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

[Nikita Popov via cfe-commits]

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

>From 6940157fa4b9c186f45b98206311b12ab78c40ff Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Fri, 10 Jan 2025 15:14:44 +0100
Subject: [PATCH 1/2] [Clang] Add release note for pointer overflow
 optimization change

---
 clang/docs/ReleaseNotes.rst | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 511a28c5554bbb..aea5eb2a04ac63 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -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``.
+
+  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
 -------------------------------------------
 

>From 53a106f43b3dcde62c25972c7f76dcab07c503b3 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Mon, 13 Jan 2025 11:10:29 +0100
Subject: [PATCH 2/2] Suggest an alternative way to write the check

---
 clang/docs/ReleaseNotes.rst | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index aea5eb2a04ac63..02967b75ff41be 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -71,7 +71,10 @@ code bases.
 
   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``.
+  ``(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