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. Sur

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.