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