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  <header>
    <copyright>
      <year>1997</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.

    </legalnotice>

    <title>HiPE</title>
    <prepared></prepared>
    <responsible></responsible>
    <docno></docno>
    <approved></approved>
    <checked></checked>
    <date></date>
    <rev></rev>
    <file>hipe.xml</file>
  </header>
  <app>HiPE</app>
  <appsummary>The HiPE Application</appsummary>
  <description>
    <note>
      <p>
	HiPE and execution of HiPE compiled code only have limited support by
	the OTP team at Ericsson. The OTP team only does limited maintenance
	of HiPE and does not actively develop HiPE. HiPE is mainly supported
	by the HiPE team at Uppsala University.
      </p>
    </note>
    <p>
      The normal way to native-compile an Erlang module using HiPE is to include the atom native 
      in the Erlang compiler options, as in:</p>
      <pre>
      1> <input>c(my_module, [native]).</input></pre>
      <p>Options to the HiPE compiler are then passed as follows:</p>
      <pre>
      1> <input>c(my_module, [native,{hipe,Options}]).</input></pre>
      <p>For on-line help in the Erlang shell, call <c>hipe:help()</c>. 
      Details on HiPE compiler options are given by <c>hipe:help_options()</c>.</p>
  </description>  
  <section>
    <title>Feature Limitations</title>
    <p>
      The HiPE compiler is in general compliant with the normal BEAM compiler,
      with respect to semantic behavior. There are however features in the BEAM compiler
      and the runtime system that have limited or no support for HiPE compiled modules.
    </p>
    <taglist>
      <tag>Stack traces</tag>
      <item><p>Stack traces returned from <seealso marker="erts:erlang#get_stacktrace/0">
      <c>erlang:get_stacktrace/0</c></seealso> or as part of <c>'EXIT'</c> terms
      can look incomplete if HiPE compiled functions are involved. Typically a stack trace
      will contain only BEAM compiled functions or only HiPE compiled functions, depending
      on where the exception was raised.</p>
      <p>Source code line numbers in stack traces are also not supported by HiPE compiled functions.</p>
      </item>

      <tag>Tracing</tag>
      <item><p>Erlang call trace is not supported by HiPE. Calling
      <seealso marker="erts:erlang#trace_pattern/3"><c>erlang:trace_pattern({M,F,A}, ...)</c></seealso>
      does not have any effect on HiPE compiled modules.</p>
      </item>

      <tag>NIFs</tag>
      <item><p>Modules compiled with HiPE can not call <seealso marker="erts:erlang#load_nif-2">
      <c>erlang:load_nif/2</c></seealso> to load NIFs.</p>
      </item>

      <tag>-on_load</tag>
      <item><p>Modules compiled with HiPE can not use
      <seealso marker="doc/reference_manual:code_loading#on_load"><c>-on_load()</c></seealso>
      directives.</p>
      </item>
    </taglist>

  </section>
  <section>
    <title>Performance Limitations</title>
    <p>
      The HiPE compiler does in general produce faster code than the
      BEAM compiler. There are however some situation when HiPE
      compiled code will perform worse than BEAM code.
    </p>
    <taglist>
      <tag>Mode switches</tag>
      <item><p>Every time a process changes from executing code in a
      HiPE compiled module to a BEAM compiled module (or vice versa),
      it will do a mode switch. This involves a certain amount of
      CPU overhead which can have a negative net impact if the
      process is switching back and forth without getting enough done in
      each mode.</p>
      </item>

      <tag>Optimization for <c>receive</c> with unique references</tag>
      <item>
	<p>
	  The BEAM compiler can do an optimization when a receive
	  statement is only waiting for messages containing a reference
	  created before the receive. All messages that existed in the
	  queue when the reference was created will be bypassed, as they
	  cannot possibly contain the reference. HiPE currently has an
	  optimization similar this, but it is not guaranteed to
	  bypass all messages. In the worst case scenario, it cannot
	  bypass any messages at all.
	</p>
	<p>
	  An example of this is when <c>gen_server:call()</c> waits for
	  the reply message.
	</p>
      </item>

      <tag>Garbage collection after BIFs</tag>
      <item>
	<p>
	  The condition for determining whether a garbage collection
	  is needed or not has changed in later releases. HiPE has not
	  been updated regarding this which may cause premature garbage
	  collections after BIF calls.
	</p>
      </item>

    </taglist>
  </section>
  <section>
    <title>Stability Issues</title>
    <taglist>
      <tag>Not checking reduction count on function returns</tag>
      <item>
	<p>
	  BEAM checks the reduction count and schedules out the executing
	  process if needed both when calling a function and when returning
	  from a function call that was not called using a tail call.
	  HiPE only checks the reduction count when calling a function.
	</p>
	<p>
	  The runtime system might need to schedule out a process
	  in order to reclaim memory. If the process isn't scheduled
	  out soon after the process has entered this state, memory
	  consumption will quickly grow. Maintaining this state is also
	  quite expensive performance wise.
	</p>
	<p>
	  Processes executing code that performs large recursions and
	  produce data after returning from recursive calls may have to
	  be scheduled out when returning from a function call. Since
	  HiPE does not check reductions on returns, processes executing
	  such HiPE compiled code may cause huge peeks in memory
	  consumption as well as severe performance degradation.
	</p>
      </item>

      <tag>Not bumping appropriate amount of reductions in <c>receive</c> statements</tag>
      <item>
	<p>
	  The process signaling improvements made in ERTS version
	  10.0 moved potentially significant amounts of work into the
	  receive statement from other places. In order to account for
	  this work, the reduction count should be bumped on the
	  executing process. Reductions are not bumped when entering
	  the <c>receive</c> statement from HiPE compiled code.
	</p>
      </item>
    </taglist>
  </section>
  <section>
    <title>SEE ALSO</title>
    <p>
      <seealso marker="stdlib:c">c(3)</seealso>, 
      <seealso marker="compiler:compile">compile(3)</seealso>
    </p>
  </section>

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