139 lines
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139 lines
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<title>GNU Compiler Collection (GCC) Internals: Number of iterations</title>
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<a name="Number-of-iterations"></a>
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<div class="header">
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<p>
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Next: <a href="Dependency-analysis.html#Dependency-analysis" accesskey="n" rel="next">Dependency analysis</a>, Previous: <a href="loop_002div.html#loop_002div" accesskey="p" rel="prev">loop-iv</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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<a name="Number-of-iterations-analysis"></a>
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<h3 class="section">15.7 Number of iterations analysis</h3>
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<a name="index-Number-of-iterations-analysis"></a>
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<p>Both on GIMPLE and on RTL, there are functions available to determine
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the number of iterations of a loop, with a similar interface. The
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number of iterations of a loop in GCC is defined as the number of
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executions of the loop latch. In many cases, it is not possible to
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determine the number of iterations unconditionally – the determined
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number is correct only if some assumptions are satisfied. The analysis
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tries to verify these conditions using the information contained in the
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program; if it fails, the conditions are returned together with the
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result. The following information and conditions are provided by the
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analysis:
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</p>
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<ul>
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<li> <code>assumptions</code>: If this condition is false, the rest of
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the information is invalid.
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</li><li> <code>noloop_assumptions</code> on RTL, <code>may_be_zero</code> on GIMPLE: If
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this condition is true, the loop exits in the first iteration.
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</li><li> <code>infinite</code>: If this condition is true, the loop is infinite.
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This condition is only available on RTL. On GIMPLE, conditions for
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finiteness of the loop are included in <code>assumptions</code>.
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</li><li> <code>niter_expr</code> on RTL, <code>niter</code> on GIMPLE: The expression
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that gives number of iterations. The number of iterations is defined as
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the number of executions of the loop latch.
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</li></ul>
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<p>Both on GIMPLE and on RTL, it necessary for the induction variable
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analysis framework to be initialized (SCEV on GIMPLE, loop-iv on RTL).
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On GIMPLE, the results are stored to <code>struct tree_niter_desc</code>
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structure. Number of iterations before the loop is exited through a
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given exit can be determined using <code>number_of_iterations_exit</code>
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function. On RTL, the results are returned in <code>struct niter_desc</code>
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structure. The corresponding function is named
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<code>check_simple_exit</code>. There are also functions that pass through
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all the exits of a loop and try to find one with easy to determine
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number of iterations – <code>find_loop_niter</code> on GIMPLE and
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<code>find_simple_exit</code> on RTL. Finally, there are functions that
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provide the same information, but additionally cache it, so that
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repeated calls to number of iterations are not so costly –
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<code>number_of_latch_executions</code> on GIMPLE and <code>get_simple_loop_desc</code>
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on RTL.
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</p>
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<p>Note that some of these functions may behave slightly differently than
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others – some of them return only the expression for the number of
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iterations, and fail if there are some assumptions. The function
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<code>number_of_latch_executions</code> works only for single-exit loops.
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The function <code>number_of_cond_exit_executions</code> can be used to
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determine number of executions of the exit condition of a single-exit
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loop (i.e., the <code>number_of_latch_executions</code> increased by one).
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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="Dependency-analysis.html#Dependency-analysis" accesskey="n" rel="next">Dependency analysis</a>, Previous: <a href="loop_002div.html#loop_002div" accesskey="p" rel="prev">loop-iv</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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