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June 27th, 2024

Misconceptions about loops in C

The paper emphasizes loop analysis in program tools, addressing challenges during transition to production. Late-discovered bugs stress the need for accurate analysis. Examples and references aid developers in improving software verification.

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Misconceptions about loops in C

The paper discusses the importance of loop analysis in program analysis tools, highlighting the challenges posed by rare edge cases as tools transition from academic prototypes to production-ready versions. Loop analysis is identified as a critical source of late-discovered algorithmic bugs, emphasizing the need to address these issues. The authors present a collection of examples and challenges in loop analysis to help developers avoid misconceptions and improve the accuracy of their analyses. Various references are provided to support the discussion, showcasing the significance of loop analysis in software verification and static analysis. The study aims to enhance the understanding of loop analysis and its implications for program correctness and optimization.

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Link Icon 14 comments
By @pornel - 5 months
If you're wondering why the paper seems to complicate "simple" constructs by converting them to control flow graphs — that's because many other kinds of static analysis, like tracking when variables are initialized and overwritten, are even more puzzling when trying to analyze very stateful code directly based on the syntax tree. The graph representation allows splitting the code into smaller blocks, and remove a lot of statefulness from them.
By @Dylan16807 - 5 months
So this is specifically from the point of view of someone analyzing control flow, and how that can be tricky or ridiculous based on how many features of C you combine. Not what I expected but quite interesting. Especially the difficulty in defining a loop.
By @mshockwave - 5 months
LLVM tries to canonicalize all the loops into one of the two canonical forms[1] before doing any useful analysis and transformations. It has been quite effectives. Not all loops can be shaped into a canonical one though.

[1]: https://llvm.org/docs/LoopTerminology.html

By @Perenti - 5 months
Best footnote for June:

returns should be thought of as domesticated gotos but domesticated in the same way that cats are domesticated

By @WalterBright - 5 months
Back in the 80's it was normal for C compilers to recognize `for` loops and do optimizations on them.

I had attended a summer class at Stanford on Data Flow Analysis, and took advantage. My compiler would break all the C flow control constructs into `if` and `goto` statements. Then, the optimizer would reconstruct the loops using graph algorithms.

Then, no matter what snarl of for, while, break, do-while, continue switch, or goto statements the programmer wrote, the optimizer could find all the loops, identify the loop variables, the loop header, etc. and then optimize it.

By @userbinator - 5 months
This reminds me of the decompilation stuff I worked on long ago, which was essentially a large recursive pattern-matching algorithm. Flows that couldn't be matched as loop constructs just got left as gotos, and breaks would be identified as such where possible, but for the most part it worked pretty well with the (very) stupid compiler output of the time, and even with much of the handwritten Asm that was far more common than it is now.
By @user- - 5 months
For the first time, I used a c style loop in a bash script today. What a coincidence
By @IshKebab - 5 months
These are all obvious but this looks very useful as a checklist if you are writing a static analysis tool, which I think is the intended purpose.
By @pizlonator - 5 months
It’s great to see this write up - should be mandatory reading for anyone getting started with compilers. But it’s not news for the experienced folks in the field.
By @eru - 5 months
First author is named as 'Martin Brain'. I wonder if that's a case of nominative determinism at work.
By @diffxx - 5 months
I think it's really unfortunate that most programmers are taught c style loops before tail recursion. But this is also a symptom of standard c's problem of not allowing one to define functions inside of functions. A loop can always be expressed with tail recursion. While the reverse is also true, there are many problems for which this is not straightforward given that a tail recursive function can have multiple exit points and these have to be coalesced into a single flag for the loop.
By @ssahoo - 5 months
Why the fuck they still write papers in 2 columns? It's a pain both for humans and computers alike. Here is the link to PDF https://dl.acm.org/doi/pdf/10.1145/3652588.3663324