Cody Boone Ferguson
5931934d8c
This includes the compilers.html and index.html file. I also threw in a slight formatting change in the thanks file.
124 lines
4.2 KiB
Markdown
124 lines
4.2 KiB
Markdown
# Conflicting compilers and optimisers that broke this entry in 2023
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As briefly noted in the [index.html](index.html) `gcc` versus `clang` caused
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different problems with this entry, depending on the optimiser being enabled or
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disabled, where if the optimiser was enabled it would work with one compiler but
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not the other; on the other hand, if you disabled the optimiser the opposite
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problem would occur: the working compiler would no longer work and the other one
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would! And then there's the problem of linux versus macOS causing problems.
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Using the [alternate source code](%%REPO_URL%%/1986/marshall/marshall.alt.c)
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demonstrates the problem so you can see if your compiler has the problem
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described below.
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In some versions of `gcc` and `clang` (this was first discovered in fedora linux 38)
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if the optimiser was enabled one compiler would generate code that segfaulted but
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the other one did not. However, when 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.
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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!
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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!
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The [marshall.c](%%REPO_URL%%/1986/marshall/marshall.c) has the original fix
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for `clang`, which works with some compilers, depending on the optimiser. Below
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we refer to the [alternate code
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marshall.alt.c](%%REPO_URL%%/1986/marshall/marshall.alt.c).
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## The conflicting compiler problems:
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Below we describe the conflicting options that resulted in the need to modify
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the original code.
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### Linux
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With the optimiser disabled `gcc` compiled the code to enter an infinite loop.
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If the optimiser was enabled it worked fine, but if the optimiser is enabled
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`clang` generates code that first prints the text twice (when it should print it
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only once) and then immediately dumps core!
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But then there's that infinite loop. What happens if we fix it?
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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 the `_exit(2)` syscall (see below).
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`clang` works fine with the loop removed if the optimiser is disabled. Once we
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enable it, though, it will generate code that prints the string twice and
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dumps core, just like before!
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<div id="exit">
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### What about the call to the `_exit(2)` syscall?
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</div>
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The call _was_:
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``` <!---c-->
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_exit(argv[(int)*argc-2/cc[1*(int)*argc]|-1<<4]);
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```
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In the fixed code, these variables are simply commented out so the code does
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`_exit(1)`.
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### macOS
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Now as for macOS, _another problem_ existed!
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If the code that resulted in an infinite loop is _NOT_ commented out _AND_ the
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optimiser is enabled it works fine; but if it [the optimiser] is disabled, it
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prints out the string and then enters that infinite loop!
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After we fix the infinite loop issue, if we change the call to the `_exit(2)`
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syscall in macOS and disable the optimiser we might get something like:
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``` <!---sh-->
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$ ./marshall
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choo choo
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Segmentation fault: 11
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```
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Observe how it doesn't print it twice. But of course it segfaults so it's not
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right either.
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If the optimiser is enabled, however, we will see that it works fine before the
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change to the call to the `_exit(2)` syscall.
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## Summary
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And that is the tale of a little <del>engine</del> train that
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could...eventually! Or perhaps it's the compilers, optimisers and OSes that
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eventually could?
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Whichever you wish it to be, it now works with both `clang` and `cc`. Choo choo!
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<hr style="width:10%;text-align:left;margin-left:0">
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Jump to: [top](#)
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