[PATCH kbuild] kbuild: Allow to reduce the number of suffixes for clang thin-lto build

[PATCH kbuild] kbuild: Allow to reduce the number of suffixes for clang thin-lto build

[Yonghong Song]

The current clang thin-lto build often produces lots of symbols with
suffix. The following is a partial list of such function call symbols:
    ...
    ethnl_module_fw_flash_ntf.llvm.7631589765585346066
    __nf_conntrack_alloc.llvm.6438426151906658917
    tcp_can_early_drop.llvm.11937612064648250727
    tcp_print_conntrack.llvm.11937612064648250727
    ...

In my particular build with current bpf-next, the number of '*.llvm.<hash>'
function calls is 1212. Such symbols make kernel live patching
difficult since
  - a minor code change will change the hash and then the '*.llvm.<hash>'
    symbol becomes another one with a different hash or no hash, and
  - a previous source-level symbol may become an one with suffix after live
    patching code.

In [1], Song Liu suggested to reduce the number of '*.llvm.<hash>' functions
to make live patch easier. In respond of this, I implemented this
in llvm ([2]). The same thin-lto build with [2] only has two symbols with
suffix:
    m_stop.llvm.14460341347352036579
    m_next.llvm.14460341347352036579
This should make live patch much easier.

To support suffix symbol reduction, a new config
    LTO_CLANG_THIN_SUFFIX_REDUCTION
is introduced and the config depends on thin-lto and llvm23 or higher.

Two lld flags are necessary to enable this feature in kernel:
    - Flag '--lto-whole-program-visibility' is needed as it ensures that all
      modules are available in the same process, which is true for kernel at
      thin-lto lld.
    - Flag '-mllvm -always-rename-promoted-locals=false' is needed to enable
      suffix reduction. Currently in llvm [1], only process mode is supported.
      There is another distributed mode (across different processes or even
      different machines) which is not supported yet ([2]).

  [1] https://lpc.events/event/19/contributions/2212
  [2] https://github.com/llvm/llvm-project/pull/178587

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 Makefile     |  3 +++
 arch/Kconfig | 15 +++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/Makefile b/Makefile
index e944c6e71e81..9d6033595615 100644
--- a/Makefile
+++ b/Makefile
@@ -1034,6 +1034,9 @@ endif
 ifdef CONFIG_LTO_CLANG
 ifdef CONFIG_LTO_CLANG_THIN
 CC_FLAGS_LTO	:= -flto=thin -fsplit-lto-unit
+ifdef CONFIG_LTO_CLANG_THIN_SUFFIX_REDUCTION
+KBUILD_LDFLAGS += --lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false
+endif
 else
 CC_FLAGS_LTO	:= -flto
 endif
diff --git a/arch/Kconfig b/arch/Kconfig
index 102ddbd4298e..e1db64a3284e 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -861,8 +861,23 @@ config LTO_CLANG_THIN
 	    https://clang.llvm.org/docs/ThinLTO.html
 
 	  If unsure, say Y.
+
 endchoice
 
+config LTO_CLANG_THIN_SUFFIX_REDUCTION
+	bool "Clang ThinLTO Suffix Reduction (EXPERIMENTAL)"
+	depends on LTO_CLANG_THIN
+	depends on CLANG_VERSION >= 230000
+	default y
+	help
+	  This option allows to reduce the number of symbols with
+	  '.llvm.<hash' suffixes. This can help KLP (kernel living
+	  patch) as symbol name can stay stable in most cases.
+
+	  See https://github.com/llvm/llvm-project/pull/178587
+
+	  If unsure, say N.
+
 config ARCH_SUPPORTS_AUTOFDO_CLANG
 	bool
 
-- 
2.47.3

Re: [PATCH kbuild] kbuild: Allow to reduce the number of suffixes for clang thin-lto build

[Josh Poimboeuf]

