[Distributed] IRGen: Don't allocate offset pointer if there is nothing to load

If underlying function does not have any parameter, then let's
exit early and avoid stack allocating the argument buffer cursor.

Author: xedin

lib/IRGen/GenDistributed.cpp

@@ -172,15 +172,20 @@ void DistributedAccessor::computeArguments(llvm::Value *argumentBuffer,
                                            Explosion &arguments) {
   auto fnType = Method->getLoweredFunctionType();
 
+  // Cover all of the arguments except to `self` of the actor.
+  auto parameters = fnType->getParameters().drop_back();
+
+  // If there are no parameters to extract, we are done.
+  if (parameters.empty())
+    return;
+
   auto offset =
       IGF.createAlloca(IGM.Int8PtrTy, IGM.getPointerAlignment(), "offset");
   IGF.Builder.CreateLifetimeStart(offset, IGM.getPointerSize());
 
   // Initialize "offset" with the address of the base of the argument buffer.
   IGF.Builder.CreateStore(argumentBuffer, offset);
 
-  // Cover all of the arguments except to `self` of the actor.
-  auto parameters = fnType->getParameters().drop_back();
   for (const auto &param : parameters) {
     auto paramTy = param.getSILStorageInterfaceType();
     const TypeInfo &typeInfo = IGF.getTypeInfo(paramTy);

test/IRGen/distributed_actor_accessors.swift

@@ -341,3 +341,17 @@ public distributed actor MyOtherActor {
 /// RESULT is returned indirectly so there is nothing to pass to `end`
 
 // CHECK: {{.*}} = call i1 (i8*, i1, ...) @llvm.coro.end.async({{.*}}, %swift.context* {{.*}}, %swift.error* {{.*}})
+
+/// ---> Thunk and distributed method for `MyOtherActor.empty`
+
+/// Let's check that there is no offset allocation here since parameter list is empty
+
+// CHECK: define internal swifttailcc void @"$s27distributed_actor_accessors12MyOtherActorC5emptyyyFTETF"
+// CHECK-NEXT: entry:
+// CHECK-NEXT: {{.*}} = alloca %swift.context*, align 8
+// CHECK-NEXT: %swifterror = alloca swifterror %swift.error*, align 8
+// CHECK-NEXT: {{.*}} = call token @llvm.coro.id.async(i32 20, i32 16, i32 0, i8* bitcast (%swift.async_func_pointer* @"$s27distributed_actor_accessors12MyOtherActorC5emptyyyFTETFTu" to i8*))
+// CHECK-NEXT: {{.*}} = call i8* @llvm.coro.begin(token %4, i8* null)
+// CHECK-NEXT: store %swift.context* {{.*}}, %swift.context** {{.*}}, align 8
+// CHECK-NEXT: store %swift.error* null, %swift.error** %swifterror, align 8
+// CHECK-NEXT: {{.*}} = load i32, i32* getelementptr inbounds (%swift.async_func_pointer, %swift.async_func_pointer* bitcast (void (%swift.context*, %T27distributed_actor_accessors12MyOtherActorC*)* @"$s27distributed_actor_accessors12MyOtherActorC5emptyyyFTE" to %swift.async_func_pointer*), i32 0, i32 0), align 8