Add PNaCl release note about how to try out Subzero / which flag to set.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4091

Review URL: https://codereview.chromium.org/1059423002

Cr-Commit-Position: refs/heads/master@{#324321}

native_client_sdk/doc_generated/reference/nacl-manifest-format.html

@@ -77,7 +77,7 @@ section for the rules on URL resolution.</p>
   }
 }
 </pre>
-<p>Portable Native Client applications can also specify an <code>optlevel</code> field.
+<p id="pnacl-nmf-optlevels">Portable Native Client applications can also specify an <code>optlevel</code> field.
 The <code>optlevel</code> field is an optimization level <em>hint</em>, which is a number
 (zero and higher). Higher numbers indicate more optimization effort.
 Setting a higher optimization level will improve the application&#8217;s

native_client_sdk/doc_generated/sdk/release-notes.html

@@ -25,6 +25,16 @@ browser (version <code>Y</code>).</li>
 deprecated and will be removed in the next release. PNaCl has used libc++
 (which is much more up-to-date, currently based on LLVM 3.6) as the default
 since Pepper 33.</li>
+<li>PNaCl&#8217;s experimental <a class="reference external" href="https://chromium.googlesource.com/native_client/pnacl-subzero/+/master/README.rst">Subzero translator</a> is available for x86-32 NaCl in
+Chrome version 43, behind a flag. To give it a try, run Chrome with the
+<code>--enable-pnacl-subzero</code> commandline flag, and use the <code>optlevel 0</code>
+<a class="reference internal" href="/native-client/reference/nacl-manifest-format.html#pnacl-nmf-optlevels"><em>NaCl manifest option</em></a>. Application startup time
+should be several times faster than the previous LLVM-based <code>optlevel 0</code>
+mode, with similar code quality. Note that x86-32 NaCl requires a 32-bit
+Chrome. On Windows, it also requires a 32-bit Windows OS, but 64-bit Linux
+OSes can run x86-32 NaCl. If you try it out, please send us feedback
+on <a class="reference external" href="https://groups.google.com/forum/#!forum/native-client-dev">native-client-dev</a>. We are working on improvements and adding
+new targets.</li>
 </ul>
 <h3 id="pepper">Pepper</h3>
 <ul class="small-gap">

native_client_sdk/src/doc/reference/nacl-manifest-format.rst

@@ -77,6 +77,8 @@ Example of a ``program`` for Portable Native Client:
     }
   }
 
+.. _pnacl_nmf_optlevels:
+
 Portable Native Client applications can also specify an ``optlevel`` field.
 The ``optlevel`` field is an optimization level *hint*, which is a number
 (zero and higher). Higher numbers indicate more optimization effort.

native_client_sdk/src/doc/sdk/release-notes.rst

@@ -33,8 +33,20 @@ PNaCl
   deprecated and will be removed in the next release. PNaCl has used libc++
   (which is much more up-to-date, currently based on LLVM 3.6) as the default
   since Pepper 33.
+* PNaCl's experimental `Subzero translator`_ is available for x86-32 NaCl in
+  Chrome version 43, behind a flag. To give it a try, run Chrome with the
+  ``--enable-pnacl-subzero`` commandline flag, and use the ``optlevel 0``
+  :ref:`NaCl manifest option<pnacl_nmf_optlevels>`. Application startup time
+  should be several times faster than the previous LLVM-based ``optlevel 0``
+  mode, with similar code quality. Note that x86-32 NaCl requires a 32-bit
+  Chrome. On Windows, it also requires a 32-bit Windows OS, but 64-bit Linux
+  OSes can run x86-32 NaCl. If you try it out, please send us feedback
+  on `native-client-dev`_. We are working on improvements and adding
+  new targets.
 
 .. _`code generation bug on ARM`: https://code.google.com/p/chromium/issues/detail?id=460432
+.. _`Subzero translator`: https://chromium.googlesource.com/native_client/pnacl-subzero/+/master/README.rst
+.. _`native-client-dev`: https://groups.google.com/forum/#!forum/native-client-dev
 
 Pepper
 ------