On Thu, Mar 05, 2026 at 07:43:25PM -0800, Yonghong Song wrote:
> The current clang thin-lto build often produces lots of symbols with
> suffix. The following is a partial list of such function call symbols:
>     ...
>     ethnl_module_fw_flash_ntf.llvm.7631589765585346066
>     __nf_conntrack_alloc.llvm.6438426151906658917
>     tcp_can_early_drop.llvm.11937612064648250727
>     tcp_print_conntrack.llvm.11937612064648250727
>     ...
> 
> In my particular build with current bpf-next, the number of '*.llvm.<hash>'
> function calls is 1212. Such symbols make kernel live patching
> difficult since
>   - a minor code change will change the hash and then the '*.llvm.<hash>'
>     symbol becomes another one with a different hash or no hash, and
>   - a previous source-level symbol may become an one with suffix after live
>     patching code.
> 
> In [1], Song Liu suggested to reduce the number of '*.llvm.<hash>' functions
> to make live patch easier. In respond of this, I implemented this
> in llvm ([2]). The same thin-lto build with [2] only has two symbols with
> suffix:
>     m_stop.llvm.14460341347352036579
>     m_next.llvm.14460341347352036579
> This should make live patch much easier.
> 
> To support suffix symbol reduction, a new config
>     LTO_CLANG_THIN_SUFFIX_REDUCTION
> is introduced and the config depends on thin-lto and llvm23 or higher.
> 
> Two lld flags are necessary to enable this feature in kernel:
>     - Flag '--lto-whole-program-visibility' is needed as it ensures that all
>       modules are available in the same process, which is true for kernel at
>       thin-lto lld.
>     - Flag '-mllvm -always-rename-promoted-locals=false' is needed to enable
>       suffix reduction. Currently in llvm [1], only process mode is supported.
>       There is another distributed mode (across different processes or even
>       different machines) which is not supported yet ([2]).
> 
>   [1] https://lpc.events/event/19/contributions/2212
>   [2] https://github.com/llvm/llvm-project/pull/178587
> 
> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
> ---
>  Makefile     |  3 +++
>  arch/Kconfig | 15 +++++++++++++++
>  2 files changed, 18 insertions(+)
> 
> diff --git a/Makefile b/Makefile
> index e944c6e71e81..9d6033595615 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1034,6 +1034,9 @@ endif
>  ifdef CONFIG_LTO_CLANG
>  ifdef CONFIG_LTO_CLANG_THIN
>  CC_FLAGS_LTO	:= -flto=thin -fsplit-lto-unit
> +ifdef CONFIG_LTO_CLANG_THIN_SUFFIX_REDUCTION
> +KBUILD_LDFLAGS += --lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false
> +endif
>  else
>  CC_FLAGS_LTO	:= -flto
>  endif
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 102ddbd4298e..e1db64a3284e 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -861,8 +861,23 @@ config LTO_CLANG_THIN
>  	    https://clang.llvm.org/docs/ThinLTO.html
>  
>  	  If unsure, say Y.
> +
>  endchoice
>  
> +config LTO_CLANG_THIN_SUFFIX_REDUCTION
> +	bool "Clang ThinLTO Suffix Reduction (EXPERIMENTAL)"
> +	depends on LTO_CLANG_THIN
> +	depends on CLANG_VERSION >= 230000
> +	default y
> +	help
> +	  This option allows to reduce the number of symbols with
> +	  '.llvm.<hash' suffixes. This can help KLP (kernel living
> +	  patch) as symbol name can stay stable in most cases.
> +
> +	  See https://github.com/llvm/llvm-project/pull/178587
> +
> +	  If unsure, say N.
> +

Thanks!  Would there be any downsides to enabling this feature
unconditionally in the kernel when the compiler supports it?

