path: root/system/doc/efficiency_guide/binaryhandling.xml

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<!DOCTYPE chapter SYSTEM "chapter.dtd">

<chapter>
  <header>
    <copyright>
      <year>2007</year>
      <year>2018</year>
      <holder>Ericsson AB, All Rights Reserved</holder>
    </copyright>
    <legalnotice>
  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
  You may obtain a copy of the License at
 
      http://www.apache.org/licenses/LICENSE-2.0

  Unless required by applicable law or agreed to in writing, software
  distributed under the License is distributed on an "AS IS" BASIS,
  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  See the License for the specific language governing permissions and
  limitations under the License.

  The Initial Developer of the Original Code is Ericsson AB.
    </legalnotice>

    <title>Constructing and Matching Binaries</title>
    <prepared>Bjorn Gustavsson</prepared>
    <docno></docno>
    <date>2007-10-12</date>
    <rev></rev>
    <file>binaryhandling.xml</file>
  </header>

  <p>Binaries can be efficiently built in the following way:</p>

  <p><em>DO</em></p>
  <code type="erl"><![CDATA[
my_list_to_binary(List) ->
    my_list_to_binary(List, <<>>).

my_list_to_binary([H|T], Acc) ->
    my_list_to_binary(T, <<Acc/binary,H>>);
my_list_to_binary([], Acc) ->
    Acc.]]></code>  

  <p>Binaries can be efficiently matched like this:</p>

  <p><em>DO</em></p>
  <code type="erl"><![CDATA[
my_binary_to_list(<<H,T/binary>>) ->
    [H|my_binary_to_list(T)];
my_binary_to_list(<<>>) -> [].]]></code>

  <section>
    <title>How Binaries are Implemented</title>

    <p>Internally, binaries and bitstrings are implemented in the same way.
    In this section, they are called <em>binaries</em> because that is what
    they are called in the emulator source code.</p>

    <p>Four types of binary objects are available internally:</p>
    <list type="bulleted">
      <item><p>Two are containers for binary data and are called:</p>
      <list type="bulleted">
	<item><em>Refc binaries</em> (short for
	<em>reference-counted binaries</em>)</item>
	<item><em>Heap binaries</em></item>
      </list></item>
      <item><p>Two are merely references to a part of a binary and
      are called:</p>
      <list type="bulleted">
	<item><em>sub binaries</em></item>
	<item><em>match contexts</em></item>
      </list></item>
    </list>

    <section>
      <marker id="refc_binary"></marker>
      <title>Refc Binaries</title>
      <p>Refc binaries consist of two parts:</p>
      <list type="bulleted">
	<item>An object stored on the process heap, called a
	<em>ProcBin</em></item>
	<item>The binary object itself, stored outside all process
	heaps</item>
      </list>

    <p>The binary object can be referenced by any number of ProcBins from any
    number of processes. The object contains a reference counter to keep track
    of the number of references, so that it can be removed when the last
    reference disappears.</p>

    <p>All ProcBin objects in a process are part of a linked list, so that
    the garbage collector can keep track of them and decrement the reference
    counters in the binary when a ProcBin disappears.</p>
    </section>

    <section>
      <marker id="heap_binary"></marker>
      <title>Heap Binaries</title>
    <p>Heap binaries are small binaries, up to 64 bytes, and are stored
    directly on the process heap. They are copied when the process is
    garbage-collected and when they are sent as a message. They do not
    require any special handling by the garbage collector.</p>
    </section>

    <section>
      <title>Sub Binaries</title>
    <p>The reference objects <em>sub binaries</em> and
    <em>match contexts</em> can reference part of
    a refc binary or heap binary.</p>

    <p><marker id="sub_binary"></marker>A <em>sub binary</em>
    is created by <c>split_binary/2</c> and when
    a binary is matched out in a binary pattern. A sub binary is a reference
    into a part of another binary (refc or heap binary, but never into another
    sub binary). Therefore, matching out a binary is relatively cheap because
    the actual binary data is never copied.</p>
    </section>

