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<div class="section" id="legalizer">
<span id="milegalizer"></span><h1>Legalizer<a class="headerlink" href="#legalizer" title="Permalink to this headline">¶</a></h1>
<p>This pass transforms the generic machine instructions such that they are legal.</p>
<p>A legal instruction is defined as:</p>
<ul class="simple">
<li><p><strong>selectable</strong> — the target will later be able to select it to a
target-specific (non-generic) instruction. This doesn’t necessarily mean that
<a class="reference internal" href="InstructionSelect.html"><span class="doc">InstructionSelect</span></a> has to handle it though. It just means that
<strong>something</strong> must handle it.</p></li>
<li><p>operating on <strong>vregs that can be loaded and stored</strong> – if necessary, the
target can select a <code class="docutils literal notranslate"><span class="pre">G_LOAD</span></code>/<code class="docutils literal notranslate"><span class="pre">G_STORE</span></code> of each gvreg operand.</p></li>
</ul>
<p>As opposed to SelectionDAG, there are no legalization phases. In particular,
‘type’ and ‘operation’ legalization are not separate.</p>
<p>Legalization is iterative, and all state is contained in GMIR. To maintain the
validity of the intermediate code, instructions are introduced:</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">G_MERGE_VALUES</span></code> — concatenate multiple registers of the same
size into a single wider register.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">G_UNMERGE_VALUES</span></code> — extract multiple registers of the same size
from a single wider register.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">G_EXTRACT</span></code> — extract a simple register (as contiguous sequences of bits)
from a single wider register.</p></li>
</ul>
<p>As they are expected to be temporary byproducts of the legalization process,
they are combined at the end of the <a class="reference internal" href="#milegalizer"><span class="std std-ref">Legalizer</span></a> pass.
If any remain, they are expected to always be selectable, using loads and stores
if necessary.</p>
<p>The legality of an instruction may only depend on the instruction itself and
must not depend on any context in which the instruction is used. However, after
deciding that an instruction is not legal, using the context of the instruction
to decide how to legalize the instruction is permitted. As an example, if we
have a <code class="docutils literal notranslate"><span class="pre">G_FOO</span></code> instruction of the form:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">%</span><span class="mi">1</span><span class="p">:</span><span class="n">_</span><span class="p">(</span><span class="n">s32</span><span class="p">)</span> <span class="o">=</span> <span class="n">G_CONSTANT</span> <span class="n">i32</span> <span class="mi">1</span>
<span class="o">%</span><span class="mi">2</span><span class="p">:</span><span class="n">_</span><span class="p">(</span><span class="n">s32</span><span class="p">)</span> <span class="o">=</span> <span class="n">G_FOO</span> <span class="o">%</span><span class="mi">0</span><span class="p">:</span><span class="n">_</span><span class="p">(</span><span class="n">s32</span><span class="p">),</span> <span class="o">%</span><span class="mi">1</span><span class="p">:</span><span class="n">_</span><span class="p">(</span><span class="n">s32</span><span class="p">)</span>
</pre></div>
</div>
<p>it’s impossible to say that G_FOO is legal iff %1 is a <code class="docutils literal notranslate"><span class="pre">G_CONSTANT</span></code> with
value <code class="docutils literal notranslate"><span class="pre">1</span></code>. However, the following:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">%</span><span class="mi">2</span><span class="p">:</span><span class="n">_</span><span class="p">(</span><span class="n">s32</span><span class="p">)</span> <span class="o">=</span> <span class="n">G_FOO</span> <span class="o">%</span><span class="mi">0</span><span class="p">:</span><span class="n">_</span><span class="p">(</span><span class="n">s32</span><span class="p">),</span> <span class="n">i32</span> <span class="mi">1</span>
</pre></div>
</div>
<p>can say that it’s legal iff operand 2 is an immediate with value <code class="docutils literal notranslate"><span class="pre">1</span></code> because
that information is entirely contained within the single instruction.</p>
<div class="section" id="api-legalizerinfo">
<span id="id1"></span><h2>API: LegalizerInfo<a class="headerlink" href="#api-legalizerinfo" title="Permalink to this headline">¶</a></h2>