-- 
Josh

Re: [PATCH kbuild] kbuild: Allow to reduce the number of suffixes for clang thin-lto build

[Yonghong Song]

On 3/6/26 10:32 AM, Josh Poimboeuf wrote:
> On Thu, Mar 05, 2026 at 07:43:25PM -0800, Yonghong Song wrote:
>> The current clang thin-lto build often produces lots of symbols with
>> suffix. The following is a partial list of such function call symbols:
>>      ...
>>      ethnl_module_fw_flash_ntf.llvm.7631589765585346066
>>      __nf_conntrack_alloc.llvm.6438426151906658917
>>      tcp_can_early_drop.llvm.11937612064648250727
>>      tcp_print_conntrack.llvm.11937612064648250727
>>      ...
>>
>> In my particular build with current bpf-next, the number of '*.llvm.<hash>'
>> function calls is 1212. Such symbols make kernel live patching
>> difficult since
>>    - a minor code change will change the hash and then the '*.llvm.<hash>'
>>      symbol becomes another one with a different hash or no hash, and
>>    - a previous source-level symbol may become an one with suffix after live
>>      patching code.
>>
>> In [1], Song Liu suggested to reduce the number of '*.llvm.<hash>' functions
>> to make live patch easier. In respond of this, I implemented this
>> in llvm ([2]). The same thin-lto build with [2] only has two symbols with
>> suffix:
>>      m_stop.llvm.14460341347352036579
>>      m_next.llvm.14460341347352036579
>> This should make live patch much easier.
>>
>> To support suffix symbol reduction, a new config
>>      LTO_CLANG_THIN_SUFFIX_REDUCTION
>> is introduced and the config depends on thin-lto and llvm23 or higher.
>>
>> Two lld flags are necessary to enable this feature in kernel:
>>      - Flag '--lto-whole-program-visibility' is needed as it ensures that all
>>        modules are available in the same process, which is true for kernel at
>>        thin-lto lld.
>>      - Flag '-mllvm -always-rename-promoted-locals=false' is needed to enable
>>        suffix reduction. Currently in llvm [1], only process mode is supported.
>>        There is another distributed mode (across different processes or even
>>        different machines) which is not supported yet ([2]).
>>
>>    [1] https://lpc.events/event/19/contributions/2212
>>    [2] https://github.com/llvm/llvm-project/pull/178587
>>
>> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
>> ---
>>   Makefile     |  3 +++
>>   arch/Kconfig | 15 +++++++++++++++
>>   2 files changed, 18 insertions(+)
>>
>> diff --git a/Makefile b/Makefile
>> index e944c6e71e81..9d6033595615 100644
>> --- a/Makefile
>> +++ b/Makefile
>> @@ -1034,6 +1034,9 @@ endif
>>   ifdef CONFIG_LTO_CLANG
>>   ifdef CONFIG_LTO_CLANG_THIN
>>   CC_FLAGS_LTO	:= -flto=thin -fsplit-lto-unit
>> +ifdef CONFIG_LTO_CLANG_THIN_SUFFIX_REDUCTION
>> +KBUILD_LDFLAGS += --lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false
>> +endif
>>   else
>>   CC_FLAGS_LTO	:= -flto
>>   endif
>> diff --git a/arch/Kconfig b/arch/Kconfig
>> index 102ddbd4298e..e1db64a3284e 100644
>> --- a/arch/Kconfig
>> +++ b/arch/Kconfig
>> @@ -861,8 +861,23 @@ config LTO_CLANG_THIN
>>   	    https://clang.llvm.org/docs/ThinLTO.html
>>   
>>   	  If unsure, say Y.
>> +
>>   endchoice
>>   
>> +config LTO_CLANG_THIN_SUFFIX_REDUCTION
>> +	bool "Clang ThinLTO Suffix Reduction (EXPERIMENTAL)"
>> +	depends on LTO_CLANG_THIN
>> +	depends on CLANG_VERSION >= 230000
>> +	default y
>> +	help
>> +	  This option allows to reduce the number of symbols with
>> +	  '.llvm.<hash' suffixes. This can help KLP (kernel living
>> +	  patch) as symbol name can stay stable in most cases.
>> +
>> +	  See https://github.com/llvm/llvm-project/pull/178587
>> +
>> +	  If unsure, say N.
>> +
> Thanks!  Would there be any downsides to enabling this feature
> unconditionally in the kernel when the compiler supports it?