    <section>
      <title>Match Context</title>
      <marker id="match_context"></marker>
      <p>A <em>match context</em> is similar to a sub binary, but is
      optimized for binary matching. For example, it contains a direct
      pointer to the binary data. For each field that is matched out of
      a binary, the position in the match context is incremented.</p>

    <p>The compiler tries to avoid generating code that
    creates a sub binary, only to shortly afterwards create a new match
    context and discard the sub binary. Instead of creating a sub binary,
    the match context is kept.</p>

    <p>The compiler can only do this optimization if it knows
    that the match context will not be shared. If it would be shared, the
    functional properties (also called referential transparency) of Erlang
    would break.</p>
    </section>
  </section>

  <section>
    <title>Constructing Binaries</title>
    <p>Appending to a binary or bitstring
    is specially optimized by the <em>runtime system</em>:</p>

  <code type="erl"><![CDATA[
<<Binary/binary, ...>>
<<Binary/bitstring, ...>>]]></code>

    <p>As the runtime system handles the optimization (instead of
    the compiler), there are very few circumstances in which the optimization
    does not work.</p>

    <p>To explain how it works, let us examine the following code line
    by line:</p>

  <code type="erl"><![CDATA[
Bin0 = <<0>>,                    %% 1
Bin1 = <<Bin0/binary,1,2,3>>,    %% 2
Bin2 = <<Bin1/binary,4,5,6>>,    %% 3
Bin3 = <<Bin2/binary,7,8,9>>,    %% 4
Bin4 = <<Bin1/binary,17>>,       %% 5 !!!
{Bin4,Bin3}                      %% 6]]></code>

  <list type="bulleted">
    <item>Line 1 (marked with the <c>%% 1</c> comment), assigns
    a <seealso marker="#heap_binary">heap binary</seealso> to
    the <c>Bin0</c> variable.</item>

    <item>Line 2 is an append operation. As <c>Bin0</c>
    has not been involved in an append operation,
    a new <seealso marker="#refc_binary">refc binary</seealso>
    is created and the contents of <c>Bin0</c> is copied
    into it. The <em>ProcBin</em> part of the refc binary has
    its size set to the size of the data stored in the binary, while
    the binary object has extra space allocated.
    The size of the binary object is either twice the
    size of <c>Bin1</c> or 256, whichever is larger. In this case
    it is 256.</item>

    <item>Line 3 is more interesting.
    <c>Bin1</c> <em>has</em> been used in an append operation,
    and it has 252 bytes of unused storage at the end, so the 3 new
    bytes are stored there.</item>

    <item>Line 4. The same applies here. There are 249 bytes left,
    so there is no problem storing another 3 bytes.</item>

    <item>Line 5. Here, something <em>interesting</em> happens. Notice
    that the result is not appended to the previous result in <c>Bin3</c>,
    but to <c>Bin1</c>. It is expected that <c>Bin4</c> will be assigned
    the value <c>&lt;&lt;0,1,2,3,17&gt;&gt;</c>. It is also expected that
    <c>Bin3</c> will retain its value
    (<c>&lt;&lt;0,1,2,3,4,5,6,7,8,9&gt;&gt;</c>).
    Clearly, the runtime system cannot write byte <c>17</c> into the binary,
    because that would change the value of <c>Bin3</c> to
    <c>&lt;&lt;0,1,2,3,4,17,6,7,8,9&gt;&gt;</c>.</item>
  </list>

    <p>The runtime system sees that <c>Bin1</c> is the result
    from a previous append operation (not from the latest append operation),
    so it <em>copies</em> the contents of <c>Bin1</c> to a new binary,
    reserve extra storage, and so on. (Here is not explained how the
    runtime system can know that it is not allowed to write into <c>Bin1</c>;
    it is left as an exercise to the curious reader to figure out how it is
    done by reading the emulator sources, primarily <c>erl_bits.c</c>.)</p>

    <section>
      <title>Circumstances That Force Copying</title>

      <p>The optimization of the binary append operation requires that
      there is a <em>single</em> ProcBin and a <em>single reference</em> to the
      ProcBin for the binary. The reason is that the binary object can be
      moved (reallocated) during an append operation, and when that happens,
      the pointer in the ProcBin must be updated. If there would be more than
      one ProcBin pointing to the binary object, it would not be possible to
      find and update all of them.</p>

