[erlang-questions] How to efficiently set the bit at position Index to 1 in a binary?
Zabrane Mickael
zabrane3@REDACTED
Sat Dec 24 23:22:37 CET 2011
One last thing.
> a(N, B) ->
> fun (<<L:N/bits, _:1, R/bits>>) -> % THIS IS LINE 266
> <<L/bits, 1:1, R/bits>>
> end(B).
How to disable the following warning only for this function:
src/foo.erl:266: Warning: NOT OPTIMIZED: sub binary is used or returned
is there a way to do it by using -compile(...) directive?
Regards,
Zabrane
On Dec 24, 2011, at 8:17 PM, Jachym Holecek wrote:
> # Zabrane Mickael 2011-12-24:
>> Hi folks,
>>
>> Here's my code:
>>
>> %%--------------------------------------------------------------------
>> %% @doc Set the bit at position Index to 1 in the binary Bin.
>> %% @end
>> %%--------------------------------------------------------------------
>> -spec set(Bin :: binary(), Index :: non_neg_integer()) -> binary().
>> set(<<1:1, _/bits>> = Bin, 0) ->
>> Bin;
>> set(<<0:1, Rest/bits>>, 0) ->
>> <<1:1, Rest/bits>>;
>> set(Bin, Index) when Index > 0 ->
>> Before = Index - 1,
>> <<Head:Before, _:1, Rest/bits>> = Bin,
>> <<Head:Before, 1:1, Rest/bits>>.
>>
>>
>> After compiling, I'm getting the following warnings:
>> src/foo.erl:248: Warning: INFO: the '=' operator will prevent delayed sub binary optimization
>> src/foo.erl:248: Warning: INFO: the '=' operator will prevent delayed sub binary optimization
>> src/foo.erl:250: Warning: NOT OPTIMIZED: sub binary is used or returned
>> src/foo.erl:254: Warning: NOT OPTIMIZED: sub binary is used or returned
>>
>> What's the best/optimized solution and how to get rid of these warnings?
>
> Binary matching is *much* more powerful than what it looks like at first:
>
> -module(a).
> -export([a/2]).
>
> a(N, B) ->
> fun (<<L:N/bits, _:1, R/bits>>) ->
> <<L/bits, 1:1, R/bits>>
> end(B).
>
> The compiler refuses the saner form (doing the binary match in a/1's head
> directly) quoting the delusion that N isn't bound at that point... pity.
>
> Have fun,
> -- Jachym
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