The only downside is for the following case:

C   file: static function foo()
Asm file: global function foo()

The thin-lto will collect all C files and with the above llvm patch,
the static function foo() may be promoted to global function foo()
if there is no other pre-existing global function foo() in all C files.

In such cases, there will be a conflict since
there are two global function foo() (one from C file, another from Asm file).
In such cases, the build will fail.

How do you think we could hit such issues in linux kernel?
Maybe should have default no for the new config?

I think the chance should be very low. The following is a grab for x86
for global symbols in asm code:

[~/work/others/linux/arch/x86 (master)]$ egrep -r globl
egrep: warning: egrep is obsolescent; using grep -E
boot/compressed/mkpiggy.c:      printf(".globl z_input_len\n");
boot/compressed/mkpiggy.c:      printf(".globl z_output_len\n");
boot/compressed/mkpiggy.c:      printf(".globl input_data, input_data_end\n");
boot/compressed/mkpiggy.c:      printf(".globl input_len\n");
boot/compressed/mkpiggy.c:      printf(".globl output_len\n");
boot/compressed/head_64.S:      .globl  verify_cpu
boot/bioscall.S:        .globl  intcall
boot/header.S:  .globl  pecompat_fstart
boot/header.S:  .globl  sentinel
boot/header.S:  .globl  hdr
boot/header.S:  .globl  _start
boot/header.S:          .globl realmode_swtch
boot/header.S:  .globl  die
entry/vdso/vdso32/sigreturn.S:  .globl __kernel_sigreturn
entry/vdso/vdso32/sigreturn.S:  .globl __kernel_rt_sigreturn
entry/vdso/vdso32/system_call.S:        .globl __kernel_vsyscall
entry/vsyscall/vsyscall_emu_64.S:       .globl __vsyscall_page
entry/entry_32.S:       .globl __irqentry_text_start
entry/entry_32.S:       .globl __irqentry_text_end
entry/entry_64.S:       .globl __irqentry_text_start
entry/entry_64.S:       .globl __irqentry_text_end
include/asm/paravirt_types.h:       ".globl " PV_THUNK_NAME(func) ";"                           \
include/asm/static_call.h:          ".globl " STATIC_CALL_TRAMP_STR(name) "             \n"     \
kernel/cpu/amd.c:       ".globl vide\n"
kernel/ftrace_32.S:.globl ftrace_call
kernel/ftrace_32.S:.globl ftrace_graph_call
kernel/ftrace_32.S:.globl return_to_handler
kernel/relocate_kernel_32.S:    .globl kexec_control_code_size
kernel/head_32.S:.globl initial_pg_pmd
kernel/head_32.S:.globl initial_page_table
kernel/head_32.S:.globl swapper_pg_dir
kernel/head_32.S:.globl empty_zero_page
lib/error-inject.c:     ".globl just_return_func\n"
math-emu/reg_round.S:.globl fpu_reg_round
math-emu/reg_round.S:.globl fpu_Arith_exit
purgatory/kexec-purgatory.S:    .globl  kexec_purgatory
purgatory/kexec-purgatory.S:    .globl  kexec_purgatory_size
um/setjmp_32.S: .globl kernel_setjmp
um/setjmp_32.S: .globl kernel_longjmp
um/setjmp_64.S: .globl kernel_setjmp
um/setjmp_64.S: .globl kernel_longjmp
xen/xen-head.S: ELFNOTE(Xen, XEN_ELFNOTE_ENTRY,          .globl xen_elfnote_entry;
[~/work/others/linux/arch/x86 (master)]$

Maybe we could collect all global symbols in asm codes before lld,
and then we add an option in lld to feed those global symbols (with a file?),
then we can be sure there won't be any conflict?

