[Clang] Add release note for pointer overflow optimization change (#1… · llvm/llvm-project@c2979c5

containing strict-aliasing violations. The new default behavior can be

disabled using ``-fno-pointer-tbaa``.

- Clang will now more aggressively use undefined behavior on pointer addition

overflow for optimization purposes. For example, a check like

``ptr + unsigned_offset < ptr`` will now optimize to ``false``, because

``ptr + unsigned_offset`` will cause undefined behavior if it overflows (or

advances past the end of the object).

Previously, ``ptr + unsigned_offset < ptr`` was optimized (by both Clang and

GCC) to ``(ssize_t)unsigned_offset < 0``. This also results in an incorrect

overflow check, but in a way that is less apparent when only testing with

pointers in the low half of the address space.

To avoid pointer addition overflow, it is necessary to perform the addition

on integers, for example using

``(uintptr_t)ptr + unsigned_offset < (uintptr_t)ptr``. Sometimes, it is also

possible to rewrite checks by only comparing the offset. For example,

``ptr + offset < end_ptr && ptr + offset >= ptr`` can be written as

``offset < (uintptr_t)(end_ptr - ptr)``.

Undefined behavior due to pointer addition overflow can be reliably detected

using ``-fsanitize=pointer-overflow``. It is also possible to use

``-fno-strict-overflow`` to opt-in to a language dialect where signed integer

and pointer overflow are well-defined.

C/C++ Language Potentially Breaking Changes

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