B.2.124.3 How the Compiler Handles an Existing Precompiled-Header File

If the compiler cannot use the precompiled-header file, under -xpch=auto and -xpch=autofirst, it generates a new precompiled-header file. If the compiler cannot use the precompiled-header file under -xpch=use, a warning is issued and the compilation is done using the real headers.