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

[Rong Xu]

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?

-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
>