 partial ⚠️
src/node-tools/profiler-edit.ts	26.08%	17 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6008      +/-   ##
==========================================
- Coverage   83.82%   83.78%   -0.04%     
==========================================
  Files         328      329       +1     
  Lines       34254    34391     +137     
  Branches     9572     9607      +35     
==========================================
+ Hits        28712    28814     +102     
- Misses       5114     5148      +34     
- Partials      428      429       +1

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This allows applying wasm symbols to existing profiles that were captured with a stripped wasm bundle. The script looks for functions with names of the shape `wasm-function[123]`, which is what Firefox uses when the wasm file doesn't have a names section. Usage: ``` yarn build-node-tools && \ node node-tools-dist/profiler-edit.js -i input.json.gz \ --symbolicate-wasm http://host/a.wasm=./a-unstripped.wasm \ --symbolicate-wasm http://host/b.wasm=./b-unstripped.wasm \ -o out.json.gz ```

canova

Nice, thanks for adding some tests! I added a bunch of nits to make things a bit clearer, but otherwise I don't see any issues.

This also made me look into the wasm spec a bit and learn more about it, so I think it was a good use of time 😄

This also made me think more about the source map support that I've been working on. These 2 things seem like solving it for 2 different use cases, so it seems very interesting. I didn't know about the name section in the wasm spec. I wonder how devtools handles it (or if it does). It looks like it's mostly looking at the dwarf info for now, but need to look deeper.

canova · 2026-05-08T13:24:14Z

+      - name: Upload node-tools-dist artifact
+        if: matrix.os == 'ubuntu-latest'
+        uses: actions/upload-artifact@v7
+        with:
+          name: node-tools-dist
+          path: node-tools-dist/
+


Oh, that's a good idea. It makes me think if we should do the same for the profiler-cli.

canova · 2026-05-08T13:50:18Z

+  path: string;
+  url?: string;


I don't immediately understand which one is from the stripped one and which one is from unstripped. How about strippedWasmUrl / unstrippedWasmPath (or symbolsWasmPath) or something?

canova · 2026-05-08T13:52:16Z

+  const symbolicateWasm: WasmSymbolicationCliSpec[] = [];
+  const rawWasmArg = argv['symbolicate-wasm'];
+  let wasmArgs: unknown[];
+  if (rawWasmArg === undefined) {
+    wasmArgs = [];
+  } else if (Array.isArray(rawWasmArg)) {
+    wasmArgs = rawWasmArg;
+  } else {
+    wasmArgs = [rawWasmArg];
+  }
+  for (const arg of wasmArgs) {
+    if (typeof arg !== 'string' || arg === '') {
+      throw new Error('--symbolicate-wasm requires a value');
+    }
+    // Accept "<url>=<path>" if the LHS looks like a URL, otherwise treat the
+    // whole string as a path and infer the URL from the profile. Split on
+    // the last `=` so URLs containing `=` (e.g. in query strings) survive
+    // intact; this assumes file paths don't contain `=`.
+    const eqIndex = arg.lastIndexOf('=');
+    if (eqIndex !== -1 && /^[a-z]+:\/\//i.test(arg.slice(0, eqIndex))) {
+      symbolicateWasm.push({
+        url: arg.slice(0, eqIndex),
+        path: arg.slice(eqIndex + 1),
+      });
+    } else {
+      symbolicateWasm.push({ path: arg });
+    }
+  }


Not for this PR, but now that we have commander in the package.json, we can possibly simplify these things by using it.

canova · 2026-05-08T13:53:22Z

+    // Accept "<url>=<path>" if the LHS looks like a URL, otherwise treat the
+    // whole string as a path and infer the URL from the profile. Split on
+    // the last `=` so URLs containing `=` (e.g. in query strings) survive
+    // intact; this assumes file paths don't contain `=`.


Can we add a comment to the WasmSymbolicationCliSpec type about this wasn't clear to me at first.

canova · 2026-05-08T13:55:26Z

    profile.meta.symbolicated = true;
  }

+  if (options.symbolicateWasm !== undefined) {


Nit: We construct symbolicateWasm as an empty array, so it should never be undefined.

canova · 2026-05-08T15:13:15Z

+// Parses the function-name subsection of a wasm "name" custom section and
+// returns a map from function index to name. Returns an empty map if the
+// module has no name section. The function index space includes imports
+// (imports come first) — same numbering used in `wasm-function[N]` strings.


Oh can we add a link to the Firefox source code where we generate this N number?

canova · 2026-05-10T18:30:46Z

+    const namesByIndex = parseWasmFunctionNames(spec.bytes);
+    if (namesByIndex.size === 0) {
+      throw new Error(
+        `${tag} has no function names — is this an unstripped wasm file?`


Nit: ${tag}: would be better for consistency. (the others seem to be that way)

canova · 2026-05-11T15:14:47Z

+      continue;
+    }
+    const sectionName = readName();
+    if (sectionName !== 'name') {


Oh I see, so this looks for the "name" section and the creates a map of the names. But this is not a dwarf info. I found some info here, maybe we can put it a link here for future reference: https://webassembly.github.io/spec/core/appendix/custom.html#name-section

canova · 2026-05-11T15:16:02Z

+      const subId = bytes[pos++];
+      const subSize = readVarUint();
+      const subEnd = pos + subSize;
+      if (subId === 1) {


It would be good to demystify 1 here, maybe we can create a constant called FUNC_NAMES_SUB_ID.

canova · 2026-05-11T15:23:27Z

+    const sectionId = bytes[pos++];
+    const sectionSize = readVarUint();
+    const sectionEnd = pos + sectionSize;
+    if (sectionId !== 0) {


Can we also demystify this section id by adding a const like CUSTOM_SECTION_ID? And possibly link: https://webassembly.github.io/exception-handling/core/binary/modules.html#sections

mstange force-pushed the symbolicate-wasm-script branch from 52ba22e to ab1e4b6 Compare May 7, 2026 19:31

mstange force-pushed the symbolicate-wasm-script branch 3 times, most recently from 0202adf to be15b28 Compare May 7, 2026 19:49

mstange requested a review from canova May 7, 2026 19:49

mstange force-pushed the symbolicate-wasm-script branch from be15b28 to 2b6584b Compare May 7, 2026 19:50

Allow downloading profiler-node-tools zip files from PRs.

a8608c4

mstange force-pushed the symbolicate-wasm-script branch from a70577c to a8608c4 Compare May 7, 2026 21:54

canova approved these changes May 11, 2026

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Add a --symbolicate-wasm arg to profiler-edit.#6008

Add a --symbolicate-wasm arg to profiler-edit.#6008
mstange wants to merge 2 commits into
firefox-devtools:mainfrom
mstange:symbolicate-wasm-script

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mstange commented May 7, 2026 •

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