# 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
@@ -1819,10 +1819,10 @@ 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}")
                     -DSWIFT_ANALYZE_CODE_COVERAGE:STRING=$(toupper "${SWIFT_ANALYZE_CODE_COVERAGE}")
@@ -2229,8 +2229,8 @@ 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
@@ -284,10 +284,10 @@ 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
@@ -450,8 +450,8 @@ 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):
--- a/swift/SwiftCompilerSources/CMakeLists.txt
+++ b/swift/SwiftCompilerSources/CMakeLists.txt
@@ -222,6 +222,11 @@ function(add_swift_compiler_modules_library name)
     list(APPEND sdk_option "-I" "${swift_exec_bin_dir}/../lib" "-I" "${sdk_path}/usr/lib")
   endif()
 
+  separate_arguments(cxxflags NATIVE_COMMAND "${CMAKE_CXX_FLAGS}")
+  foreach(flag IN LISTS cxxflags)
+    list(APPEND swift_compile_options "-Xcc" "${flag}")
+  endforeach()
+
   set(all_obj_files)
   set(all_module_targets)
   set(all_module_files)
--- a/swift/cmake/modules/AddPureSwift.cmake
+++ b/swift/cmake/modules/AddPureSwift.cmake
@@ -110,6 +110,11 @@ function(_set_swift_cxx_interop_options name)
       "-cxx-interoperability-mode=default"
     )
   endif()
+
+  separate_arguments(cxxflags NATIVE_COMMAND "${CMAKE_CXX_FLAGS}")
+  foreach(flag IN LISTS cxxflags)
+    target_compile_options(${name} PRIVATE "SHELL: -Xcc ${flag}")
+  endforeach()
 endfunction()
 
 function(_set_pure_swift_link_flags name relpath_to_lib_dir)
--- a/swift/stdlib/cmake/modules/SwiftSource.cmake
+++ b/swift/stdlib/cmake/modules/SwiftSource.cmake
@@ -138,6 +138,12 @@ function(handle_swift_sources
       endif()
     endif()
     list(APPEND swift_compile_flags "-color-diagnostics")
+
+    separate_arguments(cxxflags NATIVE_COMMAND "${CMAKE_CXX_FLAGS}")
+    foreach(flag IN LISTS cxxflags)
+      list(APPEND swift_compile_flags "-Xcc" "${flag}")
+    endforeach()
+
     _compile_swift_files(
         dependency_target
         module_dependency_target
--- a/swift-driver/Utilities/build-script-helper.py
+++ b/swift-driver/Utilities/build-script-helper.py
@@ -4,6 +4,7 @@ import argparse
 import os
 import json
 import platform
+import shlex
 import shutil
 import subprocess
 import sys
@@ -117,6 +118,10 @@ def get_swiftpm_options(args):
   if args.action == 'install':
     swiftpm_args += ['-Xswiftc', '-no-toolchain-stdlib-rpath']
 
+  cxxflags = shlex.split(os.getenv('CXXFLAGS', ''))
+  for flag in cxxflags:
+    swiftpm_args.extend(['-Xcc', flag])
+
   return swiftpm_args
 
 def install_binary(file, source_dir, install_dir, verbose):
@@ -460,9 +465,7 @@ def build_llbuild_using_cmake(args, target, swiftc_exec, build_dir, base_cmake_f
   mkdir_p(llbuild_api_dir)
   subprocess.check_call(['touch', os.path.join(llbuild_api_dir, 'codemodel-v2')])
   flags = [
-        '-DCMAKE_C_COMPILER:=clang',
-        '-DCMAKE_CXX_COMPILER:=clang++',
-        '-DCMAKE_CXX_FLAGS=-target %s' % target,
+        '-DCMAKE_CXX_FLAGS=%s -target %s' % (os.getenv('CXXFLAGS', ''), target),
         '-DLLBUILD_SUPPORT_BINDINGS:=Swift'
     ]
   llbuild_cmake_flags = base_cmake_flags + flags
@@ -544,6 +547,9 @@ def cmake_build(args, swiftc_exec, cmake_args, swift_flags, source_path,
     args.cmake_bin, '-G', 'Ninja',
     '-DCMAKE_MAKE_PROGRAM=%s' % args.ninja_bin,
     '-DCMAKE_BUILD_TYPE:=Release',
+    '-DCMAKE_C_COMPILER:=clang',
+    '-DCMAKE_CXX_COMPILER:=clang++',
+    '-DCMAKE_CXX_FLAGS=' + os.getenv('CXXFLAGS', ''),
     '-DCMAKE_Swift_FLAGS=' + ' '.join(swift_flags),
     '-DCMAKE_Swift_COMPILER:=%s' % (swiftc_exec),
   ] + cmake_args + [source_path]
--- a/swiftpm/Sources/Build/BuildDescription/ClangModuleBuildDescription.swift
+++ b/swiftpm/Sources/Build/BuildDescription/ClangModuleBuildDescription.swift
@@ -19,6 +19,7 @@ import struct PackageGraph.ResolvedModule
 import struct SPMBuildCore.BuildParameters
 import struct SPMBuildCore.BuildToolPluginInvocationResult
 import struct SPMBuildCore.CommandPluginResult
+import enum TSCBasic.ProcessEnv
 
 @available(*, deprecated, renamed: "ClangModuleBuildDescription")
 public typealias ClangTargetBuildDescription = ClangModuleBuildDescription
@@ -365,6 +366,9 @@ public final class ClangModuleBuildDescription {
             args += self.buildParameters.toolchain.extraFlags.cxxCompilerFlags
             // User arguments (from -Xcxx) should follow generated arguments to allow user overrides
             args += self.buildParameters.flags.cxxCompilerFlags
+            if let cxxflags = ProcessEnv.vars["CXXFLAGS"] {
+                args += cxxflags.split(separator: " ").map(String.init)
+            }
         }
 
         // Enable the correct lto mode if requested.
--- a/swiftpm/Utilities/bootstrap
+++ b/swiftpm/Utilities/bootstrap
@@ -22,6 +22,7 @@ import os
 import pathlib
 import platform
 import re
+import shlex
 import shutil
 import subprocess
 import sys
@@ -554,6 +555,7 @@ def build_with_cmake(args, cmake_args, ninja_args, source_path, build_dir, cmake
 
         cmd = [
             "env"] + cmake_env + ["MACOSX_DEPLOYMENT_TARGET=%s" % (g_macos_deployment_target),
+            'CXXFLAGS=%s' % os.getenv('CXXFLAGS', ''),
             args.cmake_path, "-G", "Ninja",
             "-DCMAKE_MAKE_PROGRAM=%s" % args.ninja_path,
             "-DCMAKE_BUILD_TYPE:=Debug",
@@ -881,6 +883,10 @@ def get_swiftpm_flags(args):
     # Enforce explicit target dependencies
     # build_flags.extend(["--explicit-target-dependency-import-check", "error"])
 
+    cxxflags = shlex.split(os.getenv('CXXFLAGS', ''))
+    for flag in cxxflags:
+        build_flags.extend(['-Xcc', flag])
+
     return build_flags
 
 if __name__ == '__main__':