Re: [PATCH kbuild] kbuild: Allow to reduce the number of suffixes for clang thin-lto build

[Josh Poimboeuf]

On Fri, Mar 06, 2026 at 11:40:21AM -0800, Yonghong Song wrote:
> 
> 
> On 3/6/26 10:32 AM, Josh Poimboeuf wrote:
> > On Thu, Mar 05, 2026 at 07:43:25PM -0800, Yonghong Song wrote:
> > > The current clang thin-lto build often produces lots of symbols with
> > > suffix. The following is a partial list of such function call symbols:
> > >      ...
> > >      ethnl_module_fw_flash_ntf.llvm.7631589765585346066
> > >      __nf_conntrack_alloc.llvm.6438426151906658917
> > >      tcp_can_early_drop.llvm.11937612064648250727
> > >      tcp_print_conntrack.llvm.11937612064648250727
> > >      ...
> > > 
> > > In my particular build with current bpf-next, the number of '*.llvm.<hash>'
> > > function calls is 1212. Such symbols make kernel live patching
> > > difficult since
> > >    - a minor code change will change the hash and then the '*.llvm.<hash>'
> > >      symbol becomes another one with a different hash or no hash, and
> > >    - a previous source-level symbol may become an one with suffix after live
> > >      patching code.
> > > 
> > > In [1], Song Liu suggested to reduce the number of '*.llvm.<hash>' functions
> > > to make live patch easier. In respond of this, I implemented this
> > > in llvm ([2]). The same thin-lto build with [2] only has two symbols with
> > > suffix:
> > >      m_stop.llvm.14460341347352036579
> > >      m_next.llvm.14460341347352036579
> > > This should make live patch much easier.
> > > 
> > > To support suffix symbol reduction, a new config
> > >      LTO_CLANG_THIN_SUFFIX_REDUCTION
> > > is introduced and the config depends on thin-lto and llvm23 or higher.
> > > 
> > > Two lld flags are necessary to enable this feature in kernel:
> > >      - Flag '--lto-whole-program-visibility' is needed as it ensures that all
> > >        modules are available in the same process, which is true for kernel at
> > >        thin-lto lld.
> > >      - Flag '-mllvm -always-rename-promoted-locals=false' is needed to enable
> > >        suffix reduction. Currently in llvm [1], only process mode is supported.
> > >        There is another distributed mode (across different processes or even
> > >        different machines) which is not supported yet ([2]).
> > > 
> > >    [1] https://lpc.events/event/19/contributions/2212
> > >    [2] https://github.com/llvm/llvm-project/pull/178587
> > > 
> > > Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
> > > ---
> > >   Makefile     |  3 +++
> > >   arch/Kconfig | 15 +++++++++++++++
> > >   2 files changed, 18 insertions(+)
> > > 
> > > diff --git a/Makefile b/Makefile
> > > index e944c6e71e81..9d6033595615 100644
> > > --- a/Makefile
> > > +++ b/Makefile
> > > @@ -1034,6 +1034,9 @@ endif
> > >   ifdef CONFIG_LTO_CLANG
> > >   ifdef CONFIG_LTO_CLANG_THIN
> > >   CC_FLAGS_LTO	:= -flto=thin -fsplit-lto-unit
> > > +ifdef CONFIG_LTO_CLANG_THIN_SUFFIX_REDUCTION
> > > +KBUILD_LDFLAGS += --lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false
> > > +endif
> > >   else
> > >   CC_FLAGS_LTO	:= -flto
> > >   endif
> > > diff --git a/arch/Kconfig b/arch/Kconfig
> > > index 102ddbd4298e..e1db64a3284e 100644
> > > --- a/arch/Kconfig
> > > +++ b/arch/Kconfig
> > > @@ -861,8 +861,23 @@ config LTO_CLANG_THIN
> > >   	    https://clang.llvm.org/docs/ThinLTO.html
> > >   	  If unsure, say Y.
> > > +
> > >   endchoice
> > > +config LTO_CLANG_THIN_SUFFIX_REDUCTION
> > > +	bool "Clang ThinLTO Suffix Reduction (EXPERIMENTAL)"
> > > +	depends on LTO_CLANG_THIN
> > > +	depends on CLANG_VERSION >= 230000
> > > +	default y
> > > +	help
> > > +	  This option allows to reduce the number of symbols with
> > > +	  '.llvm.<hash' suffixes. This can help KLP (kernel living
> > > +	  patch) as symbol name can stay stable in most cases.
> > > +
> > > +	  See https://github.com/llvm/llvm-project/pull/178587
> > > +
> > > +	  If unsure, say N.
> > > +
> > Thanks!  Would there be any downsides to enabling this feature
> > unconditionally in the kernel when the compiler supports it?
> 
> The only downside is for the following case:
> 
> C   file: static function foo()
> Asm file: global function foo()
> 
> The thin-lto will collect all C files and with the above llvm patch,
> the static function foo() may be promoted to global function foo()
> if there is no other pre-existing global function foo() in all C files.
> 
> In such cases, there will be a conflict since
> there are two global function foo() (one from C file, another from Asm file).
> In such cases, the build will fail.
> 
> How do you think we could hit such issues in linux kernel?
> Maybe should have default no for the new config?
> 
> I think the chance should be very low. The following is a grab for x86
> for global symbols in asm code:
> 
> [~/work/others/linux/arch/x86 (master)]$ egrep -r globl

