| 1 is not an object, but even if it was one, it would not be an object that overlaps with “* p”. You are interpreting the C standard as if it were a philosophy text. It contains a rule that says that in very precise circumstances (for an assignment from one to the other) objects must not partially overlap, and you are claiming that it means that “two pointers to the same basic type cannot overlap in memory”. The clause does not say that, sorry. The clause applies to the objects that are on one side and the other of an assignment. > I said the standard expressly allowed the optimization in the linked article. I hope that the article makes it clear that the standard expressly allows the optimization. Specific, explicit rules, cited in the article, about pointer alignment, allow the optimization. For this reason, I, “violently” as you say, disagree with the sentence “The complaint isn't about alignment at all, it's that the optimizer assumes that two pointers to the same basic type cannot overlap in memory”. This sentence gets it all wrong. It is about alignment; it is not about “pointers to basic types”, whatever that is, not being allowed to overlap in memory; they are allowed to overlap for large enough “basic types” because it is about alignment, not overlap: https://gcc.godbolt.org/z/ZAMkeH You could argue that GCC 9.3 only missed the optimization in the example in this Compiler Explorer link for some other reason and that absence of optimization doesn't mean that p and q cannot overlap. This would be correct, this aspect is one of the difficulties in studying the rules that these compilers implement. However, what I am saying is that if you reported this missed optimization to GCC developers, they would tell you that GCC can't optimize the function f because p and q can overlap. There is no clause in the C standard that prevent them to (apart from strict aliasing rules, but I used the option to tell the compiler I didn't want it to take advantage of these ones). (Please do not bother them with this, or if you do, at least leave me out of it; I have nothing better to do than to write this because it's the week-end but they have better things to do.) |