[Coroutines] Make sure that async coroutine context size is a multiple of the alignment requirement

[aschwaighofer]

Summary

This simplifies the code the allocator has to executed

[aschwaighofer]

Update the wording of a comment.

[rjmccall]

LGTM

llvm/lib/Transforms/Coroutines/CoroFrame.cpp

case coro::ABI::RetconOnce: {
     Shape.RetconLowering.IsFrameInlineInStorage
       = (B.getStructSize() <= Id->getStorageSize() &&
          B.getStructAlign() <= Id->getStorageAlignment());
     break;
   }
   case coro::ABI::Async: {
     Shape.AsyncLowering.FrameOffset =
         alignTo(Shape.AsyncLowering.ContextHeaderSize, Shape.FrameAlign);
+    // Also make the final context size a multiple of the context alignment to
+    // make allocation easier for allocators.
     Shape.AsyncLowering.ContextSize =
-        Shape.AsyncLowering.FrameOffset + Shape.FrameSize;
+        alignTo(Shape.AsyncLowering.FrameOffset + Shape.FrameSize,
+                Shape.AsyncLowering.getContextAlignment());
     if (Shape.AsyncLowering.getContextAlignment() < Shape.FrameAlign) {
       report_fatal_error(
           "The alignment requirment of frame variables cannot be higher than "
           "the alignment of the async function context");
     }
     break;
   }
   }

llvm/test/Transforms/Coroutines/coro-async.ll

entry:
   call void @some_user(i64 %val.2)
 
   tail call swiftcc void @asyncReturn(i8* %async.ctxt, %async.task* %task.2, %async.actor* %actor)
   call i1 @llvm.coro.end(i8* %hdl, i1 0)
   unreachable
 }
 
 ; Make sure we update the async function pointer
-; CHECK: @my_async_function_fp = constant <{ i32, i32 }> <{ {{.*}}, i32 168 }
-; CHECK: @my_async_function2_fp = constant <{ i32, i32 }> <{ {{.*}}, i32 168 }
+; CHECK: @my_async_function_fp = constant <{ i32, i32 }> <{ {{.*}}, i32 176 }
+; CHECK: @my_async_function2_fp = constant <{ i32, i32 }> <{ {{.*}}, i32 176 }
 
 ; CHECK-LABEL: define swiftcc void @my_async_function(i8* %async.ctxt, %async.task* %task, %async.actor* %actor) {
 ; CHECK: entry:
 ; CHECK:   [[FRAMEPTR:%.*]] = getelementptr inbounds i8, i8* %async.ctxt, i64 128
 ; CHECK:   [[ACTOR_SPILL_ADDR:%.*]] = getelementptr inbounds i8, i8* %async.ctxt, i64 152
 ; CHECK:   [[CAST1:%.*]] = bitcast i8* [[ACTOR_SPILL_ADDR]] to %async.actor**
 ; CHECK:   store %async.actor* %actor, %async.actor** [[CAST1]]
 ; CHECK:   [[ADDR1:%.*]]  = getelementptr inbounds i8, i8* %async.ctxt, i64 144