[clang] [Clang] Add release note for pointer overflow optimization change (PR #122462)

[Hans Wennborg via cfe-commits]

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@@ -58,6 +58,26 @@ code bases.
   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``.
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zmodem wrote:

In many cases it's possible to rewrite the check in the integer domain instead, checking that `unsigned_offset` is less then some ptr diff. Should we mention something like that here, so that people don't immediately just add the uintptr_t casts?

https://github.com/llvm/llvm-project/pull/122462