MS ABI: Use musttail for vtable thunks that pass arguments by value

[rnk]

Summary

This moves some memptr specific code into the generic thunk emission
codepath.

Fixes PR20053.

[rnk]

• Clean up the code a bit, add more test cases, fix bugs

[majnemer]

LGTM with comments addressed.

lib/CodeGen/CGVTables.cpp

260–261 ↗

(On Diff #11907)

Commit this separately.

317–319 ↗

(On Diff #11907)

SmallVector<llvm::Value *, 8> Args(CurFn->arg_begin(), CurFn->arg_end()); would be more efficient, it will reserve the "right" amount up-front.

lib/CodeGen/MicrosoftCXXABI.cpp

1470 ↗

(On Diff #11907)

CGF.EmitCallAndReturnForThunk(Callee, /*Thunk=*/nullptr);

[rnk]

Thanks!

lib/CodeGen/CGVTables.cpp

260–261 ↗

(On Diff #11907)

Sure

317–319 ↗

(On Diff #11907)

I need to take the address of the argument, so I need the for loop. :(

lib/CodeGen/MicrosoftCXXABI.cpp

1470 ↗

(On Diff #11907)

Sure.

[rnk]

Closed by commit rL214004 (authored by @rnk).