2 ms·

Linus has long railed against the aspects of the C standard, and how they're been interpreted by compiler writers [0]. I'm not much of a C programmer, but the

Linus has long railed against the aspects of the C standard, and how they're been interpreted by compiler writers [0]. I'm not much of a C programmer, but the C aliasing rules are incredibly confusing, and seem to be interpreted differently by GCC and Clang. e.g. GCC allows type punning via unions[1], but Clang does not [2]. [0] https://www.cl.cam.ac.uk/~srk31/research/papers/kell17some-preprint.pdf https://www.cl.cam.ac.uk/~srk31/research/papers/kell17some-p... [1] https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#Type-punning https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#Typ... [2] https://bugs.llvm.org/show_bug.cgi?id=31928 https://bugs.llvm.org/show_bug.cgi?id=31928

ajross 8y ago
That was just a clang bug, and as I read the comments they fixed it quietly without exactly admitting as much. That kind of local bitsmithery on floating point values is a pervasive idiom, clearly explained by the syntax of the language, and the compiler had no business getting smart with it.
flamedoge 8y ago
union and memcpy are known ways to do type-punning. Caveat to these is that they require at least one memory copy operation. Last I heard, clang and llvm's TBAA is still buggy.