  ROOT has several features which interact with libraries and require implicit
header inclusion. This can be triggered by reading or writing data on disk, or
user actions at the prompt. Often, the headers are immutable, and reparsing is
redundant. C++ Modules are designed to minimize the reparsing of the same
header content by providing an efficient on-disk representation of C++ Code.
ROOT has released a C++ Modules-aware technology preview which intends to
become the default for the next release.
  In this paper, we will summarize our experience with migrating C++ Modules to
LHC experiment's software code bases. We outline the challenges in C++ Modules
migration of the CMS software, including the integration of C++ Modules support
in CMS build system. We also evaluate the performance benefits that experiments
are expected to achieve.
