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WebAssembly / tool-conventions

Conventions supporting interoperatibility between tools working with WebAssembly.
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Mention elemidx and dataidx in the Linking.md documentation #193

Open matovitch opened 1 year ago

matovitch commented 1 year ago

Hello,

I am writing a wasm backend for my compiler and I was planning to output relocatable wasm modules.

In the Linking.md document, there is no mention of dataidx and elemidx renumbering such that we would need to pad the LEB128 encodings of these to 5 bytes. I believe this should be new relocation types but these do not currently exist because LLVM does not support the element section or generate any memory.init or data.drop instruction.

I lost quite a bit of time wondering why the documentation did not address this issue. So here are my questions:

First, did I understood the problem properly and there is no mention of data or element segments reordering because these are not generated/supported by LLVM/lld? Second, if this is correct, can I submit a PR updating the markdown documentation?

Thank you for your help. :)

sbc100 commented 1 year ago

Yes, llvm doesn't have any support for generating first class data or element segments, so there are no relocation types for these, and no support in the linker.

If you would like to support such things a good place to start would be looking at adding support to lld (wasm-ld), and most likely the llvm assembly format (.s format).

BTW, is the existing support for data symbols not sufficient for your language? Do you really need first class data segments and element segments? I'd like to understand your use case here.

matovitch commented 1 year ago

No it is probably sufficient for my language (it is sufficient for C++ after all ^^). I implemented the complete basic spec to generate non relocatable modules and was wondering how to adapt my code for instructions like data.drop, elem.drop, memory.init, table.init...etc. I can just avoid generating these instructions in the first place. But a mention in the documentation explaining why these types do not exist would have saved me a bit of time wondering why these were missing.

sbc100 commented 1 year ago

Yes, there is currently no support for data.drop, elem.drop, memory.init, table.init, etc in the linkable format. We should add a "limitations" section I guess.