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+<title>GNU Compiler Collection (GCC) Internals: RTL SSA Accesses</title>
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+<link href="RTL-SSA.html#RTL-SSA" rel="up" title="RTL SSA">
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+<div class="header">
+<p>
+Next: <a href="RTL-SSA-Phi-Nodes.html#RTL-SSA-Phi-Nodes" accesskey="n" rel="next">RTL SSA Phi Nodes</a>, Previous: <a href="RTL-SSA-Resources.html#RTL-SSA-Resources" accesskey="p" rel="previous">RTL SSA Resources</a>, Up: <a href="RTL-SSA.html#RTL-SSA" accesskey="u" rel="up">RTL SSA</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>
+</div>
+<hr>
+<a name="RTL-SSA-Register-and-Memory-Accesses"></a>
+<h4 class="subsection">14.21.5 RTL SSA Register and Memory Accesses</h4>
+
+<p>In the RTL SSA form, most reads or writes of a resource are
+represented as a <code>rtl_ssa::access_info</code><a name="DOCF6" href="#FOOT6"><sup>6</sup></a>.
+These <code>rtl_ssa::access_info</code>s are organized into the following
+class hierarchy:
+</p>
+<a name="index-rtl_005fssa_003a_003aaccess_005finfo"></a>
+<a name="index-rtl_005fssa_003a_003ause_005finfo"></a>
+<a name="index-rtl_005fssa_003a_003adef_005finfo"></a>
+<a name="index-rtl_005fssa_003a_003aclobber_005finfo"></a>
+<a name="index-rtl_005fssa_003a_003aset_005finfo"></a>
+<a name="index-rtl_005fssa_003a_003aphi_005finfo"></a>
+<div class="smallexample">
+<pre class="smallexample">rtl_ssa::access_info
+  |
+  +-- rtl_ssa::use_info
+  |
+  +-- rtl_ssa::def_info
+        |
+        +-- rtl_ssa::clobber_info
+        |
+        +-- rtl_ssa::set_info
+              |
+              +-- rtl_ssa::phi_info
+</pre></div>
+
+<p>A <code>rtl_ssa::use_info</code> represents a read or use of a resource and
+a <code>rtl_ssa::def_info</code> represents a write or definition of a resource.
+As in the main RTL representation, there are two basic types of
+definition: clobbers and sets.  The difference is that a clobber
+leaves the register with an unspecified value that cannot be used
+or relied on by later instructions, while a set leaves the register
+with a known value that later instructions could use if they wanted to.
+A <code>rtl_ssa::clobber_info</code> represents a clobber and
+a <code>rtl_ssa::set_info</code> represent a set.
+</p>
+<p>Each <code>rtl_ssa::use_info</code> records which single <code>rtl_ssa::set_info</code>
+provides the value of the resource; this is null if the resource is
+completely undefined at the point of use.  Each <code>rtl_ssa::set_info</code>
+in turn records all the <code>rtl_ssa::use_info</code>s that use its value.
+</p>
+<p>If a value of a resource can come from multiple sources,
+a <code>rtl_ssa::phi_info</code> brings those multiple sources together
+into a single definition (see <a href="RTL-SSA-Phi-Nodes.html#RTL-SSA-Phi-Nodes">RTL SSA Phi Nodes</a>).
+</p>
+<div class="footnote">
+<hr>
+<h4 class="footnotes-heading">Footnotes</h4>
+
+<h3><a name="FOOT6" href="#DOCF6">(6)</a></h3>
+<p>The exceptions
+are call clobbers, which are generally represented separately.
+See the comment above <code>rtl_ssa::insn_info</code> for details.</p>
+</div>
+
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