      <p>Therefore, certain operations on a binary mark it so that
      any future append operation will be forced to copy the binary.
      In most cases, the binary object will be shrunk at the same time 
      to reclaim the extra space allocated for growing.</p>

      <p>When appending to a binary as follows, only the binary returned
      from the latest append operation will support further cheap append
      operations:</p>
      
  <code type="erl"><![CDATA[
Bin = <<Bin0,...>>]]></code>

      <p>In the code fragment in the beginning of this section,
      appending to <c>Bin</c> will be cheap, while appending to <c>Bin0</c>
      will force the creation of a new binary and copying of the contents
      of <c>Bin0</c>.</p>

      <p>If a binary is sent as a message to a process or port, the binary
      will be shrunk and any further append operation will copy the binary
      data into a new binary. For example, in the following code fragment
      <c>Bin1</c> will be copied in the third line:</p>

  <code type="erl"><![CDATA[
Bin1 = <<Bin0,...>>,
PortOrPid ! Bin1,
Bin = <<Bin1,...>>  %% Bin1 will be COPIED
]]></code>

      <p>The same happens if you insert a binary into an Ets
      table, send it to a port using <c>erlang:port_command/2</c>, or
      pass it to
      <seealso marker="erts:erl_nif#enif_inspect_binary">enif_inspect_binary</seealso>
      in a NIF.</p>

      <p>Matching a binary will also cause it to shrink and the next append
      operation will copy the binary data:</p>

  <code type="erl"><![CDATA[
Bin1 = <<Bin0,...>>,
<<X,Y,Z,T/binary>> = Bin1,
Bin = <<Bin1,...>>  %% Bin1 will be COPIED
]]></code>

      <p>The reason is that a
      <seealso marker="#match_context">match context</seealso>
      contains a direct pointer to the binary data.</p>

      <p>If a process simply keeps binaries (either in "loop data" or in the
      process
      dictionary), the garbage collector can eventually shrink the binaries.
      If only one such binary is kept, it will not be shrunk. If the process
      later appends to a binary that has been shrunk, the binary object will
      be reallocated to make place for the data to be appended.</p>
    </section>
  </section>

  <section>
    <title>Matching Binaries</title>

    <p>Let us revisit the example in the beginning of the previous section:</p>

  <p><em>DO</em></p>
  <code type="erl"><![CDATA[
my_binary_to_list(<<H,T/binary>>) ->
    [H|my_binary_to_list(T)];
my_binary_to_list(<<>>) -> [].]]></code>  

  <p>The first time <c>my_binary_to_list/1</c> is called,
  a <seealso marker="#match_context">match context</seealso>
  is created. The match context points to the first
  byte of the binary. 1 byte is matched out and the match context
  is updated to point to the second byte in the binary.</p>

  <p>At this point it would make sense to create a
  <seealso marker="#sub_binary">sub binary</seealso>,
  but in this particular example the compiler sees that
  there will soon be a call to a function (in this case,
  to <c>my_binary_to_list/1</c> itself) that immediately will
  create a new match context and discard the sub binary.</p>

  <p>Therefore <c>my_binary_to_list/1</c> calls itself
  with the match context instead of with a sub binary. The instruction
  that initializes the matching operation basically does nothing
  when it sees that it was passed a match context instead of a binary.</p>

  <p>When the end of the binary is reached and the second clause matches,
  the match context will simply be discarded (removed in the next
  garbage collection, as there is no longer any reference to it).</p>

  <p>To summarize, <c>my_binary_to_list/1</c> only needs to create
  <em>one</em> match context and no sub binaries.</p>

  <p>Notice that the match context in <c>my_binary_to_list/1</c>
  was discarded when the entire binary had been traversed. What happens if
  the iteration stops before it has reached the end of the binary? Will
  the optimization still work?</p>

  <code type="erl"><![CDATA[
after_zero(<<0,T/binary>>) ->
    T;
after_zero(<<_,T/binary>>) ->
    after_zero(T);
after_zero(<<>>) ->
    <<>>.
  ]]></code>

