[wasm][IRGen] Stop attaching COMDATs to dead stub methods #85106
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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:
```mermaid
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
```
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
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compnerd
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| // Note: Windows uses internal linkage for dead stub methods, which doesn't | ||
| // use linkonce_odr or COMDAT groups. |
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Can you explain this a bit? Windows definitely uses COMDATs rather aggressively.
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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.
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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 endWhen 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.deadandC2.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
supportsCOMDATreturns false, and COFF targets don't uselinkonce_odrfor dead stub methods.The COMDAT was added in 7e8f782, and it made swift-format build for Wasm to fail