diff --git a/libcxx/test/libcxx/selftest/additional_compile_flags_if_supported/substitutes-in-compile-flags.sh.cpp b/libcxx/test/libcxx/selftest/additional_compile_flags_if_supported/substitutes-in-compile-flags.sh.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/libcxx/selftest/additional_compile_flags_if_supported/substitutes-in-compile-flags.sh.cpp
@@ -0,0 +1,18 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// This test greps for %t, which is expanded to a path with backslashes. When
+// that is passed to grep, those backslashes must be escaped. We escape those
+// within the pattern into a file and use this file with 'grep'.
+
+// Make sure that substitutions are performed inside additional compiler flags.
+
+// ADDITIONAL_COMPILE_FLAGS_IF_SUPPORTED: -I %t.1
+// ADDITIONAL_COMPILE_FLAGS_IF_SUPPORTED: -isystem %t.2 , -isysroot %t.3
+// RUN: echo "-I %t.1 -isystem %t.2 -isysroot %t.3" | sed "s/\\\/\\\\\\\/g" > %t.escaped.grep
+// RUN: echo "%{compile_flags}" | grep -e -f %t.escaped.grep
diff --git a/libcxx/test/libcxx/selftest/additional_compile_flags_if_supported/substitutes-in-run.sh.cpp b/libcxx/test/libcxx/selftest/additional_compile_flags_if_supported/substitutes-in-run.sh.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/libcxx/selftest/additional_compile_flags_if_supported/substitutes-in-run.sh.cpp
@@ -0,0 +1,15 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// Make sure that additional compiler flags are added to the %{compile_flags}
+// substitution if supported.
+
+// ADDITIONAL_COMPILE_FLAGS_IF_SUPPORTED: -Wall
+// ADDITIONAL_COMPILE_FLAGS_IF_SUPPORTED: -bar
+// ADDITIONAL_COMPILE_FLAGS_IF_SUPPORTED: -baz, -Wextra
+// RUN: echo "%{compile_flags}" | grep -e '-Wall -Wextra'
diff --git a/libcxx/utils/libcxx/test/format.py b/libcxx/utils/libcxx/test/format.py
--- a/libcxx/utils/libcxx/test/format.py
+++ b/libcxx/utils/libcxx/test/format.py
@@ -13,6 +13,7 @@
 import re
 import shutil
 import subprocess
+from libcxx.test.dsl import hasCompileFlag
 
 def _supportsVerify(config):
     """
@@ -73,6 +74,7 @@
 
     # Parse the test file, including custom directives
     additionalCompileFlags = []
+    additionalOptionalCompileFlags = []
     fileDependencies = []
     parsers = [
         lit.TestRunner.IntegratedTestKeywordParser('FILE_DEPENDENCIES:',
@@ -80,7 +82,10 @@
                                                    initial_value=fileDependencies),
         lit.TestRunner.IntegratedTestKeywordParser('ADDITIONAL_COMPILE_FLAGS:',
                                                    lit.TestRunner.ParserKind.LIST,
-                                                   initial_value=additionalCompileFlags)
+                                                   initial_value=additionalCompileFlags),
+        lit.TestRunner.IntegratedTestKeywordParser('ADDITIONAL_COMPILE_FLAGS_IF_SUPPORTED:',
+                                                   lit.TestRunner.ParserKind.LIST,
+                                                   initial_value=additionalOptionalCompileFlags),
     ]
 
     scriptInTest = lit.TestRunner.parseIntegratedTestScript(test, additional_parsers=parsers,
@@ -102,6 +107,17 @@
     substitutions = [(s, x + ' ' + ' '.join(additionalCompileFlags)) if s == '%{compile_flags}'
                             else (s, x) for (s, x) in substitutions]
 
+    # Add compile flags specified with ADDITIONAL_COMPILE_FLAGS_IF_SUPPORTED.
+    supported = [hasCompileFlag(test.config, f)
+                 for f in additionalOptionalCompileFlags]
+    # We need to remove elements in reverse order to maintain indices unchanged.
+    supported.reverse()
+    for idx, sup in enumerate(supported):
+        if not sup:
+            del additionalOptionalCompileFlags[idx]
+    substitutions = [(s, x + ' ' + ' '.join(additionalOptionalCompileFlags)) if s == '%{compile_flags}'
+                     else (s, x) for (s, x) in substitutions]
+
     # Perform substitutions in the script itself.
     script = lit.TestRunner.applySubstitutions(script, substitutions,
                                                recursion_limit=test.config.recursiveExpansionLimit)
@@ -178,6 +194,12 @@
             allows adding special compilation flags without having to use a
             .sh.cpp test, which would be more powerful but perhaps overkill.
 
+        // ADDITIONAL_COMPILE_FLAGS_IF_SUPPORTED: flag1, flag2, flag3
+
+            This directive behaves like ADDITIONAL_COMPILE_FLAGS, but only if
+            the compiler supports the flag. Unsupported flags will be silently
+            ignored.
+
 
     Additional provided substitutions and features
     ==============================================