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<title>GNU Compiler Collection (GCC) Internals: Insn Canonicalizations</title>
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<a name="Insn-Canonicalizations"></a>
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<div class="header">
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<p>
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Next: <a href="Expander-Definitions.html#Expander-Definitions" accesskey="n" rel="next">Expander Definitions</a>, Previous: <a href="Looping-Patterns.html#Looping-Patterns" accesskey="p" rel="prev">Looping Patterns</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
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<a name="Canonicalization-of-Instructions"></a>
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<h3 class="section">16.14 Canonicalization of Instructions</h3>
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<a name="index-canonicalization-of-instructions"></a>
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<a name="index-insn-canonicalization"></a>
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<p>There are often cases where multiple RTL expressions could represent an
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operation performed by a single machine instruction. This situation is
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most commonly encountered with logical, branch, and multiply-accumulate
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instructions. In such cases, the compiler attempts to convert these
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multiple RTL expressions into a single canonical form to reduce the
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number of insn patterns required.
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</p>
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<p>In addition to algebraic simplifications, following canonicalizations
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are performed:
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</p>
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<ul>
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<li> For commutative and comparison operators, a constant is always made the
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second operand. If a machine only supports a constant as the second
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operand, only patterns that match a constant in the second operand need
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be supplied.
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</li><li> For associative operators, a sequence of operators will always chain
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to the left; for instance, only the left operand of an integer <code>plus</code>
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can itself be a <code>plus</code>. <code>and</code>, <code>ior</code>, <code>xor</code>,
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<code>plus</code>, <code>mult</code>, <code>smin</code>, <code>smax</code>, <code>umin</code>, and
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<code>umax</code> are associative when applied to integers, and sometimes to
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floating-point.
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</li><li> <a name="index-neg_002c-canonicalization-of"></a>
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<a name="index-not_002c-canonicalization-of"></a>
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<a name="index-mult_002c-canonicalization-of"></a>
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<a name="index-plus_002c-canonicalization-of"></a>
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<a name="index-minus_002c-canonicalization-of"></a>
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For these operators, if only one operand is a <code>neg</code>, <code>not</code>,
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<code>mult</code>, <code>plus</code>, or <code>minus</code> expression, it will be the
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first operand.
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</li><li> In combinations of <code>neg</code>, <code>mult</code>, <code>plus</code>, and
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<code>minus</code>, the <code>neg</code> operations (if any) will be moved inside
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the operations as far as possible. For instance,
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<code>(neg (mult A B))</code> is canonicalized as <code>(mult (neg A) B)</code>, but
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<code>(plus (mult (neg B) C) A)</code> is canonicalized as
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<code>(minus A (mult B C))</code>.
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</li><li> <a name="index-compare_002c-canonicalization-of"></a>
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For the <code>compare</code> operator, a constant is always the second operand
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if the first argument is a condition code register or <code>(cc0)</code>.
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</li><li> An operand of <code>neg</code>, <code>not</code>, <code>mult</code>, <code>plus</code>, or
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<code>minus</code> is made the first operand under the same conditions as
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above.
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</li><li> <code>(ltu (plus <var>a</var> <var>b</var>) <var>b</var>)</code> is converted to
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<code>(ltu (plus <var>a</var> <var>b</var>) <var>a</var>)</code>. Likewise with <code>geu</code> instead
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of <code>ltu</code>.
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</li><li> <code>(minus <var>x</var> (const_int <var>n</var>))</code> is converted to
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<code>(plus <var>x</var> (const_int <var>-n</var>))</code>.
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</li><li> Within address computations (i.e., inside <code>mem</code>), a left shift is
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converted into the appropriate multiplication by a power of two.
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</li><li> <a name="index-ior_002c-canonicalization-of"></a>
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<a name="index-and_002c-canonicalization-of"></a>
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<a name="index-De-Morgan_0027s-law"></a>
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De Morgan’s Law is used to move bitwise negation inside a bitwise
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logical-and or logical-or operation. If this results in only one
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operand being a <code>not</code> expression, it will be the first one.
