Reduce CppInterOp Emscripten shared library size

[mcbarton]

Description

Please include a summary of changes, motivation and context for this PR.

@vgvassilev This change locally reduced the Emscripten shared library size to 47/48 Mb (depends of whether you use dir -lh or du -lh). It still passed all tests and xeus-cpp still could run all the notebook cells it could before (exception throwing is broken in CppInterOps deployment at the moment for some reason). I have cleared the cache of all llvm 20 Emscripten builds for this PR.

Fixes # (issue)

Type of change

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• Bug fix
• New feature
• Requires documentation updates

Testing

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Checklist

• I have read the contribution guide recently

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.18%. Comparing base (2585d0d) to head (de7101c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #655   +/-   ##
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  Coverage   79.18%   79.18%           
=======================================
  Files           9        9           
  Lines        3853     3853           
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  Hits         3051     3051           
  Misses        802      802           

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[anutosh491]

It's a compile time optimization flag I think.

Do we need to force it like this everywhere or can we have cmake handle this ?

[mcbarton]

It's a compile time optimization flag I think.

Do we need to force it like this everywhere or can we have cmake handle this ?

It is both a link and compile time optimisation. See https://emscripten.org/docs/tools_reference/emcc.html#emcc-oz .

[anutosh491]

Yeah but we still don't want users to look into/pass flags correct during the build correct?

We should be able to have cmake handle this.

[mcbarton]

Yeah but we still don't want users to look into/pass flags correct during the build correct?

We should be able to have cmake handle this.

I really don't understand the point your trying to make. Using EMCC_CFLAGS in this way to perfectly legitimate way of doing the Emscripten build. Emscripten is designed to be effected by modifying this flag (see https://emscripten.org/docs/tools_reference/emcc.html#environment-variables). If a user changes what we have in the documentation for this flag, and messes something up, then that is on them. I don't understand how putting it in a cmake option changes that.

[anutosh491]

My point is simple. We already have compile time and link time flags being passed through cmake, I don't see a need to pass this externally.

[vgvassilev]

I think what @anutosh491 means is that we need to make this flag as part of our default compiler flags in the CMakeLists.txt.

[mcbarton]

@vgvassilev @anutosh491 I first tried locally using -DCMAKE_CXX_FLAGS="-Dwait4=__syscall_wait4 -Oz" -DCMAKE_C_FLAGS="-Oz" -DCMAKE_STATIC_LINKER_FLAGS="-Oz" (since I thought this might be the closest thing to EMCC_CFLAGS) and the llvm build crashed 15% into the build. I then tried -DCMAKE_CXX_FLAGS="-Dwait4=__syscall_wait4 -Oz" -DCMAKE_C_FLAGS="-Oz" which although I could build llvm with this option, had no effect on the size of CppInterOps shared library, or the llvm libraries. I do not know how to proceed now. If you have any other cmake options you would like me to try, let me know. If you don't, I think we should proceed using EMCC_CFLAGS.

[mcbarton]

Putting a note here in case I forget. Using EMCC_CFLAGS="-flto" in the llvm build doesn't end up effecting the shared library size on its own. Used in conjuncation with Oz then the size falls slightly to 45/46 Mb.

[mcbarton]

I have updated this PR to use the flto flag as well, and to compile+link CppInterOp shared library with these flags too. This reduces the shared library size to 44/45 Mb.