Optimize messages if only fields are used

[jklmnn]

Sometimes message have to be returned from platform functions in session, e.g. to get an opaque value to to group multiple values. These messages are never used in another message, as a sequence element or as an opaque value:

type Blob is message
   Length : Length;
   Data : Opaque with
      Size => Length * 8;
end message;

generic
   with function Get_Blob return Blob;
session Session
...
is
begin
   state Add_Blob
   is
      Blob : Blob;
   begin
      Blob := Get_Blob;
      Message.Data_Length := Blob.Length;
      Message.Data := Blob.Data;
   end Add_Blob;
end Session;

The generated code for blob includes the returned structure that already contains the data buffer and the length field, both with the appropriate types. Now that structure is transformed into a context as it is a proper message. Since only fields of Blob are accessed only getters of that context are called. These getters simply return the same values the structure already contained.
The optimization is to remove the context creation in that case and directly use the structure. This allows to save a significant amount of memory, code size and execution time.
This optimization cannot be done when the specific properties of a real message are required:

• Calling 'Opaque.
• Appending it to a sequence.

[senier]

@jklmnn I guess your intention was to call the external function at some point in your example? Also, did you mean to call it Get_Blob?

[jklmnn]

You're right. I fixed that.

[treiher]

This optimization cannot be done when the specific properties of a real message are required:

What about setting of message fields? I think it shouldn't be needed for the use case. If we still would want to support it, we would have to ensure that no message conditions get violated.

I suppose we only want to support state-local variables. Otherwise, we also have to consider 'Read and 'Write.

[jklmnn]

I think the setting of message fields doesn't need to be supported for this optimization. The usual use case are types that are returned by platform functions.