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  25. <title>GIMPLE (GNU Compiler Collection (GCC) Internals)</title>

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

  71. <div class="header">

  72. <p>

  73. Next: <a href="Tree-SSA.html#Tree-SSA" accesskey="n" rel="next">Tree SSA</a>, Previous: <a href="GENERIC.html#GENERIC" accesskey="p" rel="prev">GENERIC</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</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="GIMPLE-1"></a>

  77. <h2 class="chapter">12 GIMPLE</h2>

  78. <a name="index-GIMPLE-2"></a>

  79. 

  80. <p>GIMPLE is a three-address representation derived from GENERIC by

  81. breaking down GENERIC expressions into tuples of no more than 3

  82. operands (with some exceptions like function calls).  GIMPLE was

  83. heavily influenced by the SIMPLE IL used by the McCAT compiler

  84. project at McGill University, though we have made some different

  85. choices.  For one thing, SIMPLE doesn&rsquo;t support <code>goto</code>.

  86. </p>

  87. <p>Temporaries are introduced to hold intermediate values needed to

  88. compute complex expressions. Additionally, all the control

  89. structures used in GENERIC are lowered into conditional jumps,

  90. lexical scopes are removed and exception regions are converted

  91. into an on the side exception region tree.

  92. </p>

  93. <p>The compiler pass which converts GENERIC into GIMPLE is referred to as

  94. the &lsquo;<samp>gimplifier</samp>&rsquo;.  The gimplifier works recursively, generating

  95. GIMPLE tuples out of the original GENERIC expressions.

  96. </p>

  97. <p>One of the early implementation strategies used for the GIMPLE

  98. representation was to use the same internal data structures used

  99. by front ends to represent parse trees. This simplified

  100. implementation because we could leverage existing functionality

  101. and interfaces. However, GIMPLE is a much more restrictive

  102. representation than abstract syntax trees (AST), therefore it

  103. does not require the full structural complexity provided by the

  104. main tree data structure.

  105. </p>

  106. <p>The GENERIC representation of a function is stored in the

  107. <code>DECL_SAVED_TREE</code> field of the associated <code>FUNCTION_DECL</code>

  108. tree node.  It is converted to GIMPLE by a call to

  109. <code>gimplify_function_tree</code>.

  110. </p>

  111. <p>If a front end wants to include language-specific tree codes in the tree

  112. representation which it provides to the back end, it must provide a

  113. definition of <code>LANG_HOOKS_GIMPLIFY_EXPR</code> which knows how to

  114. convert the front end trees to GIMPLE.  Usually such a hook will involve

  115. much of the same code for expanding front end trees to RTL.  This function

  116. can return fully lowered GIMPLE, or it can return GENERIC trees and let the

  117. main gimplifier lower them the rest of the way; this is often simpler.

  118. GIMPLE that is not fully lowered is known as &ldquo;High GIMPLE&rdquo; and

  119. consists of the IL before the pass <code>pass_lower_cf</code>.  High GIMPLE

  120. contains some container statements like lexical scopes

  121. (represented by <code>GIMPLE_BIND</code>) and nested expressions (e.g.,

  122. <code>GIMPLE_TRY</code>), while &ldquo;Low GIMPLE&rdquo; exposes all of the

  123. implicit jumps for control and exception expressions directly in

  124. the IL and EH region trees.

  125. </p>

  126. <p>The C and C++ front ends currently convert directly from front end

  127. trees to GIMPLE, and hand that off to the back end rather than first

  128. converting to GENERIC.  Their gimplifier hooks know about all the

  129. <code>_STMT</code> nodes and how to convert them to GENERIC forms.  There

  130. was some work done on a genericization pass which would run first, but

  131. the existence of <code>STMT_EXPR</code> meant that in order to convert all

  132. of the C statements into GENERIC equivalents would involve walking the

  133. entire tree anyway, so it was simpler to lower all the way.  This

  134. might change in the future if someone writes an optimization pass

  135. which would work better with higher-level trees, but currently the

  136. optimizers all expect GIMPLE.

  137. </p>

  138. <p>You can request to dump a C-like representation of the GIMPLE form

  139. with the flag <samp>-fdump-tree-gimple</samp>.

  140. </p>

  141. <table class="menu" border="0" cellspacing="0">

  142. <tr><td align="left" valign="top">&bull; <a href="Tuple-representation.html#Tuple-representation" accesskey="1">Tuple representation</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  143. </td></tr>

  144. <tr><td align="left" valign="top">&bull; <a href="Class-hierarchy-of-GIMPLE-statements.html#Class-hierarchy-of-GIMPLE-statements" accesskey="2">Class hierarchy of GIMPLE statements</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  145. </td></tr>

  146. <tr><td align="left" valign="top">&bull; <a href="GIMPLE-instruction-set.html#GIMPLE-instruction-set" accesskey="3">GIMPLE instruction set</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  147. </td></tr>

  148. <tr><td align="left" valign="top">&bull; <a href="GIMPLE-Exception-Handling.html#GIMPLE-Exception-Handling" accesskey="4">GIMPLE Exception Handling</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  149. </td></tr>

  150. <tr><td align="left" valign="top">&bull; <a href="Temporaries.html#Temporaries" accesskey="5">Temporaries</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  151. </td></tr>

  152. <tr><td align="left" valign="top">&bull; <a href="Operands.html#Operands" accesskey="6">Operands</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  153. </td></tr>

  154. <tr><td align="left" valign="top">&bull; <a href="Manipulating-GIMPLE-statements.html#Manipulating-GIMPLE-statements" accesskey="7">Manipulating GIMPLE statements</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  155. </td></tr>

  156. <tr><td align="left" valign="top">&bull; <a href="Tuple-specific-accessors.html#Tuple-specific-accessors" accesskey="8">Tuple specific accessors</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  157. </td></tr>

  158. <tr><td align="left" valign="top">&bull; <a href="GIMPLE-sequences.html#GIMPLE-sequences" accesskey="9">GIMPLE sequences</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  159. </td></tr>

  160. <tr><td align="left" valign="top">&bull; <a href="Sequence-iterators.html#Sequence-iterators">Sequence iterators</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  161. </td></tr>

  162. <tr><td align="left" valign="top">&bull; <a href="Adding-a-new-GIMPLE-statement-code.html#Adding-a-new-GIMPLE-statement-code">Adding a new GIMPLE statement code</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  163. </td></tr>

  164. <tr><td align="left" valign="top">&bull; <a href="Statement-and-operand-traversals.html#Statement-and-operand-traversals">Statement and operand traversals</a>:</td><td>&nbsp;&nbsp;</td><td align="left" valign="top">

  165. </td></tr>

  166. </table>

  167. 

  168. <hr>

  169. <div class="header">

  170. <p>

  171. Next: <a href="Tree-SSA.html#Tree-SSA" accesskey="n" rel="next">Tree SSA</a>, Previous: <a href="GENERIC.html#GENERIC" accesskey="p" rel="prev">GENERIC</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</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>

  172. </div>

  173. 

  174. 

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