Allow disabling wasm-opt while keeping clang optimizations

[anlavandier]

The webassembly optimizer wasm-opt doesn't support every proposal that clang/lld supports. This can lead to case where an option that is valid and recognized by emcc fails at the optimization stage because wasm-opt doesn't know of/support a specific feature.
For example, the custom page size proposal is support by LLVM since at least LLVM 21.0 but isn't supported by wasm-opt as of right now; see WebAssembly/binaryen#7694.
Allowing the wasm-opt step to be skipped would allow users to still get the clang optimizations in such cases and leave them the possibility of further optimizing the resulting wasm using either a newer version of wasm-opt or the competing optimizer from wasm-tools

[sbc100]

In most cases binaryen is very much on the cutting edge of WebWassembly features, whereas clang/wasm-ld is normally lagging behind.

So I'm hoping this issue with custom pages sizes is really just an anomaly, and the best fix here would be to land WebAssembly/binaryen#7694.

In the short term you maybe be able to work around this by not passing the -O2 / -O3 / etc flag to emcc at link time (while still passing it at compile time).

[anlavandier]

I'd be happy to, but how do I that ?

[kripken]

As @sbc100 said, avoid linking with -O2 or above. Linking with -O1 or below will not run binaryen optimizations.

So compile with full opts, and in the link command use emcc -O1 and nothing higher.

[kleisauke]

FWIW, Clang does support the --no-wasm-opt linker flag, see:
llvm/llvm-project#55781

However, I'm not sure whether Emscripten supports it.

[sbc100]

Ah good idea we could indeed try to support that flag. It seems a bit of a niche use case though, to pass -O3 + -no-wasm-opt to the linker, and would be just another config that we need to test. We also use wasm-opt and binaryen tools for things other than optimization, for example if you select LEGACY_VM_SUPPORT then we use binaryen for lowering? How should we handle --no-wasm-opt in that case?