How can I disable sign-extension opcode

[cardinalblues7]

Hi I'm currently working on a project which has a custom wasm loader, it does not have support for sign-extension.When I compiled my code to wasm and instantiate with its loader, it showed error message like : invalid opcode 192, which pointed to sign-extension. After some googling I found that I could add -mno-sign-ext to disable sign extension.But when I do, the generated wasm still has opcode like i32.extends_8. My question is , how do I completely disable this family of the opcodes, or at least get a warning about its location. Thank you in advance.

[tlively]

If you're already using -mno-sign-ext and that's not working, then that sounds like a bug. Was the error with that flag pointing to the exact same instruction, or was it pointing to a different instruction?

Another flag you can try is -mcpu=mvp, which will disable all new features.

[sbc100]

I don't think its enough to pass -mno-sign-ext right now since that will only effect the code generation for the code being compiled. It won't effect things like libc and other libraries that are not compiled using your custom flags.

Instead we rely on the --signext-lowering binaryen pass to lower away the sign extension operations at link time:

emscripten/emcc.py

Lines 649 to 653 in a90c5ab

	# sign-ext is enabled by default by llvm. If the target browser settings don't support
	# this we lower it away here using a binaryen pass.
	if not feature_matrix.caniuse(feature_matrix.Feature.SIGN_EXT):
	logger.debug('lowering sign-ext feature due to incompatiable target browser engines')
	passes += ['--signext-lowering']

The only way that gets triggered today (IIUC) is if you set you min browser versions to include a browser that doesn't support sign extension. For example, you could pass -sMIN_CHROME_VERSION=73. I guess we should also add a way to explicity opt out of features, rather than only implicitly opting out via targeting older browsers.

[sbc100]

@kripken pointed out that you can use -sLEGACY_VM_SUPPORT to disable all non-mvp features.

I have exactly the same problem and cannot get rid of the opcode 192 (which is part of signext as far as I understand).
wasm-opt is still called with "enable-sign-ext" which I don't want at all.

using "-mcpu=mvp" "-mno-sign-ext" for compile and link options
and those link options
"-sLEGACY_VM_SUPPORT",
"-sMIN_CHROME_VERSION=73",
"-sMIN_SAFARI_VERSION=110000",

leads to:
wasm-opt --strip-dwarf --signext-lowering --post-emscripten -O3 --low-memory-unused --zero-filled-memory --pass-arg=directize-initial-contents-immutable --strip-debug --strip-producers out/wasm/printess.wasm -o out/wasm/printess.wasm --mvp-features --enable-mutable-globals --enable-sign-ext

which outputs still i32.extend8_s and many more.

Any more ideas to try?

[sbc100]

Sounds like a bug in --signext-lowering. Any chance you could attach the input file (out/wasm/printess.wasm) so we can take a look?

Sure: https://printess-dev.s3.eu-central-1.amazonaws.com/printess.wasm

I added a post build "wasm-opt out/wasm/printess.wasm -o out/wasm/printess.wasm --signext-lowering" now to lower it.
That works fortunately :).

[tlively]

The problem is that the signext-lowering pass isn't actually disabling the sign extension feature, so it is undone by later optimize-instructions passes due to -O3. I can fix it in Binaryen.

[sbc100]

It looks like we can also fix by ensuring that --signext-lowering is the final argument on the command line.. that seems to work. Should we just do that?

[tlively]

I would consider this a real bug in Binaryen, so I'll fix it either way. Given that, it seems better to keep passing --signext-lowering first so that the lowering can potentially be optimized later.

[tlively]

WebAssembly/binaryen#5676

[Alexufo]

It's the same. Is there any news?

[sbc100]

@Alexufo what version of emscripten are you using? Can you share a repro case? Our understanding is that this bug is now fixed.

[Alexufo]

@sbc100 oh! It works on 3.1.38!

[sbc100]

Awesome. I guess we can close this issue then?