[clang][WebAssembly] Return aggregate values indirectly in swiftcc

The Swift calling convention on Wasm has historically returned aggregate
values directly at the LLVM IR level due to the use of the generic
`SwiftABIInfo` implementation. The direct return at LLVM IR level will
cause unnecessary stack allocation and memory copies for each aggregate
return value.

Author: kateinoigakukun

clang/lib/CodeGen/Targets/WebAssembly.cpp

@@ -45,13 +45,33 @@ class WebAssemblyABIInfo final : public ABIInfo {
                    AggValueSlot Slot) const override;
 };
 
+class WebAssemblySwiftABIInfo final : public SwiftABIInfo {
+  WebAssemblyABIKind K;
+public:
+  explicit WebAssemblySwiftABIInfo(CodeGen::CodeGenTypes &CGT, WebAssemblyABIKind K)
+      : SwiftABIInfo(CGT, /*SwiftErrorInRegister=*/false), K(K) {}
+
+  bool shouldPassIndirectly(ArrayRef<llvm::Type *> ComponentTys, bool AsReturnValue) const override {
+    if (AsReturnValue) {
+      if (K == WebAssemblyABIKind::ExperimentalMV) {
+        // If the MV ABI is enabled, return all values directly.
+        return false;
+      }
+      // Otherwise, check if the value occupies more than 1 scalar slot.
+      return SwiftABIInfo::occupiesMoreThan(ComponentTys, /*total=*/1);
+    }
+    // For arguments, just follow the default implementation.
+    return SwiftABIInfo::shouldPassIndirectly(ComponentTys, AsReturnValue);
+  }
+};
+
 class WebAssemblyTargetCodeGenInfo final : public TargetCodeGenInfo {
 public:
   explicit WebAssemblyTargetCodeGenInfo(CodeGen::CodeGenTypes &CGT,
                                         WebAssemblyABIKind K)
       : TargetCodeGenInfo(std::make_unique<WebAssemblyABIInfo>(CGT, K)) {
     SwiftInfo =
-        std::make_unique<SwiftABIInfo>(CGT, /*SwiftErrorInRegister=*/false);
+        std::make_unique<WebAssemblySwiftABIInfo>(CGT, K);
   }
 
   void setTargetAttributes(const Decl *D, llvm::GlobalValue *GV,

clang/test/CodeGen/WebAssembly/wasm-return-swiftcc.c

@@ -0,0 +1,50 @@
+// RUN: %clang_cc1 -triple wasm32-unknown-unknown %s -emit-llvm -o - | FileCheck %s
+
+typedef struct {
+  int aa;
+  int bb;
+} s1;
+
+// Multiple-element structs should be returned through sret.
+// CHECK: define swiftcc void @return_s1(ptr dead_on_unwind noalias writable sret(%struct.s1) align 4 %agg.result)
+__attribute__((swiftcall))
+s1 return_s1(void) {
+  s1 foo;
+  return foo;
+}
+
+typedef struct {
+  int cc;
+} s2;
+
+// Single-element structs should be returned directly.
+// CHECK: define swiftcc i32 @return_s2()
+__attribute__((swiftcall))
+s2 return_s2(void) {
+  s2 foo;
+  return foo;
+}
+
+typedef struct {
+    char c1[4];
+} s3;
+
+// CHECK: define swiftcc i32 @return_s3()
+__attribute__((swiftcall))
+s3 return_s3(void) {
+  s3 foo;
+  return foo;
+}
+
+typedef struct {
+    int bf1 : 4;
+    int bf2 : 3;
+    int bf3 : 8;
+} s4;
+
+// CHECK: define swiftcc i16 @return_s4()
+__attribute__((swiftcall))
+s4 return_s4(void) {
+  s4 foo;
+  return foo;
+}