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134 lines
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134 lines
6.3 KiB
HTML
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<title>loop-iv (GNU Compiler Collection (GCC) Internals)</title>
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Number-of-iterations.html#Number-of-iterations" rel="next" title="Number of iterations">
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<a name="loop_002div"></a>
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<div class="header">
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<p>
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Next: <a href="Number-of-iterations.html#Number-of-iterations" accesskey="n" rel="next">Number of iterations</a>, Previous: <a href="Scalar-evolutions.html#Scalar-evolutions" accesskey="p" rel="prev">Scalar evolutions</a>, Up: <a href="Loop-Analysis-and-Representation.html#Loop-Analysis-and-Representation" accesskey="u" rel="up">Loop Analysis and Representation</a> [<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>
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</div>
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<hr>
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<a name="IV-analysis-on-RTL"></a>
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<h3 class="section">16.6 IV analysis on RTL</h3>
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<a name="index-IV-analysis-on-RTL"></a>
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<p>The induction variable on RTL is simple and only allows analysis of
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affine induction variables, and only in one loop at once. The interface
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is declared in <samp>cfgloop.h</samp>. Before analyzing induction variables
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in a loop L, <code>iv_analysis_loop_init</code> function must be called on L.
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After the analysis (possibly calling <code>iv_analysis_loop_init</code> for
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several loops) is finished, <code>iv_analysis_done</code> should be called.
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The following functions can be used to access the results of the
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analysis:
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</p>
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<ul>
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<li> <code>iv_analyze</code>: Analyzes a single register used in the given
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insn. If no use of the register in this insn is found, the following
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insns are scanned, so that this function can be called on the insn
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returned by get_condition.
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</li><li> <code>iv_analyze_result</code>: Analyzes result of the assignment in the
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given insn.
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</li><li> <code>iv_analyze_expr</code>: Analyzes a more complicated expression.
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All its operands are analyzed by <code>iv_analyze</code>, and hence they must
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be used in the specified insn or one of the following insns.
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</li></ul>
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<p>The description of the induction variable is provided in <code>struct
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rtx_iv</code>. In order to handle subregs, the representation is a bit
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complicated; if the value of the <code>extend</code> field is not
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<code>UNKNOWN</code>, the value of the induction variable in the i-th
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iteration is
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</p>
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<div class="smallexample">
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<pre class="smallexample">delta + mult * extend_{extend_mode} (subreg_{mode} (base + i * step)),
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</pre></div>
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<p>with the following exception: if <code>first_special</code> is true, then the
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value in the first iteration (when <code>i</code> is zero) is <code>delta +
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mult * base</code>. However, if <code>extend</code> is equal to <code>UNKNOWN</code>,
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then <code>first_special</code> must be false, <code>delta</code> 0, <code>mult</code> 1
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and the value in the i-th iteration is
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</p>
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<div class="smallexample">
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<pre class="smallexample">subreg_{mode} (base + i * step)
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</pre></div>
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<p>The function <code>get_iv_value</code> can be used to perform these
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calculations.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Number-of-iterations.html#Number-of-iterations" accesskey="n" rel="next">Number of iterations</a>, Previous: <a href="Scalar-evolutions.html#Scalar-evolutions" accesskey="p" rel="prev">Scalar evolutions</a>, Up: <a href="Loop-Analysis-and-Representation.html#Loop-Analysis-and-Representation" accesskey="u" rel="up">Loop Analysis and Representation</a> [<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>
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</div>
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