[erlang-questions] Warning: NOT OPTIMIZED: sub binary is used or returned

[Zabrane Mickael]

Hi Björn,

On Aug 7, 2012, at 8:24 AM, Björn Gustavsson wrote:

> On Mon, Aug 6, 2012 at 6:36 PM, Zabrane Mickael <zabrane3@REDACTED> wrote:
>> Hi guys,
>> 
>> I'm communicating with linked-in driver using the following code:
>> 
>> control(Port, Command, Data) ->
>>    case port_control(Port, Command, Data) of
>>        <<0, Result/binary>> -> Result;                                           % <- LINE 468  handle a good case
>>        <<1, Error/binary>>  -> {error, binary_to_term(Error)}    %  <- LINE 469  handle error case
>>    end.
>> 
>> 
>> When compiling, I got this 02 warnings:
>> 
>> $ make
>> Recompile: src/pimco.erl
>> src/pimco.erl:468: Warning: NOT OPTIMIZED: sub binary is used or returned
>> src/pimco.erl469: Warning: NOT OPTIMIZED: sub binary used by erlang:binary_to_term/1
>> make[2]: Nothing to be done for `all'.
>> 
>> Is there a way to get rid of these warnings and avoid a full binary copies?
> 
> Yes, you can get rid of the warnings by *not* using the bin_opt_info compiler
> option.

;-)

> No, there is nothing to avoid here. The warning says that
> a sub binary is created. A sub binary is small term that
> references a part of a binary. Sub binaries is usually an
> efficient way to reference just a part of a binary.

> 
> So there is *no* copying of the binary data going on in
> your example.

Crystal clear.

This version is funny because the compiler now throws only one WARNING instead of two:
$ make
src/pimco.erl:492: Warning: NOT OPTIMIZED: sub binary is used or returned

control(Port, Command, Data) ->
    control(port_control(Port, Command, Data)).

control(<<0, Result/binary>>) ->
    control(Result, true);
control(<<1, Error/binary>>) ->
    control(Error, false).

control(<<Error/binary>>, false) ->  % <--- LINE 492
    {error, binary_to_term(Error)};
control(<<Result/binary>>, true) ->
    Result.


Anyway, thanks for your explanations Björn.

Regards,
Zabrane

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