llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT--> @llvm/pr-subscribers-clang Author: Nikita Popov (nikic) <details> <summary>Changes</summary> Add a release note for optimization change related to pointer overflow checks. I've put this in the breaking changes section to give it the best chance of being seen. --- Full diff: https://github.com/llvm/llvm-project/pull/122462.diff 1 Files Affected: - (modified) clang/docs/ReleaseNotes.rst (+20) ``````````diff 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 ------------------------------------------- `````````` </details> https://github.com/llvm/llvm-project/pull/122462 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