<p>The recommended <a class="footnote-reference brackets" href="#legalizer-legacy-footnote" id="id2">1</a> API looks like this:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">getActionDefinitionsBuilder</span><span class="p">({</span><span class="n">G_ADD</span><span class="p">,</span> <span class="n">G_SUB</span><span class="p">,</span> <span class="n">G_MUL</span><span class="p">,</span> <span class="n">G_AND</span><span class="p">,</span> <span class="n">G_OR</span><span class="p">,</span> <span class="n">G_XOR</span><span class="p">,</span> <span class="n">G_SHL</span><span class="p">})</span>
<span class="o">.</span><span class="n">legalFor</span><span class="p">({</span><span class="n">s32</span><span class="p">,</span> <span class="n">s64</span><span class="p">,</span> <span class="n">v2s32</span><span class="p">,</span> <span class="n">v4s32</span><span class="p">,</span> <span class="n">v2s64</span><span class="p">})</span>
<span class="o">.</span><span class="n">clampScalar</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="n">s32</span><span class="p">,</span> <span class="n">s64</span><span class="p">)</span>
<span class="o">.</span><span class="n">widenScalarToNextPow2</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="o">.</span><span class="n">clampNumElements</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="n">v2s32</span><span class="p">,</span> <span class="n">v4s32</span><span class="p">)</span>
<span class="o">.</span><span class="n">clampNumElements</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="n">v2s64</span><span class="p">,</span> <span class="n">v2s64</span><span class="p">)</span>
<span class="o">.</span><span class="n">moreElementsToNextPow2</span><span class="p">(</span><span class="mi">0</span><span class="p">);</span>
</pre></div>
</div>
<p>and describes a set of rules by which we can either declare an instruction legal
or decide which action to take to make it more legal.</p>
<p>At the core of this ruleset is the <code class="docutils literal notranslate"><span class="pre">LegalityQuery</span></code> which describes the
instruction. We use a description rather than the instruction to both allow other
passes to determine legality without having to create an instruction and also to
limit the information available to the predicates to that which is safe to rely
on. Currently, the information available to the predicates that determine
legality contains:</p>
<ul class="simple">
<li><p>The opcode for the instruction</p></li>
<li><p>The type of each type index (see <code class="docutils literal notranslate"><span class="pre">type0</span></code>, <code class="docutils literal notranslate"><span class="pre">type1</span></code>, etc.)</p></li>
<li><p>The size in bytes and atomic ordering for each MachineMemOperand</p></li>
</ul>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>An alternative worth investigating is to generalize the API to represent
actions using <code class="docutils literal notranslate"><span class="pre">std::function</span></code> that implements the action, instead of explicit
enum tokens (<code class="docutils literal notranslate"><span class="pre">Legal</span></code>, <code class="docutils literal notranslate"><span class="pre">WidenScalar</span></code>, …) that instruct it to call a
function. This would have some benefits, most notable being that Custom could
be removed.</p>
</div>
<p class="rubric">Footnotes</p>
<dl class="footnote brackets">
<dt class="label" id="legalizer-legacy-footnote"><span class="brackets"><a class="fn-backref" href="#id2">1</a></span></dt>
<dd><p>An API is broadly similar to
SelectionDAG/TargetLowering is available but is not recommended as a more
powerful API is available.</p>
</dd>
</dl>
<div class="section" id="rule-processing-and-declaring-rules">
<h3>Rule Processing and Declaring Rules<a class="headerlink" href="#rule-processing-and-declaring-rules" title="Permalink to this headline">¶</a></h3>
<p>The <code class="docutils literal notranslate"><span class="pre">getActionDefinitionsBuilder</span></code> function generates a ruleset for the given
opcode(s) that rules can be added to. If multiple opcodes are given, they are
all permanently bound to the same ruleset. The rules in a ruleset are executed
from top to bottom and will start again from the top if an instruction is
legalized as a result of the rules. If the ruleset is exhausted without
satisfying any rule, then it is considered unsupported.</p>
<p>When it doesn’t declare the instruction legal, each pass over the rules may
request that one type changes to another type. Sometimes this can cause multiple
types to change but we avoid this as much as possible as making multiple changes
can make it difficult to avoid infinite loops where, for example, narrowing one
type causes another to be too small and widening that type causes the first one
to be too big.</p>
<p>In general, it’s advisable to declare instructions legal as close to the top of
