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  34. <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">

  35. <link href="Peephole-Definitions.html#Peephole-Definitions" rel="up" title="Peephole Definitions">

  36. <link href="Insn-Attributes.html#Insn-Attributes" rel="next" title="Insn Attributes">

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  69. <body lang="en">

  70. <a name="define_005fpeephole2"></a>

  71. <div class="header">

  72. <p>

  73. Previous: <a href="define_005fpeephole.html#define_005fpeephole" accesskey="p" rel="prev">define_peephole</a>, Up: <a href="Peephole-Definitions.html#Peephole-Definitions" accesskey="u" rel="up">Peephole Definitions</a> &nbsp; [<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>

  74. </div>

  75. <hr>

  76. <a name="RTL-to-RTL-Peephole-Optimizers"></a>

  77. <h4 class="subsection">17.18.2 RTL to RTL Peephole Optimizers</h4>

  78. <a name="index-define_005fpeephole2"></a>

  79. 

  80. <p>The <code>define_peephole2</code> definition tells the compiler how to

  81. substitute one sequence of instructions for another sequence,

  82. what additional scratch registers may be needed and what their

  83. lifetimes must be.

  84. </p>

  85. <div class="smallexample">

  86. <pre class="smallexample">(define_peephole2

  87.   [<var>insn-pattern-1</var>

  88.    <var>insn-pattern-2</var>

  89.    &hellip;]

  90.   &quot;<var>condition</var>&quot;

  91.   [<var>new-insn-pattern-1</var>

  92.    <var>new-insn-pattern-2</var>

  93.    &hellip;]

  94.   &quot;<var>preparation-statements</var>&quot;)

  95. </pre></div>

  96. 

  97. <p>The definition is almost identical to <code>define_split</code>

  98. (see <a href="Insn-Splitting.html#Insn-Splitting">Insn Splitting</a>) except that the pattern to match is not a

  99. single instruction, but a sequence of instructions.

  100. </p>

  101. <p>It is possible to request additional scratch registers for use in the

  102. output template.  If appropriate registers are not free, the pattern

  103. will simply not match.

  104. </p>

  105. <a name="index-match_005fscratch-1"></a>

  106. <a name="index-match_005fdup-1"></a>

  107. <p>Scratch registers are requested with a <code>match_scratch</code> pattern at

  108. the top level of the input pattern.  The allocated register (initially) will

  109. be dead at the point requested within the original sequence.  If the scratch

  110. is used at more than a single point, a <code>match_dup</code> pattern at the

  111. top level of the input pattern marks the last position in the input sequence

  112. at which the register must be available.

  113. </p>

  114. <p>Here is an example from the IA-32 machine description:

  115. </p>

  116. <div class="smallexample">

  117. <pre class="smallexample">(define_peephole2

  118.   [(match_scratch:SI 2 &quot;r&quot;)

  119.    (parallel [(set (match_operand:SI 0 &quot;register_operand&quot; &quot;&quot;)

  120.                    (match_operator:SI 3 &quot;arith_or_logical_operator&quot;

  121.                      [(match_dup 0)

  122.                       (match_operand:SI 1 &quot;memory_operand&quot; &quot;&quot;)]))

  123.               (clobber (reg:CC 17))])]

  124.   &quot;! optimize_size &amp;&amp; ! TARGET_READ_MODIFY&quot;

  125.   [(set (match_dup 2) (match_dup 1))

  126.    (parallel [(set (match_dup 0)

  127.                    (match_op_dup 3 [(match_dup 0) (match_dup 2)]))

  128.               (clobber (reg:CC 17))])]

  129.   &quot;&quot;)

  130. </pre></div>

  131. 

  132. <p>This pattern tries to split a load from its use in the hopes that we&rsquo;ll be

  133. able to schedule around the memory load latency.  It allocates a single

  134. <code>SImode</code> register of class <code>GENERAL_REGS</code> (<code>&quot;r&quot;</code>) that needs

  135. to be live only at the point just before the arithmetic.

  136. </p>

  137. <p>A real example requiring extended scratch lifetimes is harder to come by,

  138. so here&rsquo;s a silly made-up example:

  139. </p>

  140. <div class="smallexample">

  141. <pre class="smallexample">(define_peephole2

  142.   [(match_scratch:SI 4 &quot;r&quot;)

  143.    (set (match_operand:SI 0 &quot;&quot; &quot;&quot;) (match_operand:SI 1 &quot;&quot; &quot;&quot;))

  144.    (set (match_operand:SI 2 &quot;&quot; &quot;&quot;) (match_dup 1))

  145.    (match_dup 4)

  146.    (set (match_operand:SI 3 &quot;&quot; &quot;&quot;) (match_dup 1))]

  147.   &quot;/* <span class="roman">determine 1 does not overlap 0 and 2</span> */&quot;

  148.   [(set (match_dup 4) (match_dup 1))

  149.    (set (match_dup 0) (match_dup 4))

  150.    (set (match_dup 2) (match_dup 4))

  151.    (set (match_dup 3) (match_dup 4))]

  152.   &quot;&quot;)

  153. </pre></div>

  154. 

  155. <p>There are two special macros defined for use in the preparation statements:

  156. <code>DONE</code> and <code>FAIL</code>.  Use them with a following semicolon,

  157. as a statement.

  158. </p>

  159. <dl compact="compact">

  160. <dd>

  161. <a name="index-DONE-2"></a>

  162. </dd>

  163. <dt><code>DONE</code></dt>

  164. <dd><p>Use the <code>DONE</code> macro to end RTL generation for the peephole.  The

  165. only RTL insns generated as replacement for the matched input insn will

  166. be those already emitted by explicit calls to <code>emit_insn</code> within

  167. the preparation statements; the replacement pattern is not used.

  168. </p>

  169. <a name="index-FAIL-2"></a>

  170. </dd>

  171. <dt><code>FAIL</code></dt>

  172. <dd><p>Make the <code>define_peephole2</code> fail on this occasion.  When a <code>define_peephole2</code>

  173. fails, it means that the replacement was not truly available for the

  174. particular inputs it was given.  In that case, GCC may still apply a

  175. later <code>define_peephole2</code> that also matches the given insn pattern.

  176. (Note that this is different from <code>define_split</code>, where <code>FAIL</code>

  177. prevents the input insn from being split at all.)

  178. </p></dd>

  179. </dl>

  180. 

  181. <p>If the preparation falls through (invokes neither <code>DONE</code> nor

  182. <code>FAIL</code>), then the <code>define_peephole2</code> uses the replacement

  183. template.

  184. </p>

  185. <p>If we had not added the <code>(match_dup 4)</code> in the middle of the input

  186. sequence, it might have been the case that the register we chose at the

  187. beginning of the sequence is killed by the first or second <code>set</code>.

  188. </p>

  189. <hr>

  190. <div class="header">

  191. <p>

  192. Previous: <a href="define_005fpeephole.html#define_005fpeephole" accesskey="p" rel="prev">define_peephole</a>, Up: <a href="Peephole-Definitions.html#Peephole-Definitions" accesskey="u" rel="up">Peephole Definitions</a> &nbsp; [<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>

  193. </div>

  194. 

  195. 

  196. 

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  198. </html>