diff --git a/CMakeLists.txt b/CMakeLists.txt
index a84adb0ca4636..dba61ecfaef14 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -630,6 +630,10 @@ if("${CMAKE_C_COMPILER_ID}" STREQUAL "MSVC" OR "${CMAKE_CXX_SIMULATE_ID}" STREQU
 endif()
 
 if(NOT SWIFT_COMPILER_IS_MSVC_LIKE)
+  # CMake's default for CMAKE_CXX_FLAGS_DBEUG is "-g". Let's add "-O0", because we want to be able
+  # to append CMAKE_CXX_FLAGS_DEBUG to a list of compile flags that already contains a -O flag.
+  set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -O0")
+
   # CMake's default for CMAKE_CXX_FLAGS_RELEASE is "-O3 -DNDEBUG". Let's avoid "-O3" for consistency
   # between Release and RelWithDebInfo. Dropping -DNDEBUG from this setting is blocked by triggering
   # a test failure of Swift-Unit :: Syntax/./SwiftSyntaxTests/TypeSyntaxTests.MetatypeTypeWithAPIs
diff --git a/cmake/modules/AddSwift.cmake b/cmake/modules/AddSwift.cmake
index 863095fdfa205..046d28bc5fbf5 100644
--- a/cmake/modules/AddSwift.cmake
+++ b/cmake/modules/AddSwift.cmake
@@ -133,7 +133,9 @@ function(_add_host_variant_c_compile_flags target)
   is_build_type_with_debuginfo("${CMAKE_BUILD_TYPE}" debuginfo)
 
   # Add -O0/-O2/-O3/-Os/-g/-momit-leaf-frame-pointer/... based on CMAKE_BUILD_TYPE.
-  target_compile_options(${target} PRIVATE "${CMAKE_CXX_FLAGS_${CMAKE_BUILD_TYPE}}")
+  string(TOUPPER ${CMAKE_BUILD_TYPE} build_type_upper)
+  string(REPLACE " " ";" cxx_flags_list "${CMAKE_CXX_FLAGS_${build_type_upper}}")
+  target_compile_options(${target} PRIVATE ${cxx_flags_list})
 
   if(optimized)
     # Omit leaf frame pointers on x86 production builds (optimized, no debug
@@ -231,12 +233,12 @@ function(_add_host_variant_c_compile_flags target)
   if(LLVM_ENABLE_ASSERTIONS)
     target_compile_options(${target} PRIVATE -UNDEBUG)
   else()
-    target_compile_definitions(${target} PRIVATE -DNDEBUG)
+    target_compile_options(${target} PRIVATE -DNDEBUG)
   endif()
 
   if(SWIFT_ENABLE_RUNTIME_FUNCTION_COUNTERS)
-    target_compile_definitions(${target} PRIVATE
-      SWIFT_ENABLE_RUNTIME_FUNCTION_COUNTERS)
+    target_compile_options(${target} PRIVATE
+      -DSWIFT_ENABLE_RUNTIME_FUNCTION_COUNTERS)
   endif()
 
   if(SWIFT_ANALYZE_CODE_COVERAGE)
diff --git a/stdlib/cmake/modules/AddSwiftStdlib.cmake b/stdlib/cmake/modules/AddSwiftStdlib.cmake
index afaf60ac15ede..826af2f76d0c2 100644
--- a/stdlib/cmake/modules/AddSwiftStdlib.cmake
+++ b/stdlib/cmake/modules/AddSwiftStdlib.cmake
@@ -167,7 +167,9 @@ function(_add_target_variant_c_compile_flags)
   is_build_type_with_debuginfo("${CFLAGS_BUILD_TYPE}" debuginfo)
 
   # Add -O0/-O2/-O3/-Os/-g/... based on CFLAGS_BUILD_TYPE.
-  list(APPEND result "${CMAKE_CXX_FLAGS_${CFLAGS_BUILD_TYPE}}")
+  string(TOUPPER ${CFLAGS_BUILD_TYPE} build_type_upper)
+  string(REPLACE " " ";" cxx_flags_list "${CMAKE_CXX_FLAGS_${build_type_upper}}")
+  list(APPEND result ${cxx_flags_list})
 
   if(optimized)
     # Omit leaf frame pointers on x86 production builds (optimized, no debug