There are actually quite a bit more than that, see SYM_CODE_START:

  $ git grep 'SYM_CODE_START(' |wc -l
  169

But still, I agree the chance of a conflict would be very low.  And
global assembly functions tend to be rather uniquely named.

> Maybe we could collect all global symbols in asm codes before lld,
> and then we add an option in lld to feed those global symbols (with a file?),
> then we can be sure there won't be any conflict?

That wouldn't be worth the effort in my opinion.

I think we should just unconditionally enable
-always-rename-promoted-locals=false when it's available.  While that
will implicitly enforce that global asm functions be uniquely named
across the tree, I don't see that as a problem.  In the rare case of a
conflict, we can just rename the function.

-- 
Josh

Re: [PATCH kbuild] kbuild: Allow to reduce the number of suffixes for clang thin-lto build

[Josh Poimboeuf]

On Fri, Mar 06, 2026 at 12:55:02PM -0800, Josh Poimboeuf wrote:
> On Fri, Mar 06, 2026 at 11:40:21AM -0800, Yonghong Song wrote:
> > > Thanks!  Would there be any downsides to enabling this feature
> > > unconditionally in the kernel when the compiler supports it?
> > 
> > The only downside is for the following case:
> > 
> > C   file: static function foo()
> > Asm file: global function foo()
> > 
> > The thin-lto will collect all C files and with the above llvm patch,
> > the static function foo() may be promoted to global function foo()
> > if there is no other pre-existing global function foo() in all C files.
> > 
> > In such cases, there will be a conflict since
> > there are two global function foo() (one from C file, another from Asm file).
> > In such cases, the build will fail.
> > 
> > How do you think we could hit such issues in linux kernel?
> > Maybe should have default no for the new config?
> > 
> > I think the chance should be very low. The following is a grab for x86
> > for global symbols in asm code:
> > 
> > [~/work/others/linux/arch/x86 (master)]$ egrep -r globl
> 
> There are actually quite a bit more than that, see SYM_CODE_START:
> 
>   $ git grep 'SYM_CODE_START(' |wc -l
>   169
> 
> But still, I agree the chance of a conflict would be very low.  And
> global assembly functions tend to be rather uniquely named.
> 
> > Maybe we could collect all global symbols in asm codes before lld,
> > and then we add an option in lld to feed those global symbols (with a file?),
> > then we can be sure there won't be any conflict?
> 
> That wouldn't be worth the effort in my opinion.
> 
> I think we should just unconditionally enable
> -always-rename-promoted-locals=false when it's available.  While that
> will implicitly enforce that global asm functions be uniquely named
> across the tree, I don't see that as a problem.  In the rare case of a
> conflict, we can just rename the function.

BTW, reading this again I realize that this would affect not only global
asm *functions*, but also global asm *data*.  But asm global data is
quite rare and also tends to be uniquely named, so my conclusion is the
same.

