[mono][wasi] Optional wasm-opt pass

[lewing]

wasm-opt which we already ship as part of the wasm-tools workload can be used to optimize wasi builds as well. We should have an optional step to run it as part of the build.

[ghost]

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Issue Details

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wasm-opt which we already ship as part of the wasm-tools workload can be used to optimize wasi builds as well. We should have an optional step to run it as part of the build.

Author:	lewing
Assignees:	-
Labels:	arch-wasm, untriaged, area-VM-meta-mono, os-wasi
Milestone:	-

[RReverser]

Note that you (.NET) are currently using Clang for linking, which already implicitly runs wasm-opt if 1) it's detected on the PATH and 2) linker optimisation is enabled.

It can be enabled via e.g. this in .csproj:

<_WasiSdkClangArgs Include="-O2" />

The problem is that currently somewhere in the pipeline .NET either doesn't emit correct target_features custom section, or maybe strips it. That secion is important for any WebAssembly tools that want to modify the binary as otherwise they don't know which features they can legally use without breaking compatibility. Correspondingly, when enabling link-time optimisation like above, wasm-opt fails validation with errors like:

  Performing WASI SDK build: "C:\Users\me\.wasi-sdk\wasi-sdk-20\bin\clang.exe" "@..."
  [wasm-validator error in function 5099] unexpected false: Bulk memory operations require bulk memory [--enable-bulk-memory], on 
  (memory.copy
   (local.get $0)
   (local.get $1)
   (local.get $2)
  )
  [wasm-validator error in function 5100] unexpected false: Bulk memory operations require bulk memory [--enable-bulk-memory], on
  (memory.copy
   (local.get $0)
   (local.get $1)
   (local.get $2)
  )
  [wasm-validator error in function 5101] unexpected false: Bulk memory operations require bulk memory [--enable-bulk-memory], on
  (memory.fill
   (local.get $0)
   (local.get $1)
   (local.get $2)
  )
  Fatal: error validating input
clang : error : linker command failed with exit code 1 (use -v to see invocation)

That is, it complains because Wasm uses bulk memory operations but doesn't have a target_features section that allows them.

I guess I could report this as a separate issue, but it seems most relevant to this one as it's pretty much the only blocker to using wasm-opt and other similar tools.

It's possible to work around that by doing something like wasm-opt -all for now but that's potentially dangerous because it allows wasm-opt to use arbitrary features regardless of whether our target engines support them.

[lewing]

I'm not sure why the section isn't being generated will take a look. At the moment --enable-bulk-memory and --enable-simd (If you pass WasmEnableSIMD) are the only post MVP features in the wasi build.

[RReverser]

At the moment --enable-bulk-memory and --enable-simd (If you pass WasmEnableSIMD) are the only post MVP features in the wasi build.

Yeah they're currently passed explicitly, but the thing is that explicitly enabling features from command line really shouldn't be necessary if target_features is preserved correctly.

[pavelsavara]

Note that because we now produce WASI components, wasm-opt can't be used on those until WebAssembly/binaryen#6728