the rule as possible and to place any expensive rules as low as possible. This
helps with performance as testing for legality happens more often than
legalization and legalization can require multiple passes over the rules.</p>
<p>As a concrete example, consider the rule:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">getActionDefinitionsBuilder</span><span class="p">({</span><span class="n">G_ADD</span><span class="p">,</span> <span class="n">G_SUB</span><span class="p">,</span> <span class="n">G_MUL</span><span class="p">,</span> <span class="n">G_AND</span><span class="p">,</span> <span class="n">G_OR</span><span class="p">,</span> <span class="n">G_XOR</span><span class="p">,</span> <span class="n">G_SHL</span><span class="p">})</span>
<span class="o">.</span><span class="n">legalFor</span><span class="p">({</span><span class="n">s32</span><span class="p">,</span> <span class="n">s64</span><span class="p">,</span> <span class="n">v2s32</span><span class="p">,</span> <span class="n">v4s32</span><span class="p">,</span> <span class="n">v2s64</span><span class="p">})</span>
<span class="o">.</span><span class="n">clampScalar</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="n">s32</span><span class="p">,</span> <span class="n">s64</span><span class="p">)</span>
<span class="o">.</span><span class="n">widenScalarToNextPow2</span><span class="p">(</span><span class="mi">0</span><span class="p">);</span>
</pre></div>
</div>
<p>and the instruction:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">%</span><span class="mi">2</span><span class="p">:</span><span class="n">_</span><span class="p">(</span><span class="n">s7</span><span class="p">)</span> <span class="o">=</span> <span class="n">G_ADD</span> <span class="o">%</span><span class="mi">0</span><span class="p">:</span><span class="n">_</span><span class="p">(</span><span class="n">s7</span><span class="p">),</span> <span class="o">%</span><span class="mi">1</span><span class="p">:</span><span class="n">_</span><span class="p">(</span><span class="n">s7</span><span class="p">)</span>
</pre></div>
</div>
<p>this doesn’t meet the predicate for the <a class="reference internal" href="#legalfor"><span class="std std-ref">.legalFor()</span></a> as <code class="docutils literal notranslate"><span class="pre">s7</span></code>
is not one of the listed types so it falls through to the
<a class="reference internal" href="#clampscalar"><span class="std std-ref">.clampScalar()</span></a>. It does meet the predicate for this rule
as the type is smaller than the <code class="docutils literal notranslate"><span class="pre">s32</span></code> and this rule instructs the legalizer
to change type 0 to <code class="docutils literal notranslate"><span class="pre">s32</span></code>. It then restarts from the top. This time it does
satisfy <code class="docutils literal notranslate"><span class="pre">.legalFor()</span></code> and the resulting output is:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">%</span><span class="mi">3</span><span class="p">:</span><span class="n">_</span><span class="p">(</span><span class="n">s32</span><span class="p">)</span> <span class="o">=</span> <span class="n">G_ANYEXT</span> <span class="o">%</span><span class="mi">0</span><span class="p">:</span><span class="n">_</span><span class="p">(</span><span class="n">s7</span><span class="p">)</span>
<span class="o">%</span><span class="mi">4</span><span class="p">:</span><span class="n">_</span><span class="p">(</span><span class="n">s32</span><span class="p">)</span> <span class="o">=</span> <span class="n">G_ANYEXT</span> <span class="o">%</span><span class="mi">1</span><span class="p">:</span><span class="n">_</span><span class="p">(</span><span class="n">s7</span><span class="p">)</span>
<span class="o">%</span><span class="mi">5</span><span class="p">:</span><span class="n">_</span><span class="p">(</span><span class="n">s32</span><span class="p">)</span> <span class="o">=</span> <span class="n">G_ADD</span> <span class="o">%</span><span class="mi">3</span><span class="p">:</span><span class="n">_</span><span class="p">(</span><span class="n">s32</span><span class="p">),</span> <span class="o">%</span><span class="mi">4</span><span class="p">:</span><span class="n">_</span><span class="p">(</span><span class="n">s32</span><span class="p">)</span>
<span class="o">%</span><span class="mi">2</span><span class="p">:</span><span class="n">_</span><span class="p">(</span><span class="n">s7</span><span class="p">)</span> <span class="o">=</span> <span class="n">G_TRUNC</span> <span class="o">%</span><span class="mi">5</span><span class="p">:</span><span class="n">_</span><span class="p">(</span><span class="n">s32</span><span class="p">)</span>
</pre></div>
</div>
<p>where the <code class="docutils literal notranslate"><span class="pre">G_ADD</span></code> is legal and the other instructions are scheduled for
