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<div class="section" id="regbankselect">
<span id="id1"></span><h1>RegBankSelect<a class="headerlink" href="#regbankselect" title="Permalink to this headline">¶</a></h1>
<p>This pass constrains the <a class="reference internal" href="GMIR.html#gmir-gvregs"><span class="std std-ref">Generic Virtual Registers</span></a> operands of generic
instructions to some <a class="reference internal" href="GMIR.html#gmir-regbank"><span class="std std-ref">Register Bank</span></a>.</p>
<p>It iteratively maps instructions to a set of per-operand bank assignment.
The possible mappings are determined by the target-provided
<a class="reference internal" href="#api-registerbankinfo"><span class="std std-ref">RegisterBankInfo</span></a>.
The mapping is then applied, possibly introducing <code class="docutils literal notranslate"><span class="pre">COPY</span></code> instructions if
necessary.</p>
<p>It traverses the <code class="docutils literal notranslate"><span class="pre">MachineFunction</span></code> top down so that all operands are already
mapped when analyzing an instruction.</p>
<p>This pass could also remap target-specific instructions when beneficial.
In the future, this could replace the ExeDepsFix pass, as we can directly
select the best variant for an instruction that’s available on multiple banks.</p>
<div class="section" id="api-registerbankinfo">
<span id="id2"></span><h2>API: RegisterBankInfo<a class="headerlink" href="#api-registerbankinfo" title="Permalink to this headline">¶</a></h2>
<p>The <code class="docutils literal notranslate"><span class="pre">RegisterBankInfo</span></code> class describes multiple aspects of register banks.</p>
<ul class="simple">
<li><p><strong>Banks</strong>: <code class="docutils literal notranslate"><span class="pre">addRegBankCoverage</span></code> — which register bank covers each
register class.</p></li>
<li><p><strong>Cross-Bank Copies</strong>: <code class="docutils literal notranslate"><span class="pre">copyCost</span></code> — the cost of a <code class="docutils literal notranslate"><span class="pre">COPY</span></code> from one bank
to another.</p></li>
<li><p><strong>Default Mapping</strong>: <code class="docutils literal notranslate"><span class="pre">getInstrMapping</span></code> — the default bank assignments for
a given instruction.</p></li>
<li><p><strong>Alternative Mapping</strong>: <code class="docutils literal notranslate"><span class="pre">getInstrAlternativeMapping</span></code> — the other
possible bank assignments for a given instruction.</p></li>
</ul>
<p><code class="docutils literal notranslate"><span class="pre">TODO</span></code>:
All this information should eventually be static and generated by TableGen,
mostly using existing information augmented by bank descriptions.</p>
<p><code class="docutils literal notranslate"><span class="pre">TODO</span></code>:
<code class="docutils literal notranslate"><span class="pre">getInstrMapping</span></code> is currently separate from <code class="docutils literal notranslate"><span class="pre">getInstrAlternativeMapping</span></code>
because the latter is more expensive: as we move to static mapping info,
both methods should be free, and we should merge them.</p>
</div>
<div class="section" id="regbankselect-modes">
<span id="id3"></span><h2>RegBankSelect Modes<a class="headerlink" href="#regbankselect-modes" title="Permalink to this headline">¶</a></h2>
<p><code class="docutils literal notranslate"><span class="pre">RegBankSelect</span></code> currently has two modes:</p>
<ul class="simple">
<li><p><strong>Fast</strong> — For each instruction, pick a target-provided “default” bank
assignment. This is the default at -O0.</p></li>
<li><p><strong>Greedy</strong> — For each instruction, pick the cheapest of several
target-provided bank assignment alternatives.</p></li>
</ul>
<p>We intend to eventually introduce an additional optimizing mode:</p>
<ul class="simple">
<li><p><strong>Global</strong> — Across multiple instructions, pick the cheapest combination of
bank assignments.</p></li>
</ul>
<p><code class="docutils literal notranslate"><span class="pre">NOTE</span></code>:
On AArch64, we are considering using the Greedy mode even at -O0 (or perhaps at
backend -O1): because <a class="reference internal" href="GMIR.html#gmir-llt"><span class="std std-ref">Low Level Type</span></a> doesn’t distinguish floating point from
integer scalars, the default assignment for loads and stores is the integer
bank, introducing cross-bank copies on most floating point operations.</p>
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