If you spend much time testing compilers, you’ll run into some strange phenomena even in apparently simple areas like computer arithmetic. For example John Cook wrote a post today explaining why IEEE floats have two different values of zero. Integer arithmetic is generally a lot simpler than floating point math, but it still contains a …
Category Archives: Software Correctness
Compilers and Termination Revisited
My earlier post C compilers Disprove Fermat’s Last Theorem generated a good amount of discussion both here and on Reddit. Unfortunately, the discussion was riddled with misunderstandings. Some of this was because the topic is subtle, but some was my fault: the post was intended to be lightweight and I failed to explain the underlying …
C Compilers Disprove Fermat’s Last Theorem
[Update: I wrote another post on this topic that may explain the underlying issues more clearly.] Obviously I’m not serious: compilers are bad at solving high-level math problems and also there is good reason to believe this theorem cannot be disproved. But I’m getting ahead of myself. Recently — for reasons that do not matter …
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Margin in Software Systems
Margin of safety is a fundamental engineering concept where a system is built to tolerate loads exceeding the maximum expected load by some factor. For example, structural elements of buildings typically have a margin of safety of 100%: they can withstand twice the expected maximum load. Pressure vessels have more margin, in the range 250%-300%, …
The Compiler Doesn’t Care About Your Intent
A misunderstanding that I sometimes run into when teaching programming is that the compiler can and should guess what the programmer means. This isn’t usually quite what people say, but it’s what they’re thinking. A great example appeared in a message sent to the avr-gcc mailing list. The poster had upgraded his version of GCC, …
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C and C++ Make It Hard to Read a Register for Its Side Effects
[ This post was co-written with Nigel Jones, who maintains an excellent embedded blog Stack Overflow. Nigel and I share an interest in volatile pitfalls in embedded C/C++ and this post resulted from an email discussion we had. Since we both have blogs, we decided to both post it. However, since comments are not enabled …
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200 Compiler Bugs
This morning I reported the 200th bug found by our compiler testing tool. It is a new way to crash GCC. The failure-inducing input is not pretty so I won’t give it here, but it can be found in GCC’s bugzilla. Although the testing tool is now entirely developed by some excellent PhD students, I …
Nine ways to break your systems code using volatile
The volatile qualifier in C/C++ is a little bit like the C preprocessor: an ugly, blunt tool that is easy to misuse but that — in a very narrow set of circumstances — gets the job done. This article will first briefly explain volatile and its history and then, through a series of examples about …
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Is That a Compiler Bug?
It’s common for a computer program to display behavior seemingly unrelated to its source code. Additionally, programs written in C/C++ frequently change their behavior when the compiler optimization options are changed. This post is a quick guide to figuring out if the compiler is at fault. This post was motivated by a blog post by …