Add a note to the release not about a potentially breaking optimization

This has come up twice already (once in pr42763 and once in the commit thread), so give warning of a new way in which UB can result in unexpected program behavior.

llvm-svn: 367941

diff --git a/llvm/docs/ReleaseNotes.rst b/llvm/docs/ReleaseNotes.rst
index 7b7972f..494efd4 100644 (file)
--- a/llvm/docs/ReleaseNotes.rst
+++ b/llvm/docs/ReleaseNotes.rst
@@ -50,6 +50,19 @@ Non-comprehensive list of changes in this release
 
    Makes programs 10x faster by doing Special New Thing.
 
+Noteworthy optimizations
+------------------------
+
+* LLVM will now remove stores to constant memory (since this is a
+  contradiction) under the assumption the code in question must be dead.  This
+  has proven to be problematic for some C/C++ code bases which expect to be
+  able to cast away 'const'.  This is (and has always been) undefined
+  behavior, but up until now had not been actively utilized for optimization
+  purposes in this exact way.  For more information, please see:
+  `bug 42763 <https://bugs.llvm.org/show_bug.cgi?id=42763>_` and
+  `post commit discussion <http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20190422/646945.html>_`.  
+
+
 Changes to the LLVM IR
 ----------------------