[c-interop] Use Swift base name for @_extern(c) without explicit name

Author: kateinoigakukun

docs/ReferenceGuides/UnderscoredAttributes.md

@@ -453,7 +453,9 @@ It's the equivalent of clang's `__attribute__((import_module("module"), import_n
 
 Indicates that a particular declaration should refer to a
 C declaration with the given name. If the optional "cName"
-string is not specified, the mangled Swift name will be used.
+string is not specified, the Swift function name is used
+without Swift name mangling. Platform-specific mangling
+rules (leading underscore on Darwin) are still applied.
 
 Similar to `@_cdecl`, but this attribute is used to reference
 C declarations from Swift, while `@_cdecl` is used to define

lib/SIL/IR/SILDeclRef.cpp

@@ -1151,6 +1151,10 @@ std::string SILDeclRef::mangle(ManglingKind MKind) const {
     if (auto *ExternA = ExternAttr::find(getDecl()->getAttrs(), ExternKind::C)) {
       if (auto cName = ExternA->Name)
         return cName->str();
+      // If no name was specified, fall back on the Swift base name without mangling.
+      assert(isa<FuncDecl>(getDecl()) && "non-FuncDecl with @_extern should be rejected by typechecker");
+      // Base name is always available and non-empty for FuncDecl.
+      return getDecl()->getBaseIdentifier().str().str();
     }
 
     // Use a given cdecl name for native-to-foreign thunks.

test/IRGen/extern_c.swift

@@ -1,16 +1,37 @@
-// RUN: %target-swift-frontend -emit-ir %s | %FileCheck %s
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// RUN: %target-swift-frontend -emit-ir %t/extern_c.swift -I%t | %FileCheck %s
+
+//--- c_types.h
+struct c_struct {
+  int field;
+};
+
+//--- module.modulemap
+module c_types {
+  header "c_types.h"
+}
+
+//--- extern_c.swift
+import c_types
 
 func test() {
   // CHECK: call void @explicit_extern_c()
   explicit_extern_c()
-  // CHECK: call void @"$s8extern_c09implicit_A2_cyyF"()
+  // CHECK: call void @implicit_extern_c()
   implicit_extern_c()
+
+  // CHECK: call i32 @"+"({{.*}})
+  _ = c_struct(field: 1) + c_struct(field: 2)
 }
 
 test()
 
 // CHECK: declare void @explicit_extern_c()
 @_extern(c, "explicit_extern_c") func explicit_extern_c()
 
-// CHECK: declare void @"$s8extern_c09implicit_A2_cyyF"()
+// CHECK: declare void @implicit_extern_c()
 @_extern(c) func implicit_extern_c()
+
+// CHECK: declare i32 @"+"({{.*}})
+@_extern(c) func +(a: c_struct, b: c_struct) -> c_struct

test/Interop/WebAssembly/extern-wasm-cdecls.swift

@@ -1,17 +1,17 @@
 // RUN: %empty-directory(%t)
 // RUN: %target-swift-frontend %s -emit-ir -module-name Extern | %FileCheck %s
 
-// CHECK: declare void @"$s6Extern7import1yyF"() [[EA1:#[0-9]+]]
+// CHECK: declare void @import1() [[EA1:#[0-9]+]]
 @_extern(c)
 @_extern(wasm, module: "m0", name: "import1")
 func import1()
 
-// CHECK: declare i32 @"$s6Extern20import2WithReturnInts5Int32VyF"() [[EA2:#[0-9]+]]
+// CHECK: declare i32 @import2WithReturnInt() [[EA2:#[0-9]+]]
 @_extern(c)
 @_extern(wasm, module: "m0", name: "import2")
 func import2WithReturnInt() -> Int32
 
-// CHECK: declare void @"$s6Extern16import3TakingIntyys5Int32VF"(i32) [[EA3:#[0-9]+]]
+// CHECK: declare void @import3TakingInt(i32) [[EA3:#[0-9]+]]
 @_extern(c)
 @_extern(wasm, module: "m0", name: "import3")
 func import3TakingInt(_: Int32)

test/SILGen/extern_c.swift

@@ -16,7 +16,7 @@ public func publicVisibility(_ x: Int) -> Int
 @_extern(c, "private_visible")
 private func privateVisiblity(_ x: Int) -> Int
 
-// CHECK-DAG: sil hidden_external @$s8extern_c12withoutCNameSiyF : $@convention(c) () -> Int
+// CHECK-DAG: sil hidden_external @withoutCName : $@convention(c) () -> Int
 @_extern(c)
 func withoutCName() -> Int
 
@@ -36,7 +36,7 @@ func main() {
   // CHECK-DAG: [[F4:%.+]] = function_ref @private_visible : $@convention(c) (Int) -> Int
   // CHECK-DAG: apply [[F4]]({{.*}}) : $@convention(c) (Int) -> Int
   _ = privateVisiblity(24)
-  // CHECK-DAG: [[F5:%.+]] = function_ref @$s8extern_c12withoutCNameSiyF : $@convention(c) () -> Int
+  // CHECK-DAG: [[F5:%.+]] = function_ref @withoutCName : $@convention(c) () -> Int
   // CHECK-DAG: apply [[F5]]() : $@convention(c) () -> Int
   _ = withoutCName()
   // CHECK-DAG: [[F6:%.+]] = function_ref @$s8extern_c10defaultArgyySiFfA_ : $@convention(thin) () -> Int