linux-kernel - Re: [PATCH 0/3] MIPS: Add support for LTO

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Message-ID: <CAKwvOdn=7iYyOg8+wikOK-6Xogf9Z6wdmkJEacYV_S1MpZ-YDw@mail.gmail.com>
Date:   Tue, 14 Dec 2021 11:34:02 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Nathan Chancellor <nathan@...nel.org>, list@...ndingux.net,
        linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH 0/3] MIPS: Add support for LTO

On Mon, Dec 13, 2021 at 2:49 PM Paul Cercueil <paul@...pouillou.net> wrote:
>
> Hi,
>
> With this set of patches, it becomes possible to build a LTO'd kernel
> with LLVM/Clang on MIPS.

Paul,
Overall, nice work pursuing this and working through all the related
issues; thanks for the series!

We should clarify whether this is LTO_CLANG_THIN or LTO_CLANG_FULL.
Getting support for either would be nice, but for each architecture
has had its own distinct challenges.

>
> This was tested on a Ingenic JZ4770 based system. It requires the
> linking step of the vmlinuz.bin to be done with binutils' LD instead of
> LLVM's ld.lld [1], but the vmlinuz ELF itself can be completely built
> with LLVM.

eh, this patch set sounds incomplete then; Can we wait to resolve
https://github.com/ClangBuiltLinux/linux/issues/1333 BEFORE this
series?  Hacking up arch/mips/boot/compressed/Makefile to force a
different linker is not something we should encourage.

>
> Strangely, the LTO'd kernel is bigger in size (3.6 MiB vs. 3.1 MiB
> without LTO), which might be completely normal and I just have wrong
> expectations.

Are you building with CC_OPTIMIZE_FOR_SIZE or CC_OPTIMIZE_FOR_SIZE?
LTO != LD_DEAD_CODE_DATA_ELIMINATION

>
> Cheers,
> -Paul
>
> [1]: https://github.com/ClangBuiltLinux/linux/issues/1333
>
> Paul Cercueil (3):
>   MIPS: boot/compressed: Disable abicalls
>   MIPS: boot/compressed: Build without LTO
>   MIPS: Add support for LTO
>
>  arch/mips/Kconfig                  | 1 +
>  arch/mips/boot/compressed/Makefile | 7 +++++--
>  2 files changed, 6 insertions(+), 2 deletions(-)
>
> --
> 2.33.0
>


-- 
Thanks,
~Nick Desaulniers