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</div><div><span class="editsection noprint plainlinks" title="Edit this template"><a rel="nofollow" class="external text" href="http://en.cppreference.com/mwiki/index.php?title=Template:cpp/language/navbar_content&amp;action=edit">&#91;edit&#93;</a></span></div></div></div></div><div class="t-navbar-sep">&#160;</div></div>
<p>Allows any and all code transformations that do not change the observable behavior of the program
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="as_if.html#Explanation"><span class="tocnumber">1</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="as_if.html#Notes"><span class="tocnumber">2</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="as_if.html#Example"><span class="tocnumber">3</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="as_if.html#See_also"><span class="tocnumber">4</span> <span class="toctext">See also</span></a></li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/as_if&amp;action=edit&amp;section=1" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></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-cxx11"><td>
<div class="t-li1"><span class="t-li">1)</span> At every <a href="eval_order.html" title="cpp/language/eval order">sequence point</a>, the values of all <a href="cv.html" title="cpp/language/cv">volatile</a> objects are stable (previous evaluations are complete, new evaluations not started)
</div></td>
<td><span class="t-mark-rev t-until-cxx11">(until C++11)</span></td></tr><tr class="t-rev t-since-cxx11"><td>
<div class="t-li1"><span class="t-li">1)</span> Accesses (reads and writes) to <a href="cv.html" title="cpp/language/cv">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.html" title="cpp/atomic/memory order">not reordered</a> with respect to other volatile accesses on the same thread.
</div></td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</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>
<div class="t-li1"><span class="t-li">4)</span> If <a href="../preprocessor/impl.html" title="cpp/preprocessor/impl"><tt>
#pragma STDC FENV_ACCESS</tt></a> is set to <code>ON</code>, the changes to the <a href="../numeric/fenv.html" title="cpp/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
<dl><dd><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 <span class="t-lc">FLT_EVAL_METHOD</span>)
</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.html" title="cpp/preprocessor/impl"><tt>
#pragma STDC FP_CONTRACT</tt></a> is <code>OFF</code>)</div>
</li></ul>
</dd></dl>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/as_if&amp;action=edit&amp;section=2" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>Because the compiler is (usually) unable to analyze the code of an external library to determine whether it does or does not perform I/O or volatile access, third-party library calls also aren't affected by optimization. However, standard library calls may be replaced by other calls, eliminated, or added to the program during optimization. Statically-linked third-party library code may be subject to link-time optimization.
</p><p>Programs with <a href="ub.html" title="cpp/language/ub">undefined behavior</a>, e.g. due to access to an array out of bounds, modification of a const object, <a href="eval_order.html" title="cpp/language/eval order">evaluation order</a> violations, etc, are free from the as-if rule: they often change observable behavior when recompiled with different optimization settings. For example, if a test for signed integer overflow relies on the result of that overflow, e.g. <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">if</span><span class="br0">&#40;</span>n<span class="sy2">+</span><span class="nu0">1</span> <span class="sy1">&lt;</span> n<span class="br0">&#41;</span> abort<span class="br0">&#40;</span><span class="br0">&#41;</span><span class="sy4">;</span></span></span>, <a rel="nofollow" class="external text" href="http://blog.llvm.org/2011/05/what-every-c-programmer-should-know_14.html">it is removed entirely by some compilers</a> because <a href="operator_arithmetic.html#Overflows" title="cpp/language/operator arithmetic">signed overflow is undefined behavior</a> and the optimizer is free to assume it never happens and the test is redundant.
</p><p><a href="copy_elision.html" title="cpp/language/copy elision">Copy elision</a> is an exception from the as-if rule: the compiler may remove calls to move- and copy-constructors and the matching calls to the destructors of temporary objects even if those calls have observable side effects.
