diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -64,8 +64,8 @@
 endif()
 
 if (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
-  message(STATUS "No build type selected, default to Debug")
-  set(CMAKE_BUILD_TYPE "Debug" CACHE STRING "Build type (default Debug)" FORCE)
+  message(STATUS "No build type selected, default to Release")
+  set(CMAKE_BUILD_TYPE "Release" CACHE STRING "Build type (default Release)" FORCE)
 endif()
 
 # Side-by-side subprojects layout: automatically set the
diff --git a/llvm/docs/ReleaseNotes.rst b/llvm/docs/ReleaseNotes.rst
--- a/llvm/docs/ReleaseNotes.rst
+++ b/llvm/docs/ReleaseNotes.rst
@@ -69,6 +69,10 @@
 Changes to building LLVM
 ------------------------
 
+* Default CMAKE_BUILD_TYPE has changed from Debug to Release in order to make
+  the default build smaller, faster and less prone to running out of RAM when
+  linking.
+
 Changes to TableGen
 -------------------