Define Linking ABI for common symbols

[dschuff]

As discussed in llvm/llvm-project#196641

So far there is not support for initialized common symbols.

[dschuff]

I have an implementation which I will post shortly.

@sbc100 This version uses the combined bitmask of BINDING_GLOBAL and BINDING_WEAK (which are mutually exclusive) to mean BINDING_COMMON. Do you think this is better than just allocating another bit at the end?

[sbc100]

Combining BINDING_GLOBAL and BINDING_WEAK makes some logical sense to me.

But I don't quite understand since WASM_SYM_BINDING_GLOBAL is zero how can you know if its included in the bitmask along with WASM_SYM_BINDING_WEAK?

[dschuff]

Sorry, I meant the combination of BINDING_LOCAL (0b01) and BINDING_WEAK(0b10), means BINDING_COMMON (0b11). Currently LOCAL is exclusive with WEAK (explicitly disallowed in the current doc), and of course both of those are exclusive with GLOBAL by the construction of the encoding.

[sbc100]

Yes, I suppose LOCAL + WEAK is less intuitive.. is it hard to add another bit to the encoding?

[dschuff]

No, I don't think it would be. With the current formulation, the first 2 bits encode one of 4 possible bindings, which become mutually exclusive by construction. That's nice; but the downside is that these are no longer one-bit "flags" in the same sense that the rest of the bits are now. And of course if we ever add another binding, then it will be even weirder, since we'll have these 4 possibilities, plus another bit.