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<p>A machine that has an instruction that performs a bitwise logical-and of one
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operand with the bitwise negation of the other should specify the pattern
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for that instruction as
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_insn ""
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[(set (match_operand:<var>m</var> 0 …)
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(and:<var>m</var> (not:<var>m</var> (match_operand:<var>m</var> 1 …))
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(match_operand:<var>m</var> 2 …)))]
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"…"
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"…")
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</pre></div>
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<p>Similarly, a pattern for a “NAND” instruction should be written
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</p>
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<div class="smallexample">
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<pre class="smallexample">(define_insn ""
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[(set (match_operand:<var>m</var> 0 …)
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(ior:<var>m</var> (not:<var>m</var> (match_operand:<var>m</var> 1 …))
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(not:<var>m</var> (match_operand:<var>m</var> 2 …))))]
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"…"
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"…")
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</pre></div>
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<p>In both cases, it is not necessary to include patterns for the many
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logically equivalent RTL expressions.
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</p>
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</li><li> <a name="index-xor_002c-canonicalization-of"></a>
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The only possible RTL expressions involving both bitwise exclusive-or
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and bitwise negation are <code>(xor:<var>m</var> <var>x</var> <var>y</var>)</code>
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and <code>(not:<var>m</var> (xor:<var>m</var> <var>x</var> <var>y</var>))</code>.
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</li><li> The sum of three items, one of which is a constant, will only appear in
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the form
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<div class="smallexample">
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<pre class="smallexample">(plus:<var>m</var> (plus:<var>m</var> <var>x</var> <var>y</var>) <var>constant</var>)
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</pre></div>
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</li><li> <a name="index-zero_005fextract_002c-canonicalization-of"></a>
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<a name="index-sign_005fextract_002c-canonicalization-of"></a>
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Equality comparisons of a group of bits (usually a single bit) with zero
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will be written using <code>zero_extract</code> rather than the equivalent
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<code>and</code> or <code>sign_extract</code> operations.
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</li><li> <a name="index-mult_002c-canonicalization-of-1"></a>
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<code>(sign_extend:<var>m1</var> (mult:<var>m2</var> (sign_extend:<var>m2</var> <var>x</var>)
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(sign_extend:<var>m2</var> <var>y</var>)))</code> is converted to <code>(mult:<var>m1</var>
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(sign_extend:<var>m1</var> <var>x</var>) (sign_extend:<var>m1</var> <var>y</var>))</code>, and likewise
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for <code>zero_extend</code>.
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</li><li> <code>(sign_extend:<var>m1</var> (mult:<var>m2</var> (ashiftrt:<var>m2</var>
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<var>x</var> <var>s</var>) (sign_extend:<var>m2</var> <var>y</var>)))</code> is converted
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to <code>(mult:<var>m1</var> (sign_extend:<var>m1</var> (ashiftrt:<var>m2</var>
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<var>x</var> <var>s</var>)) (sign_extend:<var>m1</var> <var>y</var>))</code>, and likewise for
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patterns using <code>zero_extend</code> and <code>lshiftrt</code>. If the second
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operand of <code>mult</code> is also a shift, then that is extended also.
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This transformation is only applied when it can be proven that the
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original operation had sufficient precision to prevent overflow.
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</li></ul>
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<p>Further canonicalization rules are defined in the function
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<code>commutative_operand_precedence</code> in <samp>gcc/rtlanal.c</samp>.
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</p>
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<div class="header">
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<p>
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Next: <a href="Expander-Definitions.html#Expander-Definitions" accesskey="n" rel="next">Expander Definitions</a>, Previous: <a href="Looping-Patterns.html#Looping-Patterns" accesskey="p" rel="prev">Looping Patterns</a>, Up: <a href="Machine-Desc.html#Machine-Desc" accesskey="u" rel="up">Machine Desc</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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