From 3e4e71ec9bd3d4d457c378af71af381ac2074140 Mon Sep 17 00:00:00 2001
From: Yuta Saito <kateinoigakukun@gmail.com>
Date: Sat, 14 Dec 2019 22:06:45 +0900
Subject: [PATCH] Emit thunk function for specific ABI on WebAssembly. (#6)

Changed to make thunk to convert thin-to-thick and non-throws-to-throws.
We needs it on WebAssembly host because WASM checks number of arguments strictly for indirect function call.

This patch allows you to run the Swift code below.

```swift
func f(_ a: (Int) -> Void) {
    g(a)
}

func g(_ b: (Int) throws -> Void) {
    try! b(1)
}

f { _ in }
```
---
 lib/SIL/TypeLowering.cpp | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/lib/SIL/TypeLowering.cpp b/lib/SIL/TypeLowering.cpp
index f124d37c7c4bc..4160a7716da4f 100644
--- a/lib/SIL/TypeLowering.cpp
+++ b/lib/SIL/TypeLowering.cpp
@@ -2644,12 +2644,25 @@ TypeConverter::checkFunctionForABIDifferences(SILModule &M,
       return ABIDifference::NeedsThunk;
   }
 
+  // There is no ABI compatibility between non-throws and throws on WebAssembly,
+  // so need thunk.
+  if (M.getASTContext().LangOpts.Target.isOSBinFormatWasm()) {
+    if (!fnTy1->hasErrorResult() && fnTy2->hasErrorResult()) {
+      return ABIDifference::NeedsThunk;
+    }
+  }
+
   auto rep1 = fnTy1->getRepresentation(), rep2 = fnTy2->getRepresentation();
   if (rep1 != rep2) {
     if (rep1 == SILFunctionTypeRepresentation::Thin &&
-        rep2 == SILFunctionTypeRepresentation::Thick)
+        rep2 == SILFunctionTypeRepresentation::Thick) {
+      // There is no ABI compatibility between thin and thick on WebAssembly,
+      // so need thunk.
+      if (M.getASTContext().LangOpts.Target.isOSBinFormatWasm()) {
+        return ABIDifference::NeedsThunk;
+      }
       return ABIDifference::ThinToThick;
-
+    }
     return ABIDifference::NeedsThunk;
   }