  <p>Yes, it will. The compiler will remove the building of the sub binary in
  the second clause:</p>

  <code type="erl"><![CDATA[
...
after_zero(<<_,T/binary>>) ->
    after_zero(T);
...]]></code>

  <p>But it will generate code that builds a sub binary in the first clause:</p>

  <code type="erl"><![CDATA[
after_zero(<<0,T/binary>>) ->
    T;
...]]></code>

  <p>Therefore, <c>after_zero/1</c> builds one match context and one sub binary
  (assuming it is passed a binary that contains a zero byte).</p>

  <p>Code like the following will also be optimized:</p>

  <code type="erl"><![CDATA[
all_but_zeroes_to_list(Buffer, Acc, 0) ->
    {lists:reverse(Acc),Buffer};
all_but_zeroes_to_list(<<0,T/binary>>, Acc, Remaining) ->
    all_but_zeroes_to_list(T, Acc, Remaining-1);
all_but_zeroes_to_list(<<Byte,T/binary>>, Acc, Remaining) ->
    all_but_zeroes_to_list(T, [Byte|Acc], Remaining-1).]]></code>

  <p>The compiler removes building of sub binaries in the second and third
  clauses, and it adds an instruction to the first clause that converts
  <c>Buffer</c> from a match context to a sub binary (or do nothing if
  <c>Buffer</c> is a binary already).</p>

  <p>But in more complicated code, how can one know whether the
  optimization is applied or not?</p>

  <section>
    <marker id="bin_opt_info"></marker>
    <title>Option bin_opt_info</title>

    <p>Use the <c>bin_opt_info</c> option to have the compiler print a lot of 
    information about binary optimizations. It can be given either to the
    compiler or <c>erlc</c>:</p>

  <code type="erl"><![CDATA[
erlc +bin_opt_info Mod.erl]]></code>

    <p>or passed through an environment variable:</p>

  <code type="erl"><![CDATA[
export ERL_COMPILER_OPTIONS=bin_opt_info]]></code>

    <p>Notice that the <c>bin_opt_info</c> is not meant to be a permanent
    option added to your <c>Makefile</c>s, because all messages that it
    generates cannot be eliminated. Therefore, passing the option through
    the environment is in most cases the most practical approach.</p>

    <p>The warnings look as follows:</p>

  <code type="erl"><![CDATA[
./efficiency_guide.erl:60: Warning: NOT OPTIMIZED: binary is returned from the function
./efficiency_guide.erl:62: Warning: OPTIMIZED: match context reused]]></code>

    <p>To make it clearer exactly what code the warnings refer to, the
    warnings in the following examples are inserted as comments
    after the clause they refer to, for example:</p>

  <code type="erl"><![CDATA[
after_zero(<<0,T/binary>>) ->
         %% BINARY CREATED: binary is returned from the function
    T;
after_zero(<<_,T/binary>>) ->
         %% OPTIMIZED: match context reused
    after_zero(T);
after_zero(<<>>) ->
    <<>>.]]></code>

    <p>The warning for the first clause says that the creation of a sub
    binary cannot be delayed, because it will be returned.
    The warning for the second clause says that a sub binary will not be
    created (yet).</p>
  </section>

  <section>
    <title>Unused Variables</title>

    <p>The compiler figures out if a variable is unused. The same
    code is generated for each of the following functions:</p>

  <code type="erl"><![CDATA[
count1(<<_,T/binary>>, Count) -> count1(T, Count+1);
count1(<<>>, Count) -> Count.

count2(<<H,T/binary>>, Count) -> count2(T, Count+1);
count2(<<>>, Count) -> Count.

count3(<<_H,T/binary>>, Count) -> count3(T, Count+1);
count3(<<>>, Count) -> Count.]]></code>

  <p>In each iteration, the first 8 bits in the binary will be skipped,
  not matched out.</p>
  </section>
  </section>

  <section>
    <title>Historical Note</title>

    <p>Binary handling was significantly improved in R12B. Because
    code that was efficient in R11B might not be efficient in R12B,
    and vice versa, earlier revisions of this Efficiency Guide contained
    some information about binary handling in R11B.</p>
  </section>

</chapter>