# CMake automatically reads `CFLAGS`, `CXXFLAGS`, `LDFLAGS`, etc. from the
# environment and uses them to populate its default flag variables -- but the
# Swift build system blows away `CMAKE_C{XX}_FLAGS` without picking up the
# environment. These need to be picked up again to not be ignored.

--- a/swift/utils/build-script-impl
+++ b/swift/utils/build-script-impl
@@ -1820,10 +1820,10 @@ for host in "${ALL_HOSTS[@]}"; do

                 cmake_options=(
                     "${cmake_options[@]}"
-                    -DCMAKE_C_FLAGS="$(swift_c_flags ${host})"
-                    -DCMAKE_CXX_FLAGS="$(swift_c_flags ${host})"
-                    -DCMAKE_C_FLAGS_RELWITHDEBINFO="-O2 -DNDEBUG"
-                    -DCMAKE_CXX_FLAGS_RELWITHDEBINFO="-O2 -DNDEBUG"
+                    -DCMAKE_C_FLAGS="${CFLAGS} $(swift_c_flags ${host})"
+                    -DCMAKE_CXX_FLAGS="${CXXFLAGS} $(swift_c_flags ${host})"
+                    -DCMAKE_C_FLAGS_RELWITHDEBINFO="${CFLAGS} -O2 -DNDEBUG"
+                    -DCMAKE_CXX_FLAGS_RELWITHDEBINFO="${CXXFLAGS} -O2 -DNDEBUG"
                     -DCMAKE_BUILD_TYPE:STRING="${SWIFT_BUILD_TYPE}"
                     -DLLVM_ENABLE_ASSERTIONS:BOOL=$(true_false "${SWIFT_ENABLE_ASSERTIONS}")
                     -DBOOTSTRAPPING_MODE:STRING=$(to_bootstrapping_mode "${BOOTSTRAPPING}")
@@ -2215,8 +2215,8 @@ for host in "${ALL_HOSTS[@]}"; do
                 cmake_options=(
                     "${cmake_options[@]}"
                     -C${LLDB_SOURCE_DIR}/cmake/caches/${cmake_cache}
-                    -DCMAKE_C_FLAGS="$(llvm_c_flags ${host})"
-                    -DCMAKE_CXX_FLAGS="$(llvm_c_flags ${host})"
+                    -DCMAKE_C_FLAGS="${CFLAGS} $(llvm_c_flags ${host})"
+                    -DCMAKE_CXX_FLAGS="${CXXFLAGS} $(llvm_c_flags ${host})"
                     -DCMAKE_BUILD_TYPE:STRING="${LLDB_BUILD_TYPE}"
                     -DLLDB_SWIFTC:PATH=${SWIFTC_BIN}
                     -DLLDB_SWIFT_LIBS:PATH="$(build_directory ${LOCAL_HOST} swift)/lib/swift"
--- a/swift/utils/swift_build_support/swift_build_support/products/llvm.py
+++ b/swift/utils/swift_build_support/swift_build_support/products/llvm.py
@@ -261,10 +261,10 @@ class LLVM(cmake_product.CMakeProduct):
         # llvm/tools, e.g. to build LLDB.

         llvm_c_flags = ' '.join(self.llvm_c_flags(platform, arch))
-        llvm_cmake_options.define('CMAKE_C_FLAGS', llvm_c_flags)
-        llvm_cmake_options.define('CMAKE_CXX_FLAGS', llvm_c_flags)
-        llvm_cmake_options.define('CMAKE_C_FLAGS_RELWITHDEBINFO', '-O2 -DNDEBUG')
-        llvm_cmake_options.define('CMAKE_CXX_FLAGS_RELWITHDEBINFO', '-O2 -DNDEBUG')
+        llvm_cmake_options.define('CMAKE_C_FLAGS', ' '.join([os.environ.get('CFLAGS', ''), llvm_c_flags]))
+        llvm_cmake_options.define('CMAKE_CXX_FLAGS', ' '.join([os.environ.get('CXXFLAGS', ''), llvm_c_flags]))
+        llvm_cmake_options.define('CMAKE_C_FLAGS_RELWITHDEBINFO', ' '.join([os.environ.get('CFLAGS', ''), '-O2 -DNDEBUG']))
+        llvm_cmake_options.define('CMAKE_CXX_FLAGS_RELWITHDEBINFO', ' '.join([os.environ.get('CXXFLAGS', ''), '-O2 -DNDEBUG']))
         llvm_cmake_options.define('CMAKE_BUILD_TYPE:STRING',
                                   self.args.llvm_build_variant)
         llvm_cmake_options.define('LLVM_TOOL_SWIFT_BUILD:BOOL', 'FALSE')
--- a/swift/utils/swift_build_support/swift_build_support/products/product.py
+++ b/swift/utils/swift_build_support/swift_build_support/products/product.py
@@ -440,8 +440,8 @@ class Product(object):

         (platform, arch) = host_target.split('-')
         common_c_flags = ' '.join(self.common_cross_c_flags(platform, arch))
-        self.cmake_options.define('CMAKE_C_FLAGS', common_c_flags)
-        self.cmake_options.define('CMAKE_CXX_FLAGS', common_c_flags)
+        self.cmake_options.define('CMAKE_C_FLAGS', ' '.join([os.environ.get('CFLAGS', ''), common_c_flags]))
+        self.cmake_options.define('CMAKE_CXX_FLAGS', ' '.join([os.environ.get('CXXFLAGS', ''), common_c_flags]))

         toolchain_file = None
         if self.is_darwin_host(host_target):