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Size Budgets ​

Browser transfer size and load time increase with each byte, so the WASM package has explicit size limits. Native Node, Python, and CLI builds are not subject to these WASM size gates. The WASM build still dictates the dependency policy for the whole core.

Glossary: size budget

A per-target byte ceiling for the built artifact. Builds that exceed the hard ceiling fail; builds that exceed the soft target print a warning to investigate. CI and the release workflow run the same check script as make size-check (tools/bench/wasm_size_report.sh).

Glossary: Brotli vs uncompressed

Uncompressed is the WASM file size on disk. Brotli is the compressed size served by a CDN configured for Brotli. Uncompressed size remains relevant for hosts that cannot serve Brotli.

TargetBudget
Uncompressed4.25 MiB soft target, 4.50 MiB hard ceiling
Brotli1216 KiB soft target, 1280 KiB hard ceiling

The uncompressed limit is checked for both the serial and pthread binaries. The Brotli limit is checked when the brotli command is available; otherwise the report marks that measure as skipped. make size-check uses the same ceilings for both builds.

What "budgeted" means in practice ​

Treat size-limit failures as product failures. Before adding an engine dependency, measure the resulting build size and decide whether the feature justifies the increase. Fixing it after the binary ships leaves the extra transfer and load cost in every release until the next change.

Reducing size ​

When the WASM build approaches the ceiling, look in order at:

  1. Linker-retained code — measure what the linker keeps. Formula families referenced by the dispatch table remain in the binary even when a workbook does not use them. The cold evaluator families (special functions, financial, FORECAST.ETS, format parsing, GROUPBY / PIVOTBY, regex, database, regression) and PCRE2 are compiled for size with bit-identical results.
  2. Dependency review — remove unused dependencies and consolidate duplicated implementations. Prefer in-tree implementations for the small set of spreadsheet-specific helpers.
  3. Build flag tuning — Emscripten optimization passes, link-time optimization, dead-code elimination. The binding glue is compiled for size while the engine stays at -O3.
  4. Public surface — every exported symbol forces the engine to keep its dependencies; consider whether an API can be internal.

Reading the build output ​

sh
make wasm wasm-threads
make size-check

size-check prints the current uncompressed and Brotli sizes and compares them against the budget. Install brotli to check both measures; the report marks Brotli as skipped when compression is unavailable. Use the pinned Emscripten toolchain before comparing local sizes with CI. Run the check locally before sending a PR and resolve failures.