processing by the legalizer.</p>
</div>
<div class="section" id="rule-actions">
<h3>Rule Actions<a class="headerlink" href="#rule-actions" title="Permalink to this headline">¶</a></h3>
<p>There are various rule factories that append rules to a ruleset but they have a
few actions in common:</p>
<ul class="simple" id="legalfor">
<li><p><code class="docutils literal notranslate"><span class="pre">legalIf()</span></code>, <code class="docutils literal notranslate"><span class="pre">legalFor()</span></code>, etc. declare an instruction to be legal if the
predicate is satisfied.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">narrowScalarIf()</span></code>, <code class="docutils literal notranslate"><span class="pre">narrowScalarFor()</span></code>, etc. declare an instruction to be illegal
if the predicate is satisfied and indicates that narrowing the scalars in one
of the types to a specific type would make it more legal. This action supports
both scalars and vectors.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">widenScalarIf()</span></code>, <code class="docutils literal notranslate"><span class="pre">widenScalarFor()</span></code>, etc. declare an instruction to be illegal
if the predicate is satisfied and indicates that widening the scalars in one
of the types to a specific type would make it more legal. This action supports
both scalars and vectors.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">fewerElementsIf()</span></code>, <code class="docutils literal notranslate"><span class="pre">fewerElementsFor()</span></code>, etc. declare an instruction to be
illegal if the predicate is satisfied and indicates reducing the number of
vector elements in one of the types to a specific type would make it more
legal. This action supports vectors.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">moreElementsIf()</span></code>, <code class="docutils literal notranslate"><span class="pre">moreElementsFor()</span></code>, etc. declare an instruction to be illegal
if the predicate is satisfied and indicates increasing the number of vector
elements in one of the types to a specific type would make it more legal.
This action supports vectors.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">lowerIf()</span></code>, <code class="docutils literal notranslate"><span class="pre">lowerFor()</span></code>, etc. declare an instruction to be
illegal if the predicate is satisfied and indicates that replacing
it with equivalent instruction(s) would make it more legal. Support
for this action differs for each opcode. These may provide an
optional LegalizeMutation containing a type to attempt to perform
the expansion in a different type.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">libcallIf()</span></code>, <code class="docutils literal notranslate"><span class="pre">libcallFor()</span></code>, etc. declare an instruction to be illegal if the
predicate is satisfied and indicates that replacing it with a libcall would
make it more legal. Support for this action differs for
each opcode.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">customIf()</span></code>, <code class="docutils literal notranslate"><span class="pre">customFor()</span></code>, etc. declare an instruction to be illegal if the
predicate is satisfied and indicates that the backend developer will supply
a means of making it more legal.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">unsupportedIf()</span></code>, <code class="docutils literal notranslate"><span class="pre">unsupportedFor()</span></code>, etc. declare an instruction to be illegal
if the predicate is satisfied and indicates that there is no way to make it
legal and the compiler should fail.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">fallback()</span></code> falls back on an older API and should only be used while porting
existing code from that API.</p></li>
</ul>
</div>
<div class="section" id="rule-predicates">
<h3>Rule Predicates<a class="headerlink" href="#rule-predicates" title="Permalink to this headline">¶</a></h3>
<p>The rule factories also have predicates in common:</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">legal()</span></code>, <code class="docutils literal notranslate"><span class="pre">lower()</span></code>, etc. are always satisfied.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">legalIf()</span></code>, <code class="docutils literal notranslate"><span class="pre">narrowScalarIf()</span></code>, etc. are satisfied if the user-supplied
<code class="docutils literal notranslate"><span class="pre">LegalityPredicate</span></code> function returns true. This predicate has access to the
information in the <code class="docutils literal notranslate"><span class="pre">LegalityQuery</span></code> to make its decision.
