[erlang-questions] binary optimization

[Mikael Pettersson]

On Sat, 18 Jul 2009 12:53:58 +0100, Joel Reymont wrote:

> On Jul 18, 2009, at 12:04 PM, Richard Carlsson wrote:
> 
> > You missed the important point in Mikael's mail:
> > "presumably the callee in the tailcall must be known at compile-time,
> > and must presumably only be called with a delayed sub binary."
> 
> 
> What about this one? I'm calling session:subscribe/2 and that's known
> at compile time.
> 
> ---
> src/transport.erl:98: Warning: NOT OPTIMIZED: sub binary used by =20
> session:subscribe/2
> 
> 98: handle_info({tcp, Sock, <<_:96, ?SUBSCRIBE, Topic/binary>>}, State) ->
>      inet:setopts(Sock, [{active, once}]),
>      session:subscribe(State#state.session, Topic),
>      {noreply, State};

Only the name is known, not the actual code.


> What are the problems with my matching instructions that the following
> warning is referring to?
> 
> ---
> src/transport.erl:110: Warning: NOT OPTIMIZED: the binary matching
> instruction that follows in the same function have problems that
> prevent delayed sub binary optimization (probably indicated by INFO
> warnings)
> src/transport.erl:113: Warning: NOT OPTIMIZED: sub binary used by
> topic:publish/2
> 
> 110: handle_info({tcp, Sock, <<_:96, ?PUBLISH, Len:32, Bin/binary>>}, State) ->
>      inet:setopts(Sock, [{active, once}]),
>      Len1 = Len * 8,
>      113: <<Topic:Len1/binary, Msg/binary>> = Bin,
>      topic:publish(Topic, Msg),
>      {noreply, State};

topic:publish/2 causes Topic and Msg to escape, so they'll need
full-blown term representations. The warning may refer to that, or
it may be that since they are sub-binaries of Bin the compiler gives
up on having Bin as a delayed sub-binary.