-- 
Josh

Re: [PATCH kbuild] kbuild: Allow to reduce the number of suffixes for clang thin-lto build

[Yonghong Song]

On 3/6/26 12:57 PM, Josh Poimboeuf wrote:
> On Fri, Mar 06, 2026 at 12:55:02PM -0800, Josh Poimboeuf wrote:
>> On Fri, Mar 06, 2026 at 11:40:21AM -0800, Yonghong Song wrote:
>>>> Thanks!  Would there be any downsides to enabling this feature
>>>> unconditionally in the kernel when the compiler supports it?
>>> The only downside is for the following case:
>>>
>>> C   file: static function foo()
>>> Asm file: global function foo()
>>>
>>> The thin-lto will collect all C files and with the above llvm patch,
>>> the static function foo() may be promoted to global function foo()
>>> if there is no other pre-existing global function foo() in all C files.
>>>
>>> In such cases, there will be a conflict since
>>> there are two global function foo() (one from C file, another from Asm file).
>>> In such cases, the build will fail.
>>>
>>> How do you think we could hit such issues in linux kernel?
>>> Maybe should have default no for the new config?
>>>
>>> I think the chance should be very low. The following is a grab for x86
>>> for global symbols in asm code:
>>>
>>> [~/work/others/linux/arch/x86 (master)]$ egrep -r globl
>> There are actually quite a bit more than that, see SYM_CODE_START:
>>
>>    $ git grep 'SYM_CODE_START(' |wc -l
>>    169
>>
>> But still, I agree the chance of a conflict would be very low.  And
>> global assembly functions tend to be rather uniquely named.
>>
>>> Maybe we could collect all global symbols in asm codes before lld,
>>> and then we add an option in lld to feed those global symbols (with a file?),
>>> then we can be sure there won't be any conflict?
>> That wouldn't be worth the effort in my opinion.
>>
>> I think we should just unconditionally enable
>> -always-rename-promoted-locals=false when it's available.  While that
>> will implicitly enforce that global asm functions be uniquely named
>> across the tree, I don't see that as a problem.  In the rare case of a
>> conflict, we can just rename the function.

So you mean we do not need a new config. Should just check clang and
clang versionn like below?

diff --git a/Makefile b/Makefile
index e944c6e71e81..9bd3990024c5 100644
--- a/Makefile
+++ b/Makefile
@@ -1034,6 +1034,11 @@ endif
  ifdef CONFIG_LTO_CLANG
  ifdef CONFIG_LTO_CLANG_THIN
  CC_FLAGS_LTO   := -flto=thin -fsplit-lto-unit
+ifdef CONFIG_CC_IS_CLANG
+ifeq ($(shell test $(CONFIG_CLANG_VERSION) -ge 230000; echo $$?),0)
+KBUILD_LDFLAGS += --lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false
+endif
+endif
  else
  CC_FLAGS_LTO   := -flto
  endif

I think this should work. In rare case, renaming the function should
solve the problem.

> BTW, reading this again I realize that this would affect not only global
> asm *functions*, but also global asm *data*.  But asm global data is
> quite rare and also tends to be uniquely named, so my conclusion is the
> same.
>
With thin-lto, the cross-file data inlining is a side effect of
cross-file function inlining. So the number of cross-file data inlining
is small. Compared to 1212 number of functions with suffix .llvm.<hash>,
the number of data with suffix only 9:

ffffffff84e8ad09 d __func__.net_ratelimit.llvm.12310750674033017485
ffffffff84f1d569 d .str.llvm.11541892489833206993
ffffffff87f42d40 b __cfs_bandwidth_used.llvm.9245315756205243639
ffffffff87f457c0 b __sched_clock_stable.llvm.13183765431478218167
ffffffff881968d0 b lock_chains_in_use.llvm.17116087415131265585
ffffffff88bba180 b timekeeper_data.llvm.4127123038948722377
ffffffff88bd0eb0 b slab_debugfs_root.llvm.537776758349482510
ffffffff88c0b300 b __acpi_os_prepare_sleep.llvm.3969442148434754585
ffffffff88c63b00 b handshake_rhashtbl.llvm.12810142853876704317

With newly added KBUILD_LDFLAGS flags, none of them have suffixes.
Again, the conflict for data variable should be rare too.