User-supplied predicates can also be combined using <code class="docutils literal notranslate"><span class="pre">all(P0,</span> <span class="pre">P1,</span> <span class="pre">...)</span></code>.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">legalFor()</span></code>, <code class="docutils literal notranslate"><span class="pre">narrowScalarFor()</span></code>, etc. are satisfied if the type matches one in
a given set of types. For example <code class="docutils literal notranslate"><span class="pre">.legalFor({s16,</span> <span class="pre">s32})</span></code> declares the
instruction legal if type 0 is either s16 or s32. Additional versions for two
and three type indices are generally available. For these, all the type
indices considered together must match all the types in one of the tuples. So
<code class="docutils literal notranslate"><span class="pre">.legalFor({{s16,</span> <span class="pre">s32},</span> <span class="pre">{s32,</span> <span class="pre">s64}})</span></code> will only accept <code class="docutils literal notranslate"><span class="pre">{s16,</span> <span class="pre">s32}</span></code>, or
<code class="docutils literal notranslate"><span class="pre">{s32,</span> <span class="pre">s64}</span></code> but will not accept <code class="docutils literal notranslate"><span class="pre">{s16,</span> <span class="pre">s64}</span></code>.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">legalForTypesWithMemSize()</span></code>, <code class="docutils literal notranslate"><span class="pre">narrowScalarForTypesWithMemSize()</span></code>, etc. are
similar to <code class="docutils literal notranslate"><span class="pre">legalFor()</span></code>, <code class="docutils literal notranslate"><span class="pre">narrowScalarFor()</span></code>, etc. but additionally require a
MachineMemOperand to have a given size in each tuple.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">legalForCartesianProduct()</span></code>, <code class="docutils literal notranslate"><span class="pre">narrowScalarForCartesianProduct()</span></code>, etc. are
satisfied if each type index matches one element in each of the independent
sets. So <code class="docutils literal notranslate"><span class="pre">.legalForCartesianProduct({s16,</span> <span class="pre">s32},</span> <span class="pre">{s32,</span> <span class="pre">s64})</span></code> will accept
<code class="docutils literal notranslate"><span class="pre">{s16,</span> <span class="pre">s32}</span></code>, <code class="docutils literal notranslate"><span class="pre">{s16,</span> <span class="pre">s64}</span></code>, <code class="docutils literal notranslate"><span class="pre">{s32,</span> <span class="pre">s32}</span></code>, and <code class="docutils literal notranslate"><span class="pre">{s32,</span> <span class="pre">s64}</span></code>.</p></li>
</ul>
</div>
<div class="section" id="composite-rules">
<h3>Composite Rules<a class="headerlink" href="#composite-rules" title="Permalink to this headline">¶</a></h3>
<p>There are some composite rules for common situations built out of the above facilities:</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">widenScalarToNextPow2()</span></code> is like <code class="docutils literal notranslate"><span class="pre">widenScalarIf()</span></code> but is satisfied iff the type
size in bits is not a power of 2 and selects a target type that is the next
largest power of 2.</p></li>
</ul>
<ul class="simple" id="clampscalar">
<li><p><code class="docutils literal notranslate"><span class="pre">minScalar()</span></code> is like <code class="docutils literal notranslate"><span class="pre">widenScalarIf()</span></code> but is satisfied iff the type
size in bits is smaller than the given minimum and selects the minimum as the
target type. Similarly, there is also a <code class="docutils literal notranslate"><span class="pre">maxScalar()</span></code> for the maximum and a
<code class="docutils literal notranslate"><span class="pre">clampScalar()</span></code> to do both at once.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">minScalarSameAs()</span></code> is like <code class="docutils literal notranslate"><span class="pre">minScalar()</span></code> but the minimum is taken from another
type index.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">moreElementsToNextMultiple()</span></code> is like <code class="docutils literal notranslate"><span class="pre">moreElementsToNextPow2()</span></code> but is based on
multiples of X rather than powers of 2.</p></li>
</ul>
</div>
</div>
<div class="section" id="minimum-rule-set">
<span id="min-legalizerinfo"></span><h2>Minimum Rule Set<a class="headerlink" href="#minimum-rule-set" title="Permalink to this headline">¶</a></h2>
<p>GlobalISel’s legalizer has a great deal of flexibility in how a given target
shapes the GMIR that the rest of the backend must handle. However, there are
a small number of requirements that all targets must meet.</p>
<p>Before discussing the minimum requirements, we’ll need some terminology:</p>
<dl class="simple">
<dt>Producer Type Set</dt><dd><p>The set of types which is the union of all possible types produced by at
least one legal instruction.</p>
</dd>
