devdocs/c/language%2Fas_if.html


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    <h1 id="firstHeading" class="firstHeading">As-if rule</h1>            <p>Allows any and all code transformations that do not change the observable behavior of the program.</p>
<h3 id="Explanation"> Explanation</h3> <p>The C compiler is permitted to perform any changes to the program as long as the following remains true:</p>
<table class="t-rev-begin"> <tr class="t-rev t-until-c11">
<td> <span class="t-li">1)</span> At every <a href="eval_order" title="c/language/eval order">sequence point</a>, the values of all <a href="volatile" title="c/language/volatile">volatile</a> objects are stable (previous evaluations are complete, new evaluations not started). </td> <td><span class="t-mark-rev t-until-c11">(until C11)</span></td>
</tr> <tr class="t-rev t-since-c11">
<td> <span class="t-li">1)</span> Accesses (reads and writes) to <a href="volatile" title="c/language/volatile">volatile</a> objects occur strictly according to the semantics of the expressions in which they occur. In particular, they are <a href="../atomic/memory_order" title="c/atomic/memory order">not reordered</a> with respect to other volatile accesses on the same thread. </td> <td><span class="t-mark-rev t-since-c11">(since C11)</span></td>
</tr> </table> <div class="t-li1">
<span class="t-li">2)</span> At program termination, data written to files is exactly as if the program was executed as written.</div> <div class="t-li1">
<span class="t-li">3)</span> Prompting text which is sent to interactive devices will be shown before the program waits for input.</div> <table class="t-rev-begin"> <tr class="t-rev t-since-c99">
<td> <span class="t-li">4)</span> If the pragma <a href="../preprocessor/impl#Standard_pragmas" title="c/preprocessor/impl"><code> #pragma STDC FENV_ACCESS</code></a> is supported and is set to <code>ON</code>, the changes to the <a href="../numeric/fenv" title="c/numeric/fenv">floating-point environment</a> (floating-point exceptions and rounding modes) are guaranteed to be observed by the floating-point arithmetic operators and function calls as if executed as written, except that <ul>
<li> the result of any floating-point expression other than cast and assignment may have range and precision of a floating-point type different from the type of the expression (see <code><a href="../types/limits/flt_eval_method" title="c/types/limits/FLT EVAL METHOD">FLT_EVAL_METHOD</a></code>), </li>
<li> notwithstanding the above, intermediate results of any floating-point expression may be calculated as if to infinite range and precision (unless <a href="../preprocessor/impl#Standard_pragmas" title="c/preprocessor/impl"><code> #pragma STDC FP_CONTRACT</code></a> is <code>OFF</code>).</li>
</ul> </td> <td><span class="t-mark-rev t-since-c99">(since C99)</span></td>
</tr> </table> <h3 id="Notes"> Notes</h3>  <h3 id="See_also"> See also</h3> <ul><li> <a href="eval_order" title="c/language/eval order">Order of evaluation</a> </li></ul> <table class="t-dsc-begin"> <tr class="t-dsc"> <td colspan="2"> <span><a href="https://en.cppreference.com/w/cpp/language/as_if" title="cpp/language/as if">C++ documentation</a></span> for <span class=""><span>as-if rule</span></span> </td>
</tr> </table>           <div class="_attribution">
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