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