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Document compilation options #231
Description
Activity
Hi,
I totally agree with the observation about the lack of doc on those requirements, especially the one related to optimization level.
FWIW, I've also done this kind of search on mailing lists and docs for some clear statements about the matter, and failed to find anything. The only statement I have found is the the following one: https://docs.kernel.org/bpf/clang-notes.html#jump-instructions.I think there is a place for this in the docs. I have been wanting to add more verbose sections such as guides or tutorials, and to tie that in with #8. I see this being a section about project and toolchain setup.
For -O2 it's more tricky, everybody agrees it's necessary, but it's hard to find out why, precisely
From what I recall there are BPF specific compiler passes which are only enabled under O2 that perform work to make the output pass the verifier. I also cannot recall the exact details. When adding/removing -O2 in compiler explorer it shows the following passed being added under O2:
- https://github.com/llvm/llvm-project/blob/53b26971f5192713bd3cc01ed31e48b4ea0b4d2c/llvm/lib/Target/BPF/BPFASpaceCastSimplifyPass.cpp
- https://github.com/llvm/llvm-project/blob/53b26971f5192713bd3cc01ed31e48b4ea0b4d2c/llvm/lib/Target/BPF/BPFMISimplifyPatchable.cpp
- https://github.com/llvm/llvm-project/blob/53b26971f5192713bd3cc01ed31e48b4ea0b4d2c/llvm/lib/Target/BPF/BPFMIPeephole.cpp
Not sure if these do a critical role on their own or if its the combination of other passes and these that make programs better verifiable.
Reacted by Quentin Monnet
Context
I've been looking at why
-g -O2is necessary when compiling from C to eBPF with clang. It's not documented as far as I know, and I haven't been able to find the specific reasons again. I'm sure I saw them on mailing lists in the past, but I can't find it now.Disclaimer
I realise this may be outside of the scope of this documentation, I note that you don't really mention how to compile with clang, or much of the ecosystem in general. I thought I'd open the issue as a suggestion, anyway - feel free to close if it's not relevant.
Description
So it would be nice to have some documentation about why one needs to pass
-gand-O2to clang when compiling.For
-git's easy, it's necessary to get 1) debug information about the program, in particular the C source code that we dump with bpftool or llvm-objdump, for inspection, and 2) more importantly for many use case, BTF info.For
-O2it's more tricky, everybody agrees it's necessary, but it's hard to find out why, precisely. From what I remember - and I may be totally wrong here - it may be related to: