[wasm-split] Increase in wasm initialization time after split

[ravisumit33]

Please include the following in your bug report:

Version of emscripten/emsdk:

emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 3.1.35 (a90c5abb489c516f59c0d16864d953d4bedd37da)
clang version 17.0.0 (https://github.com/llvm/llvm-project 6865cff8ea8b07d9f2385fd92cecb422404f0f35)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: ~/3.1.35/emsdk/upstream/bin

We have recently integrated module splitting in our code base with the aim of improving wasm download time. We were able to get primary module which is around 1 MB less in size that of the original module (8.5 MB). This led to decrease in wasm download time but the time after wasm download till the first line of main function is hit, has increased. This increase ranges from 10-20 ms. Is this increase expected? Does this have something to do with increase in the size of indirect table in primary split?

[tlively]

Hmm, 10-20ms does seem high. The only extra work the primary module should be doing on instantiation should be placing all the imports for the secondary functions into the table. @jkummerow, do you know if putting imports into a table via an active element segment is a particularly expensive operation? @ravisumit33, are you able to share how many imports of secondary functions your primary module has?

[ravisumit33]

@tlively Primary module has total of 506 placeholders for functions in secondary modules.

[tlively]

Interesting, that doesn't seem like a high enough number that it could account for an extra 10-20ms of work. I wonder if you could get any extra insight into where the extra time is coming from using the browser's profiling tools.