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Comment by roca | Hacker News Reader

Indeed, it's compiler implementors, not academics.

However, the performance impact of optimizations that take advantage of UB is not known, and is potentially very large. It would be a very interesting experiment to modify a C/C++ compiler so that every C/C++ program has defined semantics in terms of a simple array-of-bytes abstract machine, and see how much slower the generated code is compared to the regular compiler.

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It would be a very interesting experiment to modify a C/C++ compiler so that every C/C++ program has defined semantics in terms of a simple array-of-bytes abstract machine, and see how much slower the generated code is compared to the regular compiler.

Alternatively, look at compilers like Intel's ICC --- it has historically been one of the best at code generation, yet it's not known for having anywhere near the same level of UB-craziness as Clang (or GCC, to a lesser extent). The same has been my experience with MSVC, at least the earlier versions.

On the contrary, ICC doesn't even implement IEEE 754 correctly by default [1], for performance. This is way more aggressive than GCC or Clang.

[1]: https://hal.archives-ouvertes.fr/hal-00128124v5/document

Uh, ICC does some crazy things to get some of its performance, such as making transformations not allowed by the standard (inventing writes), in order to get better vectorization.

So in some respects ICC is very aggressive.

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