This is the first patch of a patchset to add initial support for ARM64EC. Basic documentation is available at https://docs.microsoft.com/en-us/windows/uwp/porting/arm64ec-abi . (Discourse post: https://discourse.llvm.org/t/initial-patches-for-arm64ec-windows-11-now-posted/62449 .)
The file format for ARM64EC is basically identical to normal ARM64. There are a few extra sections, but the existing code for reading ARM64 object files just works.