<dt>Consumer Type Set</dt><dd><p>The set of types which is the union of all possible types consumed by at
least one legal instruction.</p>
</dd>
</dl>
<p>Both sets are often identical but there’s no guarantee of that. For example,
it’s not uncommon to be unable to consume s64 but still be able to produce it
for a few specific instructions.</p>
<div class="section" id="minimum-rules-for-scalars">
<h3>Minimum Rules For Scalars<a class="headerlink" href="#minimum-rules-for-scalars" title="Permalink to this headline">¶</a></h3>
<ul class="simple">
<li><p>G_ANYEXT must be legal for all inputs from the producer type set and all larger
outputs from the consumer type set.</p></li>
<li><p>G_TRUNC must be legal for all inputs from the producer type set and all
smaller outputs from the consumer type set.</p></li>
</ul>
<p>G_ANYEXT, and G_TRUNC have mandatory legality since the GMIR requires a means to
connect operations with different type sizes. They are usually trivial to support
since G_ANYEXT doesn’t define the value of the additional bits and G_TRUNC is
discarding bits. The other conversions can be lowered into G_ANYEXT/G_TRUNC
with some additional operations that are subject to further legalization. For
example, G_SEXT can lower to:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">%</span><span class="mi">1</span> <span class="o">=</span> <span class="n">G_ANYEXT</span> <span class="o">%</span><span class="mi">0</span>
<span class="o">%</span><span class="mi">2</span> <span class="o">=</span> <span class="n">G_CONSTANT</span> <span class="o">...</span>
<span class="o">%</span><span class="mi">3</span> <span class="o">=</span> <span class="n">G_SHL</span> <span class="o">%</span><span class="mi">1</span><span class="p">,</span> <span class="o">%</span><span class="mi">2</span>
<span class="o">%</span><span class="mi">4</span> <span class="o">=</span> <span class="n">G_ASHR</span> <span class="o">%</span><span class="mi">3</span><span class="p">,</span> <span class="o">%</span><span class="mi">2</span>
</pre></div>
</div>
<p>and the G_CONSTANT/G_SHL/G_ASHR can further lower to other operations or target
instructions. Similarly, G_FPEXT has no legality requirement since it can lower
to a G_ANYEXT followed by a target instruction.</p>
<p>G_MERGE_VALUES and G_UNMERGE_VALUES do not have legality requirements since the
former can lower to G_ANYEXT and some other legalizable instructions, while the
latter can lower to some legalizable instructions followed by G_TRUNC.</p>
</div>
<div class="section" id="minimum-legality-for-vectors">
<h3>Minimum Legality For Vectors<a class="headerlink" href="#minimum-legality-for-vectors" title="Permalink to this headline">¶</a></h3>
<p>Within the vector types, there aren’t any defined conversions in LLVM IR as
vectors are often converted by reinterpreting the bits or by decomposing the
vector and reconstituting it as a different type. As such, G_BITCAST is the
only operation to account for. We generally don’t require that it’s legal
because it can usually be lowered to COPY (or to nothing using
replaceAllUses()). However, there are situations where G_BITCAST is non-trivial
(e.g. little-endian vectors of big-endian data such as on big-endian MIPS MSA and
big-endian ARM NEON, see <cite>_i_bitcast</cite>). To account for this G_BITCAST must be
legal for all type combinations that change the bit pattern in the value.</p>
<p>There are no legality requirements for G_BUILD_VECTOR, or G_BUILD_VECTOR_TRUNC
since these can be handled by:
* Declaring them legal.
* Scalarizing them.
* Lowering them to G_TRUNC+G_ANYEXT and some legalizable instructions.
* Lowering them to target instructions which are legal by definition.</p>
<p>The same reasoning also allows G_UNMERGE_VALUES to lack legality requirements
for vector inputs.</p>
</div>
<div class="section" id="minimum-legality-for-pointers">
<h3>Minimum Legality for Pointers<a class="headerlink" href="#minimum-legality-for-pointers" title="Permalink to this headline">¶</a></h3>
<p>There are no minimum rules for pointers since G_INTTOPTR and G_PTRTOINT can
be selected to a COPY from register class to another by the legalizer.</p>
</div>
<div class="section" id="minimum-legality-for-operations">
<h3>Minimum Legality For Operations<a class="headerlink" href="#minimum-legality-for-operations" title="Permalink to this headline">¶</a></h3>
<p>The rules for G_ANYEXT, G_MERGE_VALUES, G_BITCAST, G_BUILD_VECTOR,
G_BUILD_VECTOR_TRUNC, G_CONCAT_VECTORS, G_UNMERGE_VALUES, G_PTRTOINT, and
G_INTTOPTR have already been noted above. In addition to those, the following
operations have requirements:</p>
<ul class="simple">
<li><p>At least one G_IMPLICIT_DEF must be legal. This is usually trivial as it
requires no code to be selected.</p></li>
<li><p>G_PHI must be legal for all types in the producer and consumer typesets. This
is usually trivial as it requires no code to be selected.</p></li>
<li><p>At least one G_FRAME_INDEX must be legal</p></li>
<li><p>At least one G_BLOCK_ADDR must be legal</p></li>
</ul>
<p>There are many other operations you’d expect to have legality requirements but
they can be lowered to target instructions which are legal by definition.</p>
</div>
</div>
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