When I compile code using the LLVM GCC front end, it complains that it
cannot find libcrtend.a.
</li>
+
+ <li>
+ How can I disable all optimizations when compiling code using the LLVM GCC front end?
+ </li>
+
</ol>
</li>
<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 <iostream>?</li>
+ #include <iostream>?</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>
<div class="doc_author">
- <p>Written by <a href="http://llvm.cs.uiuc.edu">The LLVM Team</a></p>
+ <p>Written by <a href="http://llvm.org">The LLVM Team</a></p>
</div>
<div class="answer">
<p>Yes. The modified source distribution must retain the copyright notice and
follow the three bulletted conditions listed in the <a
-href="http://llvm.cs.uiuc.edu/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
+href="http://llvm.org/releases/1.3/LICENSE.TXT">LLVM license</a>.</p>
</div>
<div class="question">
<li>The GCC front end code is not as portable as the LLVM suite, so it may not
compile as well on unsupported platforms.</li>
- <li>The Python test classes are more UNIX-centric than they should be, so
- porting to non-UNIX like platforms (i.e. Windows, MacOS 9) will require some
- effort.</li>
-
<li>The LLVM build system relies heavily on UNIX shell tools, like the Bourne
Shell and sed. Porting to systems without these tools (MacOS 9, Plan 9) will
require more effort.</li>
affects projects other than LLVM. Try upgrading or downgrading your GCC.</p>
</div>
-<div class="question">
-<p>
-When I use the test suite, all of the C Backend tests fail. What is
-wrong?
-</p>
-</div>
-
-<div class="answer">
-<p>
-If you build LLVM and the C Backend tests fail in <tt>llvm/test/Programs</tt>,
-then chances are good that the directory pointed to by the LLVM_LIB_SEARCH_PATH
-environment variable does not contain the libcrtend.a library.
-</p>
-
-<p>
-To fix it, verify that LLVM_LIB_SEARCH_PATH points to the correct directory
-and that libcrtend.a is inside. For pre-built LLVM GCC front ends, this
-should be the absolute path to
-<tt>cfrontend/<<i>platform</i>>/llvm-gcc/bytecode-libs</tt>. If you've
-built your own LLVM GCC front end, then ensure that you've built and installed
-the libraries in <tt>llvm/runtime</tt> and have LLVM_LIB_SEARCH_PATH pointing
-to the <tt>LLVMGCCDIR/bytecode-libs</tt> subdirectory.
-</p>
-</div>
-
<div class="question">
<p>After CVS update, rebuilding gives the error "No rule to make target".</p>
</div>
<div class="answer">
<p>
-In order to find libcrtend.a, you must have the directory in which it lives in
-your LLVM_LIB_SEARCH_PATH environment variable. For the binary distribution of
-the LLVM GCC front end, this will be the full path of the bytecode-libs
-directory inside of the LLVM GCC distribution.
+The only way this can happen is if you haven't installed the runtime library. To
+correct this, do:</p>
+<pre>
+ % cd llvm/runtime
+ % make clean ; make install-bytecode
+</pre>
+</div>
+
+<div class="question">
+<p>
+How can I disable all optimizations when compiling code using the LLVM GCC front end?
</p>
</div>
+<div class="answer">
+<p>
+Passing "-Wa,-disable-opt -Wl,-disable-opt" will disable *all* cleanup and
+optimizations done at the llvm level, leaving you with the truly horrible
+code that you desire.
+</p>
+</div>
<!-- *********************************************************************** -->
<div class="doc_section">
</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>
<!--=========================================================================-->
-<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
<iostream>?</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>
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src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
- <a href="http://llvm.cs.uiuc.edu">LLVM Compiler Infrastructure</a><br>
+ <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
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</address>
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