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<a name="Pass-manager-1"></a>
<h3 class="section">9.3 Pass manager</h3>

<p>The pass manager is located in <samp>passes.c</samp>, <samp>tree-optimize.c</samp>
and <samp>tree-pass.h</samp>.
It processes passes as described in <samp>passes.def</samp>.
Its job is to run all of the individual passes in the correct order,
and take care of standard bookkeeping that applies to every pass.
</p>
<p>The theory of operation is that each pass defines a structure that
represents everything we need to know about that pass&mdash;when it
should be run, how it should be run, what intermediate language
form or on-the-side data structures it needs.  We register the pass
to be run in some particular order, and the pass manager arranges
for everything to happen in the correct order.
</p>
<p>The actuality doesn&rsquo;t completely live up to the theory at present.
Command-line switches and <code>timevar_id_t</code> enumerations must still
be defined elsewhere.  The pass manager validates constraints but does
not attempt to (re-)generate data structures or lower intermediate
language form based on the requirements of the next pass.  Nevertheless,
what is present is useful, and a far sight better than nothing at all.
</p>
<p>Each pass should have a unique name.
Each pass may have its own dump file (for GCC debugging purposes).
Passes with a name starting with a star do not dump anything.
Sometimes passes are supposed to share a dump file / option name.
To still give these unique names, you can use a prefix that is delimited
by a space from the part that is used for the dump file / option name.
E.g. When the pass name is &quot;ud dce&quot;, the name used for dump file/options
is &quot;dce&quot;.
</p>
<p>TODO: describe the global variables set up by the pass manager,
and a brief description of how a new pass should use it.
I need to look at what info RTL passes use first<small class="enddots">...</small>
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