Status of synchronous WASM compilation limitation in main thread (2026/07)

function uleb(n) {
  const out = [];
  do {
    let b = n & 0x7f;
    n = Math.floor(n / 128);
    if (n) b |= 0x80;
    out.push(b);
  } while (n);
  return out;
}

function paddedModule(size) {
  const bytes = new Uint8Array(size);
  bytes.set([0, 97, 115, 109, 1, 0, 0, 0]); // wasm header
  bytes[8] = 0; // custom section

  let payload = size - 10;
  while (9 + uleb(payload).length + payload !== size) {
    payload += size - (9 + uleb(payload).length + payload);
  }

  const length = uleb(payload);
  bytes.set(length, 9);
  bytes[9 + length.length] = 0; // empty custom-section name
  return bytes;
}

for (const size of [
  8 * 1024 * 1024,
  8 * 1024 * 1024 + 1,
  16 * 1024 * 1024,
  64 * 1024 * 1024,
]) {
  try {
    const module = new WebAssembly.Module(paddedModule(size));
    new WebAssembly.Instance(module);
    console.log(size, "OK");
  } catch (error) {
    console.log(size, error.toString());
  }
}

NumPy 2.4.3
2,154,517  numpy/_core/_multiarray_umath.so
1,056,155  numpy/linalg/_umath_linalg.so
  490,960  numpy/random/_generator.so
  389,101  numpy/random/mtrand.so
  260,256  numpy/fft/_pocketfft_umath.so
  207,529  numpy/linalg/lapack_lite.so
  138,648  numpy/random/_bounded_integers.so
  134,521  numpy/random/_common.so
  114,584  numpy/random/bit_generator.so
   58,123  numpy/random/_pcg64.so
   56,655  numpy/random/_mt19937.so
   53,558  numpy/random/_philox.so
   38,026  numpy/random/_sfc64.so
SciPy 1.18.0
6,456,081  scipy.libs/libopenblas.so
2,758,624  scipy/sparse/_sparsetools.so
2,377,843  scipy/optimize/_highspy/_core.so
1,419,071  scipy/special/_ufuncs_cxx.so
1,386,725  scipy/linalg/_flapack.so
  947,359  scipy/special/cython_special.so
  913,909  scipy/special/_special_ufuncs.so
  805,670  scipy/fft/_duccfft/pyduccfft.so
  705,946  scipy/io/_fast_matrix_market/_fmm_core.so
  561,884  scipy/spatial/_qhull.so
  545,850  scipy/special/_ufuncs.so
  480,765  scipy/special/_gufuncs.so
  405,566  scipy/linalg/_fblas.so
  367,781  scipy/linalg/_decomp_interpolative.so
  365,615  scipy/stats/_unuran/unuran_wrapper.so
  260,121  scipy/spatial/_ckdtree.so
  242,687  scipy/spatial/transform/_rotation_cy.so
  231,406  scipy/stats/_stats.so
  223,296  scipy/sparse/_csparsetools.so
  216,584  scipy/spatial/_distance_pybind.so
  203,624  scipy/sparse/csgraph/_shortest_path.so
  186,861  scipy/sparse/linalg/_dsolve/_superlu.so
  173,155  scipy/sparse/csgraph/_traversal.so
  162,387  scipy/optimize/_highspy/_highs_options.so
  150,585  scipy/linalg/cython_lapack.so
  147,497  scipy/linalg/_batched_linalg.so
  147,226  scipy/linalg/_decomp_update.so
  131,947  scipy/odr/__odrpack.so
  123,622  scipy/spatial/transform/_rigid_transform_cy.so
  122,553  scipy/sparse/linalg/_eigen/arpack/_arpacklib.so
  121,661  scipy/cluster/hierarchy/_hierarchy.so
  119,757  scipy/interpolate/_interpnd.so
  111,485  scipy/interpolate/_fitpack.so
  110,042  scipy/io/matlab/_mio5_utils.so
