swift

2dfd232d - Implement conditional stripping of type descriptors, protocols and protocol conformances via !llvm.used.conditional

Implement conditional stripping of type descriptors, protocols and protocol conformances via !llvm.used.conditional A new LLVM IR affordance that allows expressing conditions under which globals can be removed/dropped (even when marked with @llvm.used) is being discussed at: - <https://reviews.llvm.org/D104496> - <https://lists.llvm.org/pipermail/llvm-dev/2021-September/152656.html> This is a preliminary implementation that marks runtime lookup records (namely protocol records, type descriptors records and protocol conformance records) with the !llvm.used.conditional descriptors. That allows link-time / LTO-time removal of these records (by GlobalDCE) based on whether they're actually used within the linkage unit. Effectively, this allows libraries that have a limited and known set of clients, to be optimized against the client at LTO time, and significantly reduce the code size of that library. Parts of the implementation: - New -conditional-runtime-records frontend flag to enable using !llvm.used.conditional - IRGen code that emits these records can now emit these either as a single contiguous array (asContiguousArray = true, the old way), which is used for JIT mode, or as indivial globals (asContiguousArray = false), which is necessary for the !llvm.used.conditional stripping to work. - When records are emitted as individual globals, they have new names of "\01l_protocol_" + mangled name of the protocol descriptor, and similarly for other records. - Fixed existing tests to account for individual records instead of a single array - Added an IR level test, and an end-to-end execution test to demonstrate that the !llvm.used.conditional-based stripping actually works.