[embedded] Use __has_feature(swiftcc) to detect Swift calling convention

include/swift/Runtime/Config.h

@@ -228,8 +228,12 @@ extern uintptr_t __COMPATIBILITY_LIBRARIES_CANNOT_CHECK_THE_IS_SWIFT_BIT_DIRECTL
 
 // SWIFT_CC(swiftasync) is the Swift async calling convention.
 // We assume that it supports mandatory tail call elimination.
-#if __has_feature(swiftasynccc) && __has_attribute(swiftasynccall)
-#define SWIFT_CC_swiftasync __attribute__((swiftasynccall))
+#if __has_attribute(swiftasynccall)
+# if __has_feature(swiftasynccc) || __has_extension(swiftasynccc)
+#  define SWIFT_CC_swiftasync __attribute__((swiftasynccall))
+# else
+#  define SWIFT_CC_swiftasync SWIFT_CC_swift
+# endif
 #else
 #define SWIFT_CC_swiftasync SWIFT_CC_swift
 #endif

stdlib/public/SwiftShims/swift/shims/EmbeddedShims.h

@@ -28,8 +28,9 @@
 extern "C" {
 #endif
 
-// FIXME: Replace with __has_feature(swiftcc) once that's added to Clang.
-#if __has_feature(swiftasynccc)
+// TODO: __has_feature(swiftasynccc) is just for older clang. Remove this
+// when we no longer support older clang.
+#if __has_extension(swiftcc) || __has_feature(swiftasynccc)
 #define SWIFT_CC_swift __attribute__((swiftcall))
 #define SWIFT_CONTEXT __attribute__((swift_context))
 #define SWIFT_ERROR_RESULT __attribute__((swift_error_result))

test/embedded/classes-wasm.swift

@@ -0,0 +1,35 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// RUN: %swift-frontend -c %t/check.swift -parse-as-library -target wasm32-unknown-none-wasm \
+// RUN:   -enable-experimental-feature Embedded -Xcc -fdeclspec -disable-stack-protector \
+// RUN:   -o %t/check.o
+// RUN: %clang -target wasm32-unknown-none-wasm %t/check.o %t/rt.c -nostdlib -o %t/check.wasm
+// RUN: %target-run %t/check.wasm
+// REQUIRES: executable_test
+// REQUIRES: CPU=wasm32
+
+//--- rt.c
+
+#include <stddef.h>
+#include <stdint.h>
+
+void free(void *ptr) {}
+
+int posix_memalign(void **memptr, size_t alignment, size_t size) {
+    uintptr_t mem = __builtin_wasm_memory_grow(0, (size + 0xffff) / 0x10000);
+    if (mem == -1) {
+        return -1;
+    }
+    *memptr = (void *)(mem * 0x10000);
+    *memptr = (void *)(((uintptr_t)*memptr + alignment - 1) & -alignment);
+    return 0;
+}
+
+//--- check.swift
+
+class Foo {}
+
+@_cdecl("_start")
+func main() {
+  _ = Foo()
+}

utils/swift_build_support/swift_build_support/products/wasmstdlib.py

@@ -101,7 +101,7 @@ def build(self, host_target):
         # Test configuration
         self.cmake_options.define('SWIFT_INCLUDE_TESTS:BOOL', 'TRUE')
         self.cmake_options.define('SWIFT_ENABLE_SOURCEKIT_TESTS:BOOL', 'FALSE')
-        lit_test_paths = ['IRGen', 'stdlib', 'Concurrency/Runtime']
+        lit_test_paths = ['IRGen', 'stdlib', 'Concurrency/Runtime', 'embedded']
         lit_test_paths = [os.path.join(
             self.build_dir, 'test-wasi-wasm32', path) for path in lit_test_paths]
         self.cmake_options.define('SWIFT_LIT_TEST_PATHS:STRING',