Re: [PATCH] kbuild: Fix Propeller build option

[Nathan Chancellor <nathan@kernel.org>]

Hi Rong,

Thanks for the quick patch!

On Thu, Nov 07, 2024 at 09:10:30PM -0800, Rong Xu wrote:
> The '-fbasic-block-sections=labels' option has been deprecated with
> the following llvm patch:
> https://github.com/llvm/llvm-project/pull/110039
> 
> The old option still works, but with a warning like
>   clang: warning: argument '-fbasic-block-sections=labels' \
>   is deprecated, use '-fbasic-block-address-map' instead [-Wdeprecated]
> 
> Currently, the option change is only in the ToT clang (v20) and not yet
> released in v19.

I think this whole block could be a little easier to read and understand
with some more standard kernel commit message practices. I would combine
the first and third block into one sentence and I would unwrap the
warning text (it is okay to be long for a little clarity). Perhaps
something like this?

  | The '-fbasic-block-sections=labels' option has been deprecated in tip of
  | tree clang (20.0.0) [1]. While the option still works, a warning is
  | emitted:
  |
  |   clang: warning: argument '-fbasic-block-sections=labels' is deprecated, use '-fbasic-block-address-map' instead [-Wdeprecated]

then putting:

  | Link: https://github.com/llvm/llvm-project/pull/110039 [1]

right above your Signoff.

> This patch adds a check to the Makefile to set the proper option.

Try to avoid saying "this patch" in kernel commit messages, it is
implicit. It would be cleaner to say something like:

  | Add a version check to set the proper option.

You can see some more commit message tips in sections 4.2.3 to 4.2.6 in
the tip documentation:

https://docs.kernel.org/process/maintainer-tip.html#changelog

> If the option change is later integrated in v19.x, we need to update the
> check accordingly.

I think it is worth dropping this sentence entirely, as I would argue
that it is pretty unlikely that this option gets backported to
release/19.x, since it would basically accelerate the deprecation
timeline, which seems unreasonable to do in a stable release.

> Signed-off-by: Rong Xu <xur@google.com>
> Reported-by: Nathan Chancellor <nathan@kernel.org>
> ---
>  scripts/Makefile.propeller | 14 ++++++++++++--
>  1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/scripts/Makefile.propeller b/scripts/Makefile.propeller
> index 344190717e47..1a68ea0d1fe4 100644
> --- a/scripts/Makefile.propeller
> +++ b/scripts/Makefile.propeller
> @@ -5,7 +5,13 @@ ifdef CLANG_PROPELLER_PROFILE_PREFIX
>    CFLAGS_PROPELLER_CLANG := -fbasic-block-sections=list=$(CLANG_PROPELLER_PROFILE_PREFIX)_cc_profile.txt -ffunction-sections
>    KBUILD_LDFLAGS += --symbol-ordering-file=$(CLANG_PROPELLER_PROFILE_PREFIX)_ld_profile.txt --no-warn-symbol-ordering
>  else
> -  CFLAGS_PROPELLER_CLANG := -fbasic-block-sections=labels
> +  # Staring with clang v20, the '-fbasic-block-sections=labels' option is
> +  # deprecated. Use the recommended '-fbasic-block-address-map' option.

Might be worth linking the specific change here for archeology sake.

> +  ifeq ($(call clang-min-version, 200000),y)
> +    CFLAGS_PROPELLER_CLANG := -fbasic-block-address-map
> +  else
> +    CFLAGS_PROPELLER_CLANG := -fbasic-block-sections=labels
> +  endif
>  endif
>  
>  # Propeller requires debug information to embed module names in the profiles.
> @@ -21,7 +27,11 @@ ifdef CONFIG_LTO_CLANG_THIN
>    ifdef CLANG_PROPELLER_PROFILE_PREFIX
>      KBUILD_LDFLAGS += --lto-basic-block-sections=$(CLANG_PROPELLER_PROFILE_PREFIX)_cc_profile.txt
>    else
> -    KBUILD_LDFLAGS += --lto-basic-block-sections=labels
> +    ifeq ($(call clang-min-version, 200000),y)

Would it be better/more accurate to check the LLD version here? I doubt
it would really happen but it is possible for someone to have LLVM=1
(using their system wide ld.lld and LLVM tools) and CC=clang-20 to just
use an updated clang. Perhaps:

  ifeq ($(call test-ge, $(CONFIG_LLD_VERSION), 200000),y)

here?

> +       KBUILD_LDFLAGS += --lto-basic-block-address-map
> +    else
> +       KBUILD_LDFLAGS += --lto-basic-block-sections=labels
> +    endif
>    endif
>  endif
>  
> 
> base-commit: 0dcc2d1066150787017a71f035145c566597dec7
> -- 
> 2.47.0.277.g8800431eea-goog
>