Legion Release 25.09.0 (September 30, 2025) – Grand Refactor, Realm Split

[Elliott Slaughter]

Hi everyone,

This release covers two major changes in Legion.

First, the "grandrefactor" branch of Legion has been merged, which includes major internal code restructuring and a rewrite of every user-visible error message.

Second, in this release Realm has been split from the main Legion codebase, and now lives in https://github.com/StanfordLegion/realm. A read-only copy of Realm is maintained inside the Legion repo under the /realm subdirectory to simplify the build process for existing Legion users. Note that while hypothetically Legion may work with a range of Realm versions, only the version checked into the repo is guaranteed to work. Realm now uses a standalone build system which may shake loose a variety of bugs. Please check for and report issues with your applications and report any issues that arise.

The full list of changes is below.

• Build
  • The Realm build system has been split off, see below for details
• Legion
  • Large-scale refactoring of the Legion code base
  • All Legion error messages have been rewritten for clarity
  • Legion now checks for typos in command-line flags and issues errors if any flags are misspelled
  • Legion source code is now formatted via clang-format
• Realm
  • Realm has been split into its own repository: https://github.com/StanfordLegion/realm
  • A read-only copy of Realm is still maintained under the Legion repository in the /realm directory
  • The Realm build system has been split and is now invoked via CPM in the Legion CMake build, but must be built separately in the Makefile build. The location of a pre-existing Realm installation can be specified in both builds via Realm_ROOT

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Elliott Slaughter

"Don't worry about what anybody else is going to do. The best way to predict the future is to invent it." - Alan Kay