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I’m surprised this is necessary at all. Wouldn’t a compiler at any reasonable optimisation level optimise __cmp_once into __cmp automatically (and then into the resulting max constant) if used with constants? Seems like very basic constant propagation.

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As explained in the article linked at the top ("LWN recently looked at the kernel's max() macro"), the issue isn't one of optimization, it's one of syntax. Sure, a decently optimizing compiler can optimize __cmp_once in to a constant value. However, C99 requires that non-VLA arrays have a size that is a constant expression, which is a syntax thing; even though GCC knows that __cmp_once evaluates to a constant value, the resulting C coming out of the preprocessor is of the wrong syntactic shape.

The problem is cmp and cmp_once are actually different if x and y are function calls. In that case cmp calls the functions twice.

I think you've misread the question. Grandparent isn't asking why use cmp_once at all; they're asking, why not always force the single evaluation of x and y (i.e., always use cmp_once and let the compiler optimizer reduce the resulting expression)? A responsive answer is given in https://news.ycombinator.com/item?id=16720118 .

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