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Emscripten is aware at compile time whether an object file will be used in a dynamic linking context, so we are within our rights to apply that flag in those situations, when doing LTO.

Hopefully this does not run into any LLVM LTO bugs...

This reduces 35% of call_indirect instructions in the Bullet benchmark (making them direct calls, some of whom end up inlined), so it may be very useful in principle.

I added a new test here, in the first commit which is before the optimization. The second commit shows the effect on that test: just a few bytes of size difference, so on simple C++ programs without large vtable hierarchies this is not expected to help much I suppose.

Unfortunately this fails many tests in LTO and ThinLTO, e.g. thinltoz.test_EXPORT_EXCEPTION_HANDLING_HELPERS crashes in wasm-ld:

wasm-ld: /b/s/w/ir/cache/builder/emscripten-releases/llvm-project/llvm/lib/MC/WasmObjectWriter.cpp:1814: uint64_t (anonymous namespace)::WasmObjectWriter::writeOneObject(MCAssembler &, const MCAsmLayout &, DwoMode): Assertion `DataLocations.count(&WS) > 0' failed.                                                                                                                    

At least it is an error in the wasm backend and not generic code.

@sbc100 any idea? Or if not, who knows WasmObjectWriter best?

Unfortunately this fails many tests in LTO and ThinLTO, e.g. thinltoz.test_EXPORT_EXCEPTION_HANDLING_HELPERS crashes in wasm-ld:

wasm-ld: /b/s/w/ir/cache/builder/emscripten-releases/llvm-project/llvm/lib/MC/WasmObjectWriter.cpp:1814: uint64_t (anonymous namespace)::WasmObjectWriter::writeOneObject(MCAssembler &, const MCAsmLayout &, DwoMode): Assertion `DataLocations.count(&WS) > 0' failed.                                                                                                                    

At least it is an error in the wasm backend and not generic code.

@sbc100 any idea? Or if not, who knows WasmObjectWriter best?

Thats probably me yes. This looks like a basic assumption of the writer if failing, but I can't imagine why. I'll try to take a look soon.