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llvm-svn: 12529
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Chris Lattner 2004-03-29 19:14:35 +00:00
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@ -71,6 +71,9 @@
<li>What is this <tt>__main()</tt> call that gets inserted into <li>What is this <tt>__main()</tt> call that gets inserted into
<tt>main()</tt>?</li> <tt>main()</tt>?</li>
<li>Where did all of my code go??</li> <li>Where did all of my code go??</li>
<li>What is this <tt>llvm.global_ctors</tt> and
<tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I
#include &lt;iostream&gt;?</li>
</ol> </ol>
</li> </li>
</ol> </ol>
@ -449,6 +452,42 @@ you can read from and assign to <tt>volatile</tt> global variables.
</p> </p>
</div> </div>
<!--=========================================================================-->
<div class="question"><p>
What is this <tt>llvm.global_ctors</tt> and <tt>_GLOBAL__I__tmp_webcompile...</tt> stuff that happens when I #include &lt;iostream&gt;?
</p></div>
<div class="answer">
<p>
If you #include the &lt;iostream&gt; header into a C++ translation unit, the
file will probably use the <tt>std::cin</tt>/<tt>std::cout</tt>/... global
objects. However, C++ does not guarantee an order of initialization between
static objects in different translation units, so if a static ctor/dtor in your
.cpp file used <tt>std::cout</tt>, for example, the object would not necessarily
be automatically initialized before your use.
</p>
<p>
To make <tt>std::cout</tt> and friends work correctly in these scenarios, the
STL that we use declares a static object that gets created in every translation
unit that includes &lt;iostream&gt;. This object has a static constructor and
destructor that initializes and destroys the global iostream objects before they
could possibly be used in the file. The code that you see in the .ll file
corresponds to the constructor and destructor registration code.
</p>
<p>
If you would like to make it easier to <b>understand</b> the LLVM code generated
by the compiler in the demo page, consider using printf instead of iostreams to
print values.
</p>
</div>
<li>
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