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document instalias.

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@ -1957,6 +1957,55 @@ on the current instruction set.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">Instruction Aliases</div>
<div class="doc_text">
<p>The most general phase of alias processing occurs while matching is
happening: it provides new forms for the matcher to match along with a specific
instruction to generate. An instruction alias has two parts: the string to
match and the instruction to generate. For example:
</p>
<div class="doc_code">
<pre>
def : InstAlias&lt;"movsx $src, $dst", (MOVSX16rr8W GR16:$dst, GR8 :$src)&gt;;
def : InstAlias&lt;"movsx $src, $dst", (MOVSX16rm8W GR16:$dst, i8mem:$src)&gt;;
def : InstAlias&lt;"movsx $src, $dst", (MOVSX32rr8 GR32:$dst, GR8 :$src)&gt;;
def : InstAlias&lt;"movsx $src, $dst", (MOVSX32rr16 GR32:$dst, GR16 :$src)&gt;;
def : InstAlias&lt;"movsx $src, $dst", (MOVSX64rr8 GR64:$dst, GR8 :$src)&gt;;
def : InstAlias&lt;"movsx $src, $dst", (MOVSX64rr16 GR64:$dst, GR16 :$src)&gt;;
def : InstAlias&lt;"movsx $src, $dst", (MOVSX64rr32 GR64:$dst, GR32 :$src)&gt;;
</pre>
</div>
<p>This shows a powerful example of the instruction aliases, matching the
same mnemonic in multiple different ways depending on what operands are present
in the assembly. The result of instruction aliases can include operands in a
different order than the destination instruction, and can use an input
multiple times, for example:</p>
<div class="doc_code">
<pre>
def : InstAlias&lt;"clrb $reg", (XOR8rr GR8 :$reg, GR8 :$reg)&gt;;
def : InstAlias&lt;"clrw $reg", (XOR16rr GR16:$reg, GR16:$reg)&gt;;
def : InstAlias&lt;"clrl $reg", (XOR32rr GR32:$reg, GR32:$reg)&gt;;
def : InstAlias&lt;"clrq $reg", (XOR64rr GR64:$reg, GR64:$reg)&gt;;
</pre>
</div>
<p>This example also shows that tied operands are only listed once. In the X86
backend, XOR8rr has two input GR8's and one output GR8 (where an input is tied
to the output). InstAliases take a flattened operand list without duplicates
for tied operands.</p>
<p>Instruction aliases can also have a Requires clause to make them
subtarget specific.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection" id="na_matching">Instruction Matching</div>