Landon Curt Noll
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<img src="../../png/ioccc.png" alt="IOCCC image by Matt Zucker" width=300 height=110>
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<h1>The International Obfuscated C Code Contest</h1>
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<h2>compilers.md for 1986/marshall</h2>
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<a class="Left" href="index.html">↑ jump to index.html ↑</a>
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<h1 id="conflicting-compilers-and-optimisers-that-broke-this-entry-in-2023">Conflicting compilers and optimisers that broke this entry in 2023</h1>
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<p>As briefly noted in the <a href="README.md">README.md</a> gcc and clang caused different
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problems with this entry depending on the optimiser being enabled or not, where
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if the optimiser was enabled it would work with one compiler and not the other.
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But if you then disabled the optimiser the opposite problem would occur: the
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working compiler would no longer work and the non-functional one would start to
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work! And then there’s the problem of linux versus macOS causing problems.</p>
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<p>The <a href="marshall.alt.c">alternate code</a> (source code) demonstrates the problem so
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you can see if your compiler has the problem as described below.</p>
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<p>In some versions of gcc and clang (this was first discovered in fedora linux 38)
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if the compiler was enabled one compiler would generate code that segfaulted but
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the other one would not. However, when then disabling the optimiser the compiler
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that had no problem suddenly did and the one that did suddenly did not. But
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segfaulting was not by any means the only problem.</p>
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<p>Another problem was that depending on the optimiser an infinite loop would be
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entered. But then removing it would cause other problems as well, including a
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segfault.</p>
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<p>It was not only one compiler that dumped core. Both dumped core with other
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behaviour, depending on what changes in code were made and the optimiser being
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on or off. Clang in macOS also dumped core!</p>
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<p>The <a href="README.md#alternate-code">alternate code</a> (README.md) has the original fix
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for clang which works with some compilers depending on the optimiser. When we
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refer to the code below we refer to the alternate code.</p>
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<h2 id="the-conflicting-compiler-problems">The conflicting compiler problems:</h2>
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<p>Below we describe the conflicting options that resulted in the need to modify
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the original code.</p>
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<h3 id="linux">Linux</h3>
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<p>With the optimiser disabled GCC compiled the code to enter an infinite loop.
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Enabling the optimiser and it worked fine. But if the optimiser is enabled clang
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generates code that first prints the text twice (it should print it once) and
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then immediately dumps core!</p>
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<p>But then there’s that infinite loop. What happens if we remove it?</p>
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<p>Once the loop is removed if we compile with clang and the optimiser is disabled
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all is okay. If however we compile with gcc and the optimiser is disabled the
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string is printed once and then the program dumps core! Why is this? It is
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because of the call to <code>_exit()</code> (see below).</p>
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<p>Clang works fine with the loop removed and the optimiser disabled. If however we
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enable the optimiser clang will generate code that prints the string twice and
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dumps core just like before! What about the call to <code>_exit()</code>?</p>
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<p>The call was:</p>
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<pre><code> _exit(argv[(int)*argc-2/cc[1*(int)*argc]|-1<<4]);</code></pre>
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<p>Changing it to not refer to these variables and just exit 1 and the problems
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disappear!</p>
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<h3 id="macos">macOS</h3>
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<p>But then there was macOS which had another problem!</p>
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<p>If the infinite loop is not removed/commented out and the optimiser is enabled
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it works fine but if it’s disabled it prints out the string and then enters an
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infinite loop!</p>
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<p>After we fix the infinite loop if we change the <code>_exit()</code> call in macOS and
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disable the optimiser we might get something like:</p>
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<pre><code> $ ./marshall
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choo choo
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Segmentation fault: 11</code></pre>
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<p>Observe how it doesn’t print it twice. But of course it segfaults so it’s not
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right either.</p>
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<p>If the optimiser is enabled, however, we will see that it works fine before the
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change to <code>_exit()</code>.</p>
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<h2 id="summary">Summary</h2>
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<p>And that is the tale of a little <strike>engine</strike> train that
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could…eventually! Or perhaps it’s the compilers, optimisers and OSes that
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eventually could?</p>
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<p>Whichever you wish it to be, it now works with both clang and gcc! Choo choo!</p>
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Copyright © 1984-2024 by Landon Curt Noll. All Rights Reserved.
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You are free to share and adapt this file under the terms of this license:
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Copyright © 1984-2024 by Landon Curt Noll. All Rights Reserved.
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You are free to share and adapt this file under the terms of this license:
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Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)
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<b> Copyright © 1984-2024 by Landon Curt Noll. <a href="https://creativecommons.org/faq/#what-does-some-rights-reserved-mean">Some Rights Reserved</a>.</b>
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<h2>Coda</h2>
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