I will repost the patch with Makefile change once llvm23 release
is cut.

Thanks!

Re: [PATCH kbuild] kbuild: Allow to reduce the number of suffixes for clang thin-lto build

[Nathan Chancellor]

On Fri, Mar 06, 2026 at 02:12:44PM -0800, Yonghong Song wrote:
> So you mean we do not need a new config. Should just check clang and
> clang versionn like below?
> 
> diff --git a/Makefile b/Makefile
> index e944c6e71e81..9bd3990024c5 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1034,6 +1034,11 @@ endif
>  ifdef CONFIG_LTO_CLANG
>  ifdef CONFIG_LTO_CLANG_THIN
>  CC_FLAGS_LTO   := -flto=thin -fsplit-lto-unit
> +ifdef CONFIG_CC_IS_CLANG
> +ifeq ($(shell test $(CONFIG_CLANG_VERSION) -ge 230000; echo $$?),0)

This is the same as

  ifeq ($(call clang-min-version,230000),y)

That being said, could we not just do a dynamic check so that it always
works when these flags are supported?

> +KBUILD_LDFLAGS += --lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false

  KBUILD_LDFLAGS += $(call ld-option,--lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false)

> +endif
> +endif
>  else
>  CC_FLAGS_LTO   := -flto
>  endif
> 
> I think this should work. In rare case, renaming the function should
> solve the problem.

I agree with Josh that having this be the default behavior with the
workarounds that we have available to us to avoid problems from it is
worth it. Thanks for chasing this after the discussions at LPC.

Cheers,
Nathan

Re: [PATCH kbuild] kbuild: Allow to reduce the number of suffixes for clang thin-lto build

[Yonghong Song]

On 3/6/26 2:45 PM, Nathan Chancellor wrote:
> On Fri, Mar 06, 2026 at 02:12:44PM -0800, Yonghong Song wrote:
>> So you mean we do not need a new config. Should just check clang and
>> clang versionn like below?
>>
>> diff --git a/Makefile b/Makefile
>> index e944c6e71e81..9bd3990024c5 100644
>> --- a/Makefile
>> +++ b/Makefile
>> @@ -1034,6 +1034,11 @@ endif
>>   ifdef CONFIG_LTO_CLANG
>>   ifdef CONFIG_LTO_CLANG_THIN
>>   CC_FLAGS_LTO   := -flto=thin -fsplit-lto-unit
>> +ifdef CONFIG_CC_IS_CLANG
>> +ifeq ($(shell test $(CONFIG_CLANG_VERSION) -ge 230000; echo $$?),0)
> This is the same as
>
>    ifeq ($(call clang-min-version,230000),y)
>
> That being said, could we not just do a dynamic check so that it always
> works when these flags are supported?

Thanks for the suggestion! What you had below with 'call ld-option' is perfect
to satisfy this requirement: <= llvm22, two flags will be ignored, and
>= llvm23, two flags will be used.

v2: https://lore.kernel.org/linux-kbuild/20260307050250.3767489-1-yonghong.song@linux.dev/

>
>> +KBUILD_LDFLAGS += --lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false
>    KBUILD_LDFLAGS += $(call ld-option,--lto-whole-program-visibility -mllvm -always-rename-promoted-locals=false)
>
>> +endif
>> +endif
>>   else
>>   CC_FLAGS_LTO   := -flto
>>   endif
>>
>> I think this should work. In rare case, renaming the function should
>> solve the problem.
> I agree with Josh that having this be the default behavior with the
> workarounds that we have available to us to avoid problems from it is
> worth it. Thanks for chasing this after the discussions at LPC.
>
> Cheers,
> Nathan