[SR-15399] async function pointer handling does not account for GOT/IAT

[compnerd]

Previous ID	SR-15399
Radar	None
Original Reporter	@compnerd
Type	Bug
Status	Closed
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, Windows
Assignee	@compnerd
Priority	Medium

md5: 60d118265670b2fd126a9cf46ade5005

Issue Description:

Cross-module async function pointer references are made directly without consulting the GOT. This results in an unsatisfiable link request on Windows.

// %swiftc -emit-module -emit-library -o L.dll %s -DL
// %swiftc -emit-executable -o E.exe %s -I. -lL

#if L
open class C {
 open func f() async throws {}
}
#else
import L
class D: C {
}
#endif

When linking we observe the following:
reduced-e4271c.o : error LNK2001: unresolved external symbol $s1L1CC1fyyYaKFTu

Due to the unresolved symbol reference.

The full type metadata for D is emitted thusly:

$s4main1DCMf:
        .quad   ($s4main1DCfD)
        .quad   0
        .quad   0
        .quad   0
        .long   2
        .long   0
        .long   16
        .short  7
        .short  0
        .long   88
        .long   16
        .quad   ($s4main1DCMn)
        .quad   0
        .quad   ($s1L1CC1fyyYaKFTu)
        .quad   ($s1L1CCACycfC)

Because the member is not overridden, the async function pointer reference is emitted directly. However, because the symbol is module external, it needs to be indirected on Windows via the IAT. The address of the variable is not known at compile time and will need to be resolved at runtime.

[compnerd]

#39998

Thanks to @DougGregor @aschwaighofer for the help with pointers and brain storming