  103,251  scipy/linalg/_cythonized_array_utils.so
   99,335  scipy/special/_specfun.so
   97,671  scipy/interpolate/_ppoly.so
   96,634  scipy/interpolate/_rbfinterp_pythran.so
   95,854  scipy/sparse/csgraph/_tools.so
   94,811  scipy/optimize/_trlib/_trlib.so
   93,021  scipy/optimize/_bglu_dense.so
   93,010  scipy/signal/_upfirdn_apply.so
   86,301  scipy/sparse/csgraph/_matching.so
   85,685  scipy/stats/_sobol.so
   85,072  scipy/stats/_biasedurn.so
   83,937  scipy/optimize/_pava_pybind.so
   80,920  scipy/ndimage/_nd_image.so
   80,426  scipy/sparse/csgraph/_flow.so
   79,516  scipy/ndimage/_ni_label.so
   78,631  scipy/_cyutility.so
   75,398  scipy/sparse/csgraph/_reordering.so
   74,278  scipy/integrate/_quadpack.so
   71,940  scipy/cluster/hierarchy/_optimal_leaf_ordering.so
   59,781  scipy/linalg/cython_blas.so
   57,701  scipy/signal/_peak_finding_utils.so
   56,861  scipy/optimize/_moduleTNC.so
   56,589  scipy/linalg/_internal_matfuncs.so
   56,564  scipy/io/matlab/_streams.so
   56,213  scipy/sparse/linalg/_propack.so
   53,251  scipy/linalg/_solve_toeplitz.so
   52,259  scipy/integrate/_vode.so
   52,070  scipy/cluster/vq/_vq.so
   51,853  scipy/fftpack/convolve.so
   50,983  scipy/interpolate/_rgi_cython.so
   49,628  scipy/signal/_sosfilt.so
   49,497  scipy/signal/_sigtools.so
   48,249  scipy/stats/_qmc_cy.so
   47,473  scipy/stats/_stats_pythran.so
   47,398  scipy/stats/_qmvnt_cy.so
   45,816  scipy/special/_ellip_harm_2.so
   45,577  scipy/interpolate/_dierckx.so
   45,347  scipy/stats/_ansari_swilk_statistics.so
   43,951  scipy/sparse/csgraph/_min_spanning_tree.so
   43,212  scipy/linalg/_matfuncs_sqrtm_triu.so
   42,146  scipy/_lib/_ccallback_c.so
   41,866  scipy/stats/_rcont/rcont.so
   40,553  scipy/optimize/cython_optimize/_zeros.so
   38,586  scipy/spatial/_hausdorff.so
   38,261  scipy/optimize/_minpack.so
   38,193  scipy/linalg/_linalg_pythran.so
   38,089  scipy/_lib/_uarray/_uarray.so
   36,467  scipy/spatial/_voronoi.so
   36,225  scipy/ndimage/_cytest.so
   36,090  scipy/special/_test_internal.so
   33,211  scipy/_lib/messagestream.so
   32,833  scipy/integrate/_odepack.so
   29,327  scipy/io/matlab/_mio_utils.so
   25,605  scipy/optimize/_lsq/givens_elimination.so
   24,959  scipy/stats/_levy_stable/levyst.so
   23,984  scipy/optimize/_slsqplib.so
   23,868  scipy/spatial/_distance_wrap.so
   23,092  scipy/optimize/_group_columns.so
   22,725  scipy/special/_comb.so
   21,574  scipy/optimize/_lbfgsb.so
   19,533  scipy/signal/_spline.so
   17,656  scipy/optimize/_direct.so
   16,934  scipy/_lib/_test_deprecation_call.so
   15,644  scipy/integrate/_dop.so
   14,658  scipy/optimize/_lsap.so
   13,781  scipy/signal/_max_len_seq_inner.so
   10,171  scipy/ndimage/_rank_filter_1d.so
    8,500  scipy/_lib/_test_deprecation_def.so
    7,539  scipy/optimize/_zeros.so
    4,461  scipy/_lib/_test_ccallback.so
    1,972  scipy/ndimage/_ctest.so
    1,222  scipy/integrate/_test_multivariate.so
      747  scipy/_lib/_fpumode.so