char* pointer comparisons and clang12

[Jacob Lifshay]

On Tue, May 18, 2021, 00:21 Alan Modra via Binutils <binutils@sourceware.org>
wrote:

> Is it true that char* pointers cannot be compared using the relational
> operators <, >, <=, or >= except when they point into the same object
> or array (or one past)?  Reading the C standard section 6.5.8 seems to
> say that.  "In all other cases, the behavior is undefined."  (I know
> enough computer architecture to know why the standard is written that
> way, so the question is rhetorical.  The real question is below.)
>
> Are there compilers that optimize making use of that UB on flat memory
> architectures?  Specifically, clang12.  See pr27836.
>

Idk about when clang is in C mode, but at least for C++ mode clang gives
defined semantics to the < operator, since that's used to implement
std::less<T *>, which is required by the C++ standard to give a strict
total order on all pointers.

https://en.cppreference.com/w/cpp/utility/functional/less

libc++ 12 implementation of std::less:
https://github.com/llvm/llvm-project/blob/llvmorg-12.0.0/libcxx/include/__functional_base#L54

Jacob Lifshay