diff --git a/libc/include/__llvm-libc-common.h b/libc/include/__llvm-libc-common.h
index d9d70aff771c0..c63eb134a5e5d 100644
--- a/libc/include/__llvm-libc-common.h
+++ b/libc/include/__llvm-libc-common.h
@@ -50,7 +50,14 @@
 #define __END_C_DECLS
 
 #undef __restrict
-#define __restrict restrict // C99 and above support the restrict keyword.
+#if __STDC_VERSION__ >= 199901L
+// C99 and above support the restrict keyword.
+#define __restrict restrict
+#elif !defined(__GNUC__)
+// GNU-compatible compilers accept the __ spelling in all modes.
+// Otherwise, omit the qualifier for pure C89 compatibility.
+#define __restrict
+#endif
 
 #undef _Noreturn
 #if __STDC_VERSION__ >= 201112L