<div dir="ltr">Hi Chris,<div><br></div><div>I just had the joy of reading Regehr's and your articles on undefined behavior in C and C++. Thank you for those. They sort of put words on things that I have felt for a decade or so - that C and C++ are too unsafe languages to be used in all but the most "masochistic" environments (sorry, you guys who love C++ out there - please don't get offended just because I personally happen to have grown very tired of the C/C++ universe long ago, I once lived and breathed C and C++ like you do now!). </div>
<div><br></div><div>Anyways, I just wanted to suggest something that I have been doing for a few decades. And that is to increase the reliability of the output program by using at least four different compilers. Not three times GNU and one time MSVC, but four different vendors' compilers. I believe LLVM already does this, but I also know of many projects that do the "fatal" mistake of tying themselves in with a single compiler, like MSVC, and then sort of lean on the implied behavior in undefined cases. Obviously, such projects are in for a rough ride the first time they ever get ported to another compiler. </div>
<div><br></div><div>Like I always say: Windows didn't become stable until the time that Microsoft was forced, by business plans, into porting Windows to four different architectures. And the same goes with many software projects that only use a single compiler on a single platform - they are, in your words, mines waiting to explode.</div>
<div><br></div><div>I'm confident that you know all this well, but you write "There is No Reliable Way to Determine if a Large Codebase Contains Undefined Behavior". True indeed, but if you use 4+ compilers, perhaps combined with building on both big-endian and little-endian platforms, and do both debug and release builds, you get much closer to the magic point where you can actually sleep soundly knowing that your code is mostly okay :-)</div>
<div><br></div><div><br></div><div>-- Mikael</div></div><div class="gmail_extra"><br><br><div class="gmail_quote">2013/12/4 Chris Lattner <span dir="ltr"><<a href="mailto:clattner@apple.com" target="_blank">clattner@apple.com</a>></span><br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word"><br><div><div>On Nov 28, 2013, at 6:07 PM, Mikael Lyngvig <<a href="mailto:mikael@lyngvig.org" target="_blank">mikael@lyngvig.org</a>> wrote:</div>
<br><blockquote type="cite"><div dir="ltr">Hi,<div><br></div><div>It will probably take a few weeks or a month before the "Mapping High-Level Constructs to LLVM IR" document is ready for prime time. Until then, you can review and study it at this URL:</div>
<div><br></div><div> <a href="https://github.com/archfrog/llvm-doc/blob/master/MappingHighLevelConstructsToLLVMIR.rst" target="_blank">https://github.com/archfrog/llvm-doc/blob/master/MappingHighLevelConstructsToLLVMIR.rst</a></div>
<div>
<br></div><div><br></div><div>Please notice that I specifically do not advocate reviewing the document for a week or two. But feel free to give me any feedback, comments, and criticism that you may have to share. </div>
<div><br></div></div></blockquote><br></div><div>This looks really awesome! Great idea starting this, and thank you for pushing it forward. Some thoughts:</div><div><br></div><div>- In "local variables", it would be great to talk about how the "alloca trick" avoids forcing your frontend to build SSA. You could even include an example.</div>
<div><br></div><div>- In the "constants" section, it is probably best to say that "constants that allocate memory" are just global variables in LLVM IR, marked with the 'constant' keyword. It would also be great to mention constant exprs here, since they are a point of confusion (and you introduce them in sizeof).</div>
<div><br></div><div>- Having something that talks about lowering C-style unions to llvm IR would be great :-)</div><div><br></div><div>- A nice new top-level section would be "interoperating with a runtime library", pointing out that not everything needs to be emitted as inline LLVM IR: a frontend can either just call into a runtime library, or it can even emit a call to a runtime library (whose body is also available as IR) and then have the optimizer inline it if run.</div>
<div><br></div><div><br></div><div><blockquote type="cite"><div dir="ltr">Once the document has been finalized and comitted to LLVM, I'll delete the repository at Github - or, perhaps even better, simply make a small page that refers to the official copy in LLVM.</div>
</blockquote><br></div><div>Are you interested in just building it in <a href="http://llvm.org/docs?" target="_blank">llvm.org/docs?</a> Unless your workflow is better on github, it seems easier to do it on <a href="http://llvm.org" target="_blank">llvm.org</a> - it would make it easier for other people to chip in.</div>
<span class="HOEnZb"><font color="#888888"><div><br></div><div>-Chris</div></font></span></div></blockquote></div><br></div>