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add an entry, add links to entries for demo page.

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llvm-svn: 20328
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Chris Lattner 2005-02-25 20:30:21 +00:00
parent 9340ba4bf9
commit ea8835f35b
1 changed files with 53 additions and 27 deletions
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docs/FAQ.html
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@ -72,12 +72,13 @@
<li><a href="#cfe_code">Questions about code generated by the GCC front-end</a>
<ol>
<li>What is this <tt>__main()</tt> call that gets inserted into
<tt>main()</tt>?</li>
<li>Where did all of my code go??</li>
<li>What is this <tt>llvm.global_ctors</tt> and
<li><a href="#__main">What is this <tt>__main()</tt> call that gets inserted into
<tt>main()</tt>?</a></li>
<li><a href="#iosinit">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>
#include &lt;iostream&gt;?</a></li>
<li><a href="#codedce">Where did all of my code go??</a></li>
<li><a href="#undef">What is this "<tt>undef</tt>" thing that shows up in my code?</a></li>
</ol>
</li>
</ol>
@ -448,6 +449,7 @@ correct this, do:</p>
</div>
<div class="question"><p>
<a name="__main"></a>
What is this <tt>__main()</tt> call that gets inserted into <tt>main()</tt>?
</p></div>
@ -469,29 +471,8 @@ linked in automatically when you link the program.
<!--=========================================================================-->
<div class="question"><p>
Where did all of my code go??
</p></div>
<div class="answer">
<p>
If you are using the LLVM demo page, you may often wonder what happened to all
of the code that you typed in. Remember that the demo script is running the
code through the LLVM optimizers, so if your code doesn't actually do anything
useful, it might all be deleted.
</p>
<p>
To prevent this, make sure that the code is actually needed. For example, if
you are computing some expression, return the value from the function instead of
leaving it in a local variable. If you really want to constrain the optimizer,
you can read from and assign to <tt>volatile</tt> global variables.
</p>
</div>
<!--=========================================================================-->
<div class="question">
<a name="iosinit"></a>
<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>
@ -520,6 +501,51 @@ instead of <tt>iostream</tt>s to print values.</p>
</div>
<!--=========================================================================-->
<div class="question"><p>
<a name="codedce"></a>
Where did all of my code go??
</p></div>
<div class="answer">
<p>
If you are using the LLVM demo page, you may often wonder what happened to all
of the code that you typed in. Remember that the demo script is running the
code through the LLVM optimizers, so if your code doesn't actually do anything
useful, it might all be deleted.
</p>
<p>
To prevent this, make sure that the code is actually needed. For example, if
you are computing some expression, return the value from the function instead of
leaving it in a local variable. If you really want to constrain the optimizer,
you can read from and assign to <tt>volatile</tt> global variables.
</p>
</div>
<!--=========================================================================-->
<div class="question"><p>
<a name="undef"></a>
<p>What is this "<tt>undef</tt>" thing that shows up in my code?
</p></div>
<div class="answer">
<p>
<a href="LangRef.html#undef"><tt>undef</tt></a> is the LLVM way of representing
a value that is not defined. You can get these if you do not initialize a
variable before you use it. For example, the C function:</p>
<div class="doc_code">
<tt>int X() { int i; return i; }</tt>
</div>
<p>Is compiled to "<tt>ret int undef</tt>" because "i" never has a value
specified for it.
</p>
</div>
<!-- *********************************************************************** -->
<hr>