LLVM Embedded Toolchains Working Group sync up

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2022-09-15: Dev Meeting round table, LTO, libc initialization

Participants

  1. Teresa Johnson

  2. Todd Snider

  3. Prabhu Rajasekaran

  4. rouxy

  5. Simon Wallis

  6. Michael Jones

  7. Siva Chandra

  8. Daniel Thornburgh

  9. Stephen Hines

  10. Guillot Tony

  11. Petr Hosek

  12. Tue Ly

  13. Pirama

  14. Peter Smith

  15. Simon Butcher

  16. Zoltan Lipp

  17. Volodymyr Turanskyy

Agenda

Discussion

Multilib support

  • No updates.

Round table in LLVM Dev Meeting

  • The LLVM Compiler Infrastructure Project Nov 8-9, 2022

  • Who is going?

    • Siva, Prabhu, Petr, others - would be nice to have a discussion in person.

    • Arm: Volodymyr will check, if anyone is going.

  • Action item for all to highlight to colleagues and suggest to join.

  • Volodymyr to cancel the call in the same week - done, removed from the Google calendar.

LTO for embedded

  • Background - link to slides in the agenda.

  • Recent discussion Linker generated attributes for LTO - does lld already do this?

  • Issues to address:

    • Cannot inline across output sections => add information about output sections to inform the linker.

    • How to place sections at particular addresses => extend IR with information about named sections to pass to LTO and take into account.

    • So changes are likely required in both IR and LLD.

  • There are known downstream implementations, but nothing in upstream yet.

  • Teresa (co-author of ThinLTO):

    • Many changes are not LTO specific - are required in other generic passes.

    • The patches from the 2017 presentation were not published, so need to start from scratch? Or check if anyone from Qualcomm can share the patches.

    • The changes are expected to be accepted upstream without major concerns.

  • Todd (author of the Discrose thread above): Work done at TI, IR is extended to implement mentioned features.

  • Teresa: would be easier to go without IR modification in the linker to keep the existing interface.

  • Todd: would it make sense to have a special “embedded” type of LTO?

  • Teresa/Petr: a recent relevant discussion about fat-lto-objects [RFC] -ffat-lto-objects support + WIP patch ⚙ D131618 [WIP][Do NOT review] LLD related changes for -ffat-lto-objects support

  • Arm linker wraps IR into ELF files to make library selection easier, but this seems to be a too specific solution for the Arm Compiler toolchain.

LLVM libc initialization (Peter)

  • Discourse post in the agenda.

  • picolibc approach presented there can be reused for simple configurations and may be a good starting point for LLVM libc enablement.

  • Eventually it would be good to have a sort of template for different types of embedded targets.

  • Siva: Looks good. Now we need to provide the examples.

  • Question: Testing strategy for such code? We could use QEMU.