3e05ab6322
Currently for thin-lto, the imported static global values (functions, variables, etc) will be promoted/renamed from e.g., foo() to foo.llvm.(). Such a renaming caused difficulties in live patching since function name is changed ([1]). It is possible that some global value names have to be promoted to avoid name collision and linker failure. But in practice, majority of name promotions can be avoided. In [2], the suggestion is that thin-lto pre-link decides whether a particular global value needs name promotion or not. If yes, later on in thinBackend() the name will be promoted. I compiled a particular linux kernel version (latest bpf-next tree) and found 1216 global values with suffix .llvm.. With this patch, the number of promoted functions is 2, 98% reduction from the original kernel build. If some native objects are not participating with LTO, name promotions have to be done to avoid potential linker issues. So the current implementation cannot be on by default. But in certain cases, e.g., linux kernel build, people can enable lld flag --lto-whole-program-visibility to reduce the number of functions like foo.llvm.(). For ThinLTOCodeGenerator.cpp which is used by llvm-lto tool and a few other rare cases, reducing the number of renaming due to promotion, is not implemented as lld flag '-lto-whole-program-visibility' is not supported in ThinLTOCodeGenerator.cpp for now. In summary, this pull request only supports llvm-lto2 style workflow. The feature is off by default. To enable the future, lld flag '-lto-whole-program-visibility' and llvm flag '-always-rename-promoted-locals=false' are needed. The link [3] has more context for the pull request discussions. [1] https://lpc.events/event/19/contributions/2212 [2] https://discourse.llvm.org/t/rfc-avoid-functions-like-foo-llvm-for-kernel-live-patch/89400 [3] https://github.com/llvm/llvm-project/pull/178587
LLVM Linker (lld)
This directory and its subdirectories contain source code for the LLVM Linker, a modular cross platform linker which is built as part of the LLVM compiler infrastructure project.
lld is open source software. You may freely distribute it under the terms of the license agreement found in LICENSE.txt.
Benchmarking
In order to make sure various developers can evaluate patches over the same tests, we create a collection of self contained programs.
It is hosted at https://s3-us-west-2.amazonaws.com/linker-tests/lld-speed-test.tar.xz
The current sha256 is 10eec685463d5a8bbf08d77f4ca96282161d396c65bd97dc99dbde644a31610f.