The blog post "It is not a compiler error (2017)" explores a subtle bug related to floating-point comparisons in C++. The author demonstrates how seemingly innocuous code, involving comparing a floating-point value against zero after decrementing it in a loop, can lead to unexpected infinite loops. This arises because floating-point numbers have limited precision, and repeated subtraction of a small value from a larger one might never exactly reach zero. The post emphasizes the importance of understanding floating-point limitations and suggests using alternative comparison methods, like checking if the value is within a small tolerance of zero (epsilon comparison), or restructuring the loop condition to avoid direct equality checks with floating-point numbers.
This paper demonstrates how seemingly harmless data races in C/C++ programs, specifically involving non-atomic operations on padding bytes, can lead to miscompilation by optimizing compilers. The authors show that compilers can exploit the assumption of data-race freedom to perform transformations that change program behavior when races are actually present. They provide concrete examples where races on padding bytes within structures cause compilers like GCC and Clang to generate incorrect code, leading to unexpected outputs or crashes. This highlights the subtle ways in which undefined behavior due to data races can manifest, even when the races appear to involve data irrelevant to program logic. Ultimately, the paper reinforces the importance of avoiding data races entirely, even those that might seem benign, to ensure predictable program behavior.
Hacker News users discussed the implications of Boehm's paper on benign data races. Several commenters pointed out the difficulty in truly defining "benign," as seemingly harmless races can lead to unexpected behavior in complex systems, especially with compiler optimizations. Some highlighted the importance of tools and methodologies to detect and prevent data races, even if deemed benign. One commenter questioned the practical applicability of the paper's proposed relaxed memory model, expressing concern that relying on "benign" races would make debugging significantly harder. Others focused on the performance implications, suggesting that allowing benign races could offer speed improvements but might not be worth the potential instability. The overall sentiment leans towards caution regarding the exploitation of benign data races, despite acknowledging the potential benefits.
Summary of Comments ( 74 )
https://news.ycombinator.com/item?id=43112187
HN users discuss integer overflow in C/C++, focusing on its undefined behavior and the security implications. Some highlight the dangers, especially in situations where the compiler optimizes away overflow checks based on the assumption that it can't happen. Others point out that
-fwrapvcan enforce predictable wrapping behavior, making code safer but potentially slower. The discussion also touches on how static analyzers can help catch these issues, and the inherent difficulties in ensuring complete safety in C/C++ due to the language's flexibility. A few commenters mention alternatives like Rust, which offer stricter memory safety and overflow handling. One commenter shares a personal anecdote about an integer underflow vulnerability they found in a C++ program, emphasizing the real-world impact of these seemingly theoretical problems.The Hacker News post "It is not a compiler error (2017)" linking to a blog post about subtle C++ template issues generated a moderate amount of discussion, with a number of commenters sharing their own related experiences and insights.
Several commenters agreed with the author's premise that template errors can be incredibly obtuse and difficult to decipher. One commenter highlighted the frustration of encountering such errors, especially when they manifest as seemingly unrelated issues far from the actual source of the problem. They recounted an experience where a template error caused a cascade of cryptic error messages throughout their codebase, making it a nightmare to debug. Another commenter echoed this sentiment, emphasizing the sheer volume and complexity of error messages that can arise from even minor template mishaps. They pointed out that these errors often require a deep understanding of template metaprogramming and the C++ type system to unravel.
Some commenters offered practical advice for mitigating the pain of template errors. One suggestion involved using concepts (C++20 and later) to provide more descriptive and targeted error messages when template parameters don't meet the required constraints. Another commenter recommended employing static analysis tools and compiler extensions to catch potential template issues early in the development process. They also suggested breaking down complex templates into smaller, more manageable components to simplify debugging.
A few commenters discussed the trade-offs between the power and flexibility of C++ templates and the complexity they introduce. While acknowledging the potential for difficult-to-debug errors, they argued that the benefits of generic programming and code reusability offered by templates outweigh the drawbacks. One commenter specifically mentioned how templates enable writing highly performant code by allowing the compiler to perform optimizations tailored to specific types.
One comment thread delved into the specific example presented in the blog post, analyzing the underlying causes of the error and discussing alternative approaches to achieve the desired functionality. This discussion highlighted the intricacies of template argument deduction and the importance of carefully considering the interactions between different parts of a template.
Finally, some commenters simply expressed their shared frustration with C++ template errors, offering commiseration and solidarity with the author and other developers who have wrestled with similar issues. They lamented the steep learning curve associated with mastering C++ templates and the occasional feeling of helplessness when faced with an avalanche of incomprehensible error messages.