SILGen: Consistently use the buildMainActorExecutorRef built-in.

SILGen and IRGen would disagree on the return type of the
`swift_task_getMainExecutor()` runtime function if
`SILGenModule::getGetMainExecutor()` had ever been called. To address this,
consistently use the `buildMainActorExecutorRef` built-in and get rid of
`SILGenModule::getGetMainExecutor()`.

Resolves rdar://116472583

Author: tshortli

lib/SILGen/SILGen.cpp

@@ -503,11 +503,6 @@ FuncDecl *SILGenModule::getAsyncMainDrainQueue() {
                                     "_asyncMainDrainQueue");
 }
 
-FuncDecl *SILGenModule::getGetMainExecutor() {
-  return lookupConcurrencyIntrinsic(getASTContext(), GetMainExecutor,
-                                    "_getMainExecutor");
-}
-
 FuncDecl *SILGenModule::getSwiftJobRun() {
   return lookupConcurrencyIntrinsic(getASTContext(), SwiftJobRun,
                                     "_swiftJobRun");

lib/SILGen/SILGen.h

@@ -570,8 +570,6 @@ class LLVM_LIBRARY_VISIBILITY SILGenModule : public ASTVisitor<SILGenModule> {
 
   /// Retrieve the _Concurrency._asyncMainDrainQueue intrinsic.
   FuncDecl *getAsyncMainDrainQueue();
-  /// Retrieve the _Concurrency._getMainExecutor intrinsic.
-  FuncDecl *getGetMainExecutor();
   /// Retrieve the _Concurrency._swiftJobRun intrinsic.
   FuncDecl *getSwiftJobRun();
   // Retrieve the _SwiftConcurrencyShims.exit intrinsic.

lib/SILGen/SILGenProlog.cpp

@@ -1448,36 +1448,12 @@ void SILGenFunction::emitProlog(
 }
 
 SILValue SILGenFunction::emitMainExecutor(SILLocation loc) {
-  // Get main executor
-  FuncDecl *getMainExecutorFuncDecl = SGM.getGetMainExecutor();
-  if (!getMainExecutorFuncDecl) {
-    // If it doesn't exist due to an SDK-compiler mismatch, we can conjure one
-    // up instead of crashing:
-    // @available(SwiftStdlib 5.1, *)
-    // @_silgen_name("swift_task_getMainExecutor")
-    // internal func _getMainExecutor() -> Builtin.Executor
-    auto &ctx = getASTContext();
-
-    ParameterList *emptyParams = ParameterList::createEmpty(ctx);
-    getMainExecutorFuncDecl = FuncDecl::createImplicit(
-        ctx, StaticSpellingKind::None,
-        DeclName(
-            ctx,
-            DeclBaseName(ctx.getIdentifier("_getMainExecutor")),
-            /*Arguments*/ emptyParams),
-        {}, /*async*/ false, /*throws*/ false, {}, emptyParams,
-        ctx.TheExecutorType,
-        getModule().getSwiftModule());
-    getMainExecutorFuncDecl->getAttrs().add(
-        new (ctx) SILGenNameAttr("swift_task_getMainExecutor", /*raw*/ false,
-                                 /*implicit*/ true));
-  }
+  auto &ctx = getASTContext();
+  auto builtinName = ctx.getIdentifier(
+      getBuiltinName(BuiltinValueKind::BuildMainActorExecutorRef));
+  auto resultType = SILType::getPrimitiveObjectType(ctx.TheExecutorType);
 
-  auto fn = SGM.getFunction(
-      SILDeclRef(getMainExecutorFuncDecl, SILDeclRef::Kind::Func),
-      NotForDefinition);
-  SILValue fnRef = B.createFunctionRefFor(loc, fn);
-  return B.createApply(loc, fnRef, {}, {});
+  return B.createBuiltin(loc, builtinName, resultType, {}, {});
 }
 
 SILValue SILGenFunction::emitGenericExecutor(SILLocation loc) {

test/Concurrency/async_main.swift

@@ -79,12 +79,10 @@ func asyncFunc() async {
 // CHECK-SIL-NEXT:  %12 = function_ref @swift_job_run : $@convention(thin) (UnownedJob, UnownedSerialExecutor) -> ()
 // CHECK-SIL-NEXT:  %13 = builtin "convertTaskToJob"(%11 : $Builtin.NativeObject) : $Builtin.Job
 // CHECK-SIL-NEXT:  %14 = struct $UnownedJob (%13 : $Builtin.Job)
-// CHECK-SIL-NEXT:  // function_ref swift_task_getMainExecutor
-// CHECK-SIL-NEXT:  %15 = function_ref @swift_task_getMainExecutor : $@convention(thin) () -> Builtin.Executor
-// CHECK-SIL-NEXT:  %16 = apply %15() : $@convention(thin) () -> Builtin.Executor
-// CHECK-SIL-NEXT:  %17 = struct $UnownedSerialExecutor (%16 : $Builtin.Executor)
-// CHECK-SIL-NEXT:  %18 = apply %12(%14, %17) : $@convention(thin) (UnownedJob, UnownedSerialExecutor) -> ()
+// CHECK-SIL-NEXT:  %15 = builtin "buildMainActorExecutorRef"() : $Builtin.Executor
+// CHECK-SIL-NEXT:  %16 = struct $UnownedSerialExecutor (%15 : $Builtin.Executor)
+// CHECK-SIL-NEXT:  %17 = apply %12(%14, %16) : $@convention(thin) (UnownedJob, UnownedSerialExecutor) -> ()
 // CHECK-SIL-NEXT:  // function_ref swift_task_asyncMainDrainQueue
-// CHECK-SIL-NEXT:  %19 = function_ref @swift_task_asyncMainDrainQueue : $@convention(thin) () -> Never
-// CHECK-SIL-NEXT:  %20 = apply %19() : $@convention(thin) () -> Never
+// CHECK-SIL-NEXT:  %18 = function_ref @swift_task_asyncMainDrainQueue : $@convention(thin) () -> Never
+// CHECK-SIL-NEXT:  %19 = apply %18() : $@convention(thin) () -> Never
 // CHECK-SIL-NEXT:  unreachable

test/SILGen/toplevel_globalactorvars.swift

@@ -5,9 +5,7 @@
 
 // CHECK-LABEL: sil private [ossa] @async_Main
 // CHECK: bb0:
-// CHECK-NEXT: // function_ref
-// CHECK-NEXT: [[GET_MAIN:%.*]] = function_ref @swift_task_getMainExecutor
-// CHECK-NEXT: [[MAIN:%.*]] = apply [[GET_MAIN]]()
+// CHECK-NEXT: [[MAIN:%.*]] = builtin "buildMainActorExecutorRef"() : $Builtin.Executor
 // CHECK-NEXT: [[MAIN_OPTIONAL:%[0-9]+]] = enum $Optional<Builtin.Executor>, #Optional.some!enumelt, [[MAIN]]
 
 actor MyActorImpl {}