Avoid illegal function pointer operation for async funcs

[kateinoigakukun]

When compiling Concurrency module targeting wasm32, it fails with Function addresses with offsets not supported.
It's a limitation of wasm32 for security reason. In wasm32, function and data address spaces are separated, so additive operation to function address as data pointer is invalid. Therefore if it's statically known to be an additive operation to function address, LLVM fails with the above error.

<unknown>:0: note: Function addresses with offsets not supported
Please submit a bug report (https://swift.org/contributing/#reporting-bugs) and include the project and the crash backtrace.
Stack dump:
0.      Program arguments: /Users/kateinoigakukun/projects/swiftwasm-source/host-build/Ninja-Release/swift-macosx-x86_64/bin/swift-frontend -frontend -c /Users/kateinoigakukun/projects/swiftwasm-source/swift/stdlib/public/Concurrency/Actor.swift /Users/kateinoigakukun/projects/swiftwasm-source/swift/stdlib/public/Concurrency/PartialAsyncTask.swift /Users/kateinoigakukun/projects/swiftwasm-source/swift/stdlib/public/Concurrency/Task.swift /Users/kateinoigakukun/projects/swiftwasm-source/swift/stdlib/public/Concurrency/TaskCancellation.swift /Users/kateinoigakukun/projects/swiftwasm-source/swift/stdlib/public/Concurrency/_TimeTypes.swift /Users/kateinoigakukun/projects/swiftwasm-source/swift/stdlib/public/Concurrency/TaskGroup.swift -supplementary-output-file-map /var/folders/br/7sr_p7cj7v549x_gtkb33df00000gp/T/supplementaryOutputs-aa56e1 -disable-objc-attr-requires-foundation-module -target wasm32-unknown-wasi -disable-objc-interop -sdk /Users/kateinoigakukun/projects/swiftwasm-source/wasi-sdk/share/wasi-sysroot -I /Users/kateinoigakukun/projects/swiftwasm-source/target-build/swift-stdlib-wasi-wasm32/./lib/swift/wasi -warn-implicit-overrides -enable-library-evolution -module-cache-path /Users/kateinoigakukun/projects/swiftwasm-source/target-build/swift-stdlib-wasi-wasm32/./module-cache -module-link-name swift_Concurrency -parse-stdlib -resource-dir /Users/kateinoigakukun/projects/swiftwasm-source/target-build/swift-stdlib-wasi-wasm32/./lib/swift -swift-version 5 -O -D SWIFT_RUNTIME_OS_VERSIONING -assume-single-threaded -enforce-exclusivity=unchecked -enable-experimental-concurrency -disable-implicit-concurrency-module-import -Xcc -D_WASI_EMULATED_MMAN -parse-as-library -module-name _Concurrency -o /Users/kateinoigakukun/projects/swiftwasm-source/target-build/swift-stdlib-wasi-wasm32/stdlib/public/Concurrency/WASI/wasm32/_Concurrency.o -runtime-compatibility-version none -use-static-resource-dir -disable-autolinking-runtime-compatibility-dynamic-replacements
1.      SwiftWasm Swift version 5.3-dev (LLVM d8483794c7cfb2f, Swift 8e50d4d7dbda580)
2.      Running pass 'Function Pass Manager' on module '/Users/kateinoigakukun/projects/swiftwasm-source/target-build/swift-stdlib-wasi-wasm32/stdlib/public/Concurrency/WASI/wasm32/_Concurrency.o'.
3.      Running pass 'WebAssembly Assembly Printer' on function '@"$s12_Concurrency16_runAsyncHandler9operationyyyYc_tF"'

define dso_local swiftcc void @"$s12_Concurrency16_runAsyncHandler9operationyyyYc_tF"(i8* %0, %swift.refcounted* %1) #3 {
entry:
  %2 = load i32, i32* getelementptr inbounds (%swift.async_func_pointer, %swift.async_func_pointer* @"$ss5Error_pIegHzo_ytsAA_pIegHrzo_TRTu", i32 0, i32 1), align 4
  %3 = tail call noalias %swift.refcounted* @swift_allocObject(%swift.type* getelementptr inbounds (%swift.full_boxmetadata, %swift.full_boxmetadata* @metadata, i32 0, i32 2), i32 20, i32 3) #5
  %4 = getelementptr inbounds %swift.refcounted, %swift.refcounted* %3, i32 1
  %5 = bitcast %swift.refcounted* %4 to i32*
  store i32 %2, i32* %5, align 4
  %6 = getelementptr inbounds %swift.refcounted, %swift.refcounted* %3, i32 1, i32 1
  %.fn = bitcast i32* %6 to i8**
  store i8* %0, i8** %.fn, align 4
  %.data = getelementptr inbounds i32, i32* %6, i32 1
  %7 = bitcast i32* %.data to %swift.refcounted**
  store %swift.refcounted* %1, %swift.refcounted** %7, align 4
  %.not = icmp eq %swift.refcounted* %3, null
  %8 = tail call %swift.refcounted* @swift_nonatomic_retain(%swift.refcounted* returned %1) #5
  %9 = tail call %swift.refcounted* @swift_nonatomic_retain(%swift.refcounted* returned %3) #5
  br i1 %.not, label %10, label %14

10:                                               ; preds = %entry
  %11 = load i32, i32* getelementptr inbounds (%swift.async_func_pointer, %swift.async_func_pointer* bitcast (void (%swift.task*, %swift.executor*, %swift.context*)* @"$ss5Error_pIegHzo_ytsAA_pIegHrzo_TRTA" to %swift.async_func_pointer*), i32 0, i32 0), align 4
  %12 = add i32 %11, ptrtoint (void (%swift.task*, %swift.executor*, %swift.context*)* @"$ss5Error_pIegHzo_ytsAA_pIegHrzo_TRTA" to i32)
  %13 = inttoptr i32 %12 to i8*
  ; !!Here is the additive operation!!
  %.pre = load i32, i32* getelementptr inbounds (%swift.async_func_pointer, %swift.async_func_pointer* bitcast (void (%swift.task*, %swift.executor*, %swift.context*)* @"$ss5Error_pIegHzo_ytsAA_pIegHrzo_TRTA" to %swift.async_func_pointer*), i32 0, i32 1), align 4
  br label %14

14:                                               ; preds = %entry, %10
  %15 = phi i32 [ %.pre, %10 ], [ %2, %entry ]
  %16 = phi i8* [ %13, %10 ], [ bitcast (void (%swift.task*, %swift.executor*, %swift.context*)* @"$ss5Error_pIegHzo_ytsAA_pIegHrzo_TRTA" to i8*), %entry ]

As far as I understand, if function pointer is a direct pointer to a function, it could not be an async function pointer. So this PR changes to emit undefs instead of offset operations.

I'm not familiar with this area, so I would appreciate it if you could tell me if I'm doing in wrong way.

CC: @nate-chandler

[MaxDesiatov]

@swift-ci please smoke test

[MaxDesiatov]

@swift-ci please test macOS platform

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 3a6e33f

[kateinoigakukun]

@MaxDesiatov Could you re-trigger CI again?

[MaxDesiatov]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 3a6e33f

[nate-chandler]

Hi @kateinoigakukun , thanks for looking at this! We're planning to change the way that "references" to async functions get passed (they will always be async function pointers) and that will most likely result in this function (irgen::getAsyncFunctionAndSize) getting deleted. I don't fully understand the ramifications of this change, so I'd like to hold off on taking it for now.

[kateinoigakukun]

Thanks for your response!

they will always be async function pointers

It seems simpler way! OK, I'll close this and take a look after the change again 👍