mmjjpp wrote: > > This change has no description and no tests - it needs both, and possibly > > an RFC for a change as big as this. At first glance, a couple of high level > > comments: LTO should not be directly calling lld. Also, regular LTO already > > has support for split codegen, can that be leveraged? In particular, look > > at how that adds the split modules to the link, which is via the AddStream > > callback. > > Looking at the existing Regular LTO split-codegen path, I think the ThinLTO > handling has two distinct cases. > > ### 1. Linker-driven In-process ThinLTO > Since `lld` is already the LTO client, I agree this should follow the Regular > LTO model: > > * The ThinLTO backend should emit each split partition through `AddStream`. > * `lld` should receive those object buffers from `LTO::run()`, create > `InputFiles` from them, and add them to the final link. > * **Key constraint:** There should be no recursive linker invocation from > `LTOBackend` in this path. > * **Note:** The main extra piece is task numbering / `getMaxTasks()`, since > one ThinLTO backend task can produce multiple partition objects. > > ### 2. Distributed ThinLTO Backend Compilation (`clang -fthinlto-index -c`) > Here, there is no in-process `lld` client receiving `AddStream` buffers. > `cc1` is just producing backend objects, while the clang driver still has the > normal `-c -o foo.o` contract to produce a single relocatable object. > > **Proposed direction:** > > 1. `cc1` uses `AddStream` to emit the split partition objects and writes an > output list. > 2. The `clang` driver performs a follow-up merge: `ld.lld -r @list -o foo.o`. > > _Benefit:_ This keeps the linker invocation out of the LTO library/backend, > while preserving the existing distributed backend single-output contract. > > ### Summary of Responsibilities > Does that split of responsibilities sound reasonable to you? In other words: > > * **In-process ThinLTO:** `LTO` -> `AddStream` -> `lld` consumes partition > objects directly. > * **Distributed `-fthinlto-index -c`:** `LTO`/`cc1` -> `AddStream` writes > partition objects -> `clang` driver merges them with `ld.lld -r`.
pr： https://github.com/mmjjpp/llvm-project/pull/16 [ThinLTO][Driver] Move split-codegen partition merge out of LTOBackend https://github.com/llvm/llvm-project/pull/198702 _______________________________________________ cfe-commits mailing list [email protected] https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
