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170 lines
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4 years ago
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<title>Insn Canonicalizations - GNU Compiler Collection (GCC) Internals</title>
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<link rel="prev" href="Looping-Patterns.html#Looping-Patterns" title="Looping Patterns">
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Permission is granted to copy, distribute and/or modify this document
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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<div class="node">
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<a name="Insn-Canonicalizations"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Expander-Definitions.html#Expander-Definitions">Expander Definitions</a>,
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Previous: <a rel="previous" accesskey="p" href="Looping-Patterns.html#Looping-Patterns">Looping Patterns</a>,
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Up: <a rel="up" accesskey="u" href="Machine-Desc.html#Machine-Desc">Machine Desc</a>
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<hr>
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</div>
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<h3 class="section">16.14 Canonicalization of Instructions</h3>
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<p><a name="index-canonicalization-of-instructions-3679"></a><a name="index-insn-canonicalization-3680"></a>
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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>In addition to algebraic simplifications, following canonicalizations
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are performed:
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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>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><a name="index-g_t_0040code_007bneg_007d_002c-canonicalization-of-3681"></a><a name="index-g_t_0040code_007bnot_007d_002c-canonicalization-of-3682"></a><a name="index-g_t_0040code_007bmult_007d_002c-canonicalization-of-3683"></a><a name="index-g_t_0040code_007bplus_007d_002c-canonicalization-of-3684"></a><a name="index-g_t_0040code_007bminus_007d_002c-canonicalization-of-3685"></a>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>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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<p><a name="index-g_t_0040code_007bcompare_007d_002c-canonicalization-of-3686"></a><li>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>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><code>(ltu (plus </code><var>a</var> <var>b</var><code>) </code><var>b</var><code>)</code> is converted to
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<code>(ltu (plus </code><var>a</var> <var>b</var><code>) </code><var>a</var><code>)</code>. Likewise with <code>geu</code> instead
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of <code>ltu</code>.
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<li><code>(minus </code><var>x</var><code> (const_int </code><var>n</var><code>))</code> is converted to
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<code>(plus </code><var>x</var><code> (const_int </code><var>-n</var><code>))</code>.
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<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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<p><a name="index-g_t_0040code_007bior_007d_002c-canonicalization-of-3687"></a><a name="index-g_t_0040code_007band_007d_002c-canonicalization-of-3688"></a><a name="index-De-Morgan_0027s-law-3689"></a><li>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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<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>
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<p class="noindent">Similarly, a pattern for a “NAND” instruction should be written
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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>
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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><a name="index-g_t_0040code_007bxor_007d_002c-canonicalization-of-3690"></a><li>The only possible RTL expressions involving both bitwise exclusive-or
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and bitwise negation are <code>(xor:</code><var>m</var> <var>x</var> <var>y</var><code>)</code>
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and <code>(not:</code><var>m</var><code> (xor:</code><var>m</var> <var>x</var> <var>y</var><code>))</code>.
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<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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<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>
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<p><a name="index-g_t_0040code_007bzero_005fextract_007d_002c-canonicalization-of-3691"></a><a name="index-g_t_0040code_007bsign_005fextract_007d_002c-canonicalization-of-3692"></a><li>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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<p><a name="index-g_t_0040code_007bmult_007d_002c-canonicalization-of-3693"></a><li><code>(sign_extend:</code><var>m1</var><code> (mult:</code><var>m2</var><code> (sign_extend:</code><var>m2</var> <var>x</var><code>)
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(sign_extend:</code><var>m2</var> <var>y</var><code>)))</code> is converted to <code>(mult:</code><var>m1</var><code>
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(sign_extend:</code><var>m1</var> <var>x</var><code>) (sign_extend:</code><var>m1</var> <var>y</var><code>))</code>, and likewise
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for <code>zero_extend</code>.
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<li><code>(sign_extend:</code><var>m1</var><code> (mult:</code><var>m2</var><code> (ashiftrt:</code><var>m2</var>
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<var>x</var> <var>s</var><code>) (sign_extend:</code><var>m2</var> <var>y</var><code>)))</code> is converted
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to <code>(mult:</code><var>m1</var><code> (sign_extend:</code><var>m1</var><code> (ashiftrt:</code><var>m2</var>
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<var>x</var> <var>s</var><code>)) (sign_extend:</code><var>m1</var> <var>y</var><code>))</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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</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><span class="file">gcc/rtlanal.c</span></samp>.
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</body></html>
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