[wasm][IRGen] Stop attaching COMDATs to dead stub methods

[kateinoigakukun]

When generating dead stub methods for vtable entries, we should not attach COMDATs to their definitions. This is because such stubs may be referenced only through aliases, and the presence of a COMDAT on the stub definition would cause the aliased symbol, we want to keep, to be discarded from the symbol table when other object files also have a dead stub method.

Given the following two object files generated:

graph TD
    subgraph O0[A.swift.o]
        subgraph C0[COMDAT Group swift_dead_method_stub]
            D0_0["[Def] swift_dead_method_stub"]
        end
        S0_0["[Symbol] swift_dead_method_stub"] --> D0_0
        S1["[Symbol] C1.dead"] --alias--> D0_0
    end

    subgraph O1[B.swift.o]
        subgraph C1[COMDAT Group swift_dead_method_stub]
            D0_1["[Def] swift_dead_method_stub"]
        end
        S0_1["[Symbol] swift_dead_method_stub"] --> D0_1
        S2["[Symbol] C2.beef"] --alias--> D0_1
    end

Loading

When linking these two object files, the linker will pick one of the COMDAT groups of swift_dead_method_stub. The other COMDAT group will be discarded, along with all symbols aliased to the discarded definition, even though those aliased symbols (C1.dead and C2.beef) are the ones we want to keep to construct vtables.

This change stops attaching COMDATs to dead stub method definitions. This effectively only affects WebAssembly targets because MachO's supportsCOMDAT returns false, and COFF targets don't use linkonce_odr for dead stub methods.

The COMDAT was added in 7e8f782, and it made swift-format build for Wasm to fail

[kateinoigakukun]

@swift-ci smoke test

[kateinoigakukun]

@swift-ci smoke test

[compnerd]

Can you explain this a bit? Windows definitely uses COMDATs rather aggressively.

[kateinoigakukun]

Maybe I wasn’t clear, in this case, IRGenModule::emitVTableStubs uses internal linkage instead of linkonce_odr only for COFF, so we don’t attach COMDATs in ApplyIRLinkage::to in the first place.