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx14"><td>
<p><a href="new.html#Allocation" title="cpp/language/new">New-expression</a> has another exception from the as-if rule: the compiler may remove calls to the replaceable allocation functions even if a user-defined replacement is provided and has observable side-effects.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx14">(since C++14)</span></td></tr>
</table>
<p>The count and order of floating-point exceptions can be changed by optimization as long as the state as observed by the next floating-point operation is as if no optimization took place:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#pragma STDC FENV_ACCESS ON</span>
<span class="kw1">for</span> <span class="br0">&#40;</span>i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> i <span class="sy1">&lt;</span> n<span class="sy4">;</span> i<span class="sy2">++</span><span class="br0">&#41;</span> x <span class="sy2">+</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// x+1 is dead code, but may raise FP exceptions</span>
<span class="co1">// (unless the optimizer can prove otherwise). However, executing it n times will</span>
<span class="co1">// raise the same exception over and over. So this can be optimized to:</span>
<span class="kw1">if</span> <span class="br0">&#40;</span><span class="nu0">0</span> <span class="sy1">&lt;</span> n<span class="br0">&#41;</span> x <span class="sy2">+</span> <span class="nu0">1</span><span class="sy4">;</span></pre></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/as_if&amp;action=edit&amp;section=3" title="Edit section: Example">edit</a>]</span> <span class="mw-headline" id="Example">Example</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span><span class="sy3">&amp;</span> preinc<span class="br0">&#40;</span><span class="kw4">int</span><span class="sy3">&amp;</span> n<span class="br0">&#41;</span> <span class="br0">&#123;</span> <span class="kw1">return</span> <span class="sy2">++</span>n<span class="sy4">;</span> <span class="br0">&#125;</span>
<span class="kw4">int</span> add<span class="br0">&#40;</span><span class="kw4">int</span> n, <span class="kw4">int</span> m<span class="br0">&#41;</span> <span class="br0">&#123;</span> <span class="kw1">return</span> n<span class="sy2">+</span>m<span class="sy4">;</span> <span class="br0">&#125;</span>
&#160;
<span class="co1">// volatile input to prevent constant folding</span>
<span class="kw4">volatile</span> <span class="kw4">int</span> input <span class="sy1">=</span> <span class="nu0">7</span><span class="sy4">;</span>
&#160;
<span class="co1">// volatile output to make the result a visible side-effect</span>
<span class="kw4">volatile</span> <span class="kw4">int</span> result<span class="sy4">;</span>
&#160;
<span class="kw4">int</span> main<span class="br0">&#40;</span><span class="br0">&#41;</span>
<span class="br0">&#123;</span>
    <span class="kw4">int</span> n <span class="sy1">=</span> input<span class="sy4">;</span>
<span class="co1">// using built-in operators would invoke undefined behavior</span>
<span class="co1">//    int m = ++n + ++n;</span>
<span class="co1">// but using functions makes sure the code executes as-if </span>
<span class="co1">// the functions were not overlapped</span>
    <span class="kw4">int</span> m <span class="sy1">=</span> add<span class="br0">&#40;</span>preinc<span class="br0">&#40;</span>n<span class="br0">&#41;</span>, preinc<span class="br0">&#40;</span>n<span class="br0">&#41;</span><span class="br0">&#41;</span><span class="sy4">;</span>
    result <span class="sy1">=</span> m<span class="sy4">;</span>
<span class="br0">&#125;</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1"># full code of the main() function as produced by the GCC compiler
# x86 (Intel) platform:
        movl    input(%rip),&#160;%eax   # eax = input
        leal    3(%rax,%rax),&#160;%eax  # eax = 3 + eax + eax
        movl   &#160;%eax, result(%rip)  # result = eax
        xorl   &#160;%eax,&#160;%eax          # eax = 0 (the return value of main())
        ret
&#160;
# PowerPC (IBM) platform:
        lwz 9,LC..1(2)
        li 3,0          # r3 = 0 (the return value of main())
        lwz 11,0(9)     # r11 = input;
        slwi 11,11,1    # r11 = r11 &lt;&lt; 1;
        addi 0,11,3     # r0 = r11 + 3;
        stw 0,4(9)      # result = r0;
        blr
&#160;
# Sparc (Sun) platform:
        sethi  &#160;%hi(result),&#160;%g2
        sethi  &#160;%hi(input),&#160;%g1
        mov     0,&#160;%o0                 # o0 = 0 (the return value of main)
        ld      [%g1+%lo(input)],&#160;%g1  # g1 = input
        add    &#160;%g1,&#160;%g1,&#160;%g1          # g1 = g1 + g1
        add    &#160;%g1, 3,&#160;%g1            # g1 = 3 + g1
        st     &#160;%g1, [%g2+%lo(result)] # result = g1
        jmp    &#160;%o7+8
        nop
&#160;
# in all cases, the side effects of preinc() were eliminated, and the
# entire main() function was reduced to the equivalent of result = 2*input + 3;</pre></div></div> 
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/as_if&amp;action=edit&amp;section=4" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span></h3>
<ul><li> <a href="copy_elision.html" title="cpp/language/copy elision">copy elision</a>
</li></ul>

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