Re: [PATCH v9 3/3] kbuild: distributed build support for Clang ThinLTO

[Yonghong Song]

On 5/22/26 8:32 AM, Rong Xu wrote:
> On Thu, May 21, 2026 at 2:57 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
> > On 5/21/26 11:31 AM, Rong Xu wrote:
> > > Yonghong, thanks for the update.
> > >
> > > Regarding this guard: ther is a period of Clang (before this patch and
> > > after your first patch), even though ld.lld having these options
> > > (specifically --lto-whole-program-visibility -mllvm
> > > -always-rename-promoted-locals=false), distributed ThinLTO mode
> > > remains unsupported, correct? What the behvior of using this options
> > > in distributed mode with these compilers? nop or it will lead to
> > > error?
> > The in-process thin-lto support is landed on Feb 27.
> > The distributed thin-lto support is landed on Apr 24.
> >
> > If people are using distributed thin-lto in kernel between Feb 27 and
> > Apr 24, there will be some issues. But people typically use released
> > compiler, so we should be fine.
> This is not the case for us (google). We do use compiler b/w releases,
> and we build our own.
>
> What is the issue if we use the compiler in b/w Feb27 and Apr24?

If you use the custom compiler between Feb27 and Apr24 and your kernel
will do distributed thin-lto, you can remove
   $(call ld-option,--lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false)
from your kernel. Since you have custom compiler, you can
do some customization for your kernel as well.


> -Rong
>
> > > I would assume there will be errors; otherwise, you would not ask me
> > > to change my patch last time. In this case, I would keep this guard
> > > and remove it when the minimum llvm version passes llvm23. What do you
> > > think?
> > There is no need to keep compiler version guard.
> >
> > Before llvm23, the below will be a noop:
> >      $(call ld-option,--lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false)
> > since '-mllvm -always-rename-promoted-locals=false' is a new flag and the compiler won't
> > recognize it so the kernel will resolve above 'call ...' option as noop.
> >
> > With llvm23 and later, the kernel will be able to recognize above options and
> > things should be okay.
> >
> > > Best,
> > >
> > > -Rong
> > >
> > >
> > > On Thu, May 21, 2026 at 1:57 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
> > > > On 3/31/26 9:27 AM, Nathan Chancellor wrote:
> > > > > Hi Rong,
> > > > >
> > > > > On Tue, Mar 31, 2026 at 03:48:27PM +0000, xur@xxxxxxxxxx wrote:
> > > > > > diff --git a/Makefile b/Makefile
> > > > > > index 69ccf9b8507d..26005c64016d 100644
> > > > > > --- a/Makefile
> > > > > > +++ b/Makefile
> > > > > > @@ -1047,11 +1047,13 @@ export CC_FLAGS_SCS
> > > > > >     endif
> > > > > >
> > > > > >     ifdef CONFIG_LTO_CLANG
> > > > > > -ifdef CONFIG_LTO_CLANG_THIN
> > > > > > +ifdef CONFIG_LTO_CLANG_FULL
> > > > > > +CC_FLAGS_LTO        := -flto
> > > > > > +else
> > > > > >     CC_FLAGS_LTO       := -flto=thin -fsplit-lto-unit
> > > > > > +if CONFIG_LTO_CLANG_THIN
> > > > > This should be an 'ifdef', not an 'if'. You copied Yonghong's mistake:
> > > > >
> > > > >      https://lore.kernel.org/abgRRX3PH9IaExi8@xxxxxxxxxxxxx/
> > > > >      https://lore.kernel.org/6db3a2f6-d61c-42f1-9b9d-0aca021cc2d7@xxxxxxxxx/
> > > > >
> > > > > Please slow down and test build your changes before sending them. Each
> > > > > revision adds four new emails to everyone's inbox, which is just noise
> > > > > when there are obvious, basic problems. 'b4 diff' shows no actual
> > > > > difference from v8 and v9, which should have been caught by a simple
> > > > > build test right before 'git send-email'.
> > > > >
> > > > > >     KBUILD_LDFLAGS += $(call ld-option,--lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false)
> > > > > > -else
> > > > > > -CC_FLAGS_LTO        := -flto
> > > > > > +endif
> > > > The above patch has a guard CONFIG_LTO_CLANG_THIN, which can be removed.
> > > > See llvm patch
> > > >       https://github.com/llvm/llvm-project/pull/188074
> > > > which supports distributed thin-lto mode too for reducing the number
> > > > of renaming. In other words, for llvm23, both in-process and
> > > > distributed-process are supported for thin-lto.
> > > >
> > > > > >     endif
> > > > > >     CC_FLAGS_LTO       += -fvisibility=hidden
> > > > > >
> > > > > > @@ -1657,6 +1659,7 @@ endif # CONFIG_MODULES
> > > > > >     CLEAN_FILES += vmlinux.symvers modules-only.symvers \
> > > > > >               modules.builtin modules.builtin.modinfo modules.nsdeps \
> > > > > >               modules.builtin.ranges vmlinux.o.map vmlinux.unstripped \
> > > > > > +           vmlinux.thinlto-index builtin.order \
> > > > > >               compile_commands.json rust/test \
> > > > > >               rust-project.json .vmlinux.objs .vmlinux.export.c \
> > > > > >                    .builtin-dtbs-list .builtin-dtb.S
> > > > > > @@ -2118,7 +2121,7 @@ clean: $(clean-dirs)
> > > > > >        $(call cmd,rmfiles)
> > > > > >        @find . $(RCS_FIND_IGNORE) \
> > > > > >                \( -name '*.[aios]' -o -name '*.rsi' -o -name '*.ko' -o -name '.*.cmd' \
> > > > > > -            -o -name '*.ko.*' \
> > > > > > +            -o -name '*.ko.*' -o -name '*.o.thinlto.bc' \
> > > > > >                -o -name '*.dtb' -o -name '*.dtbo' \
> > > > > >                -o -name '*.dtb.S' -o -name '*.dtbo.S' \
> > > > > >                -o -name '*.dt.yaml' -o -name 'dtbs-list' \
> > > > > With that addressed above, the series survives my basic LLVM 22.1.2
> > > > > build test with my distribution configuration. I'll provide formal tags
> > > > > on a properly tested and fixed revision.
> > > > >
> > > > > Cheers,
> > > > > Nathan