Re: [PATCH 03/13] Makefile: Generate CLANG_FLAGS even in GCC builds

[Nathan Chancellor <nathan@kernel.org>]

Hi Miguel,

On Wed, Apr 14, 2021 at 08:45:54PM +0200, ojeda@kernel.org wrote:
> From: Miguel Ojeda <ojeda@kernel.org>
> 
> To support Rust under GCC-built kernels, we need to save the flags that
> would have been passed if the kernel was being compiled with Clang.
> 
> The reason is that bindgen -- the tool we use to generate Rust bindings
> to the C side of the kernel -- relies on libclang to parse C. Ideally:
> 
>   - bindgen would support a GCC backend (requested at [1]),
> 
>   - or the Clang driver would be perfectly compatible with GCC,
>     including plugins. Unlikely, of course, but perhaps a big
>     subset of configs may be possible to guarantee to be kept
>     compatible nevertheless.
> 
> This is also the reason why GCC builds are very experimental and some
> configurations may not work (e.g. GCC_PLUGIN_RANDSTRUCT). However,
> we keep GCC builds working (for some example configs) in the CI
> to avoid diverging/regressing further, so that we are better prepared
> for the future when a solution might become available.
> 
> [1] https://github.com/rust-lang/rust-bindgen/issues/1949
> 
> Link: https://github.com/Rust-for-Linux/linux/issues/167
> 
> Co-developed-by: Alex Gaynor <alex.gaynor@gmail.com>
> Signed-off-by: Alex Gaynor <alex.gaynor@gmail.com>
> Co-developed-by: Geoffrey Thomas <geofft@ldpreload.com>
> Signed-off-by: Geoffrey Thomas <geofft@ldpreload.com>
> Co-developed-by: Finn Behrens <me@kloenk.de>
> Signed-off-by: Finn Behrens <me@kloenk.de>
> Co-developed-by: Adam Bratschi-Kaye <ark.email@gmail.com>
> Signed-off-by: Adam Bratschi-Kaye <ark.email@gmail.com>
> Co-developed-by: Wedson Almeida Filho <wedsonaf@google.com>
> Signed-off-by: Wedson Almeida Filho <wedsonaf@google.com>
> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
> ---
>  Makefile | 27 ++++++++++++++++-----------
>  1 file changed, 16 insertions(+), 11 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index d4784d181123..9c75354324ed 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -559,26 +559,31 @@ ifdef building_out_of_srctree
>  	{ echo "# this is build directory, ignore it"; echo "*"; } > .gitignore
>  endif
>  
> -# The expansion should be delayed until arch/$(SRCARCH)/Makefile is included.
> -# Some architectures define CROSS_COMPILE in arch/$(SRCARCH)/Makefile.
> -# CC_VERSION_TEXT is referenced from Kconfig (so it needs export),
> -# and from include/config/auto.conf.cmd to detect the compiler upgrade.
> -CC_VERSION_TEXT = $(shell $(CC) --version 2>/dev/null | head -n 1 | sed 's/\#//g')
> +TENTATIVE_CLANG_FLAGS := -Werror=unknown-warning-option
>  
> -ifneq ($(findstring clang,$(CC_VERSION_TEXT)),)
>  ifneq ($(CROSS_COMPILE),)
> -CLANG_FLAGS	+= --target=$(notdir $(CROSS_COMPILE:%-=%))
> +TENTATIVE_CLANG_FLAGS	+= --target=$(notdir $(CROSS_COMPILE:%-=%))
>  GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))

Shuffling this around will cause this issue (I never saw you CC'd on the
thread).

https://lore.kernel.org/r/f6218ac526a04fa4d4406f935bcc4eb4a7df65c4.1617917438.git.msuchanek@suse.de/

Perhaps that patch should be added to this series?

> -CLANG_FLAGS	+= --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
> +TENTATIVE_CLANG_FLAGS	+= --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
>  GCC_TOOLCHAIN	:= $(realpath $(GCC_TOOLCHAIN_DIR)/..)
>  endif
>  ifneq ($(GCC_TOOLCHAIN),)
> -CLANG_FLAGS	+= --gcc-toolchain=$(GCC_TOOLCHAIN)
> +TENTATIVE_CLANG_FLAGS	+= --gcc-toolchain=$(GCC_TOOLCHAIN)
>  endif
>  ifneq ($(LLVM_IAS),1)
> -CLANG_FLAGS	+= -no-integrated-as
> +TENTATIVE_CLANG_FLAGS	+= -no-integrated-as
>  endif
> -CLANG_FLAGS	+= -Werror=unknown-warning-option
> +
> +export TENTATIVE_CLANG_FLAGS
> +
> +# The expansion should be delayed until arch/$(SRCARCH)/Makefile is included.
> +# Some architectures define CROSS_COMPILE in arch/$(SRCARCH)/Makefile.
> +# CC_VERSION_TEXT is referenced from Kconfig (so it needs export),
> +# and from include/config/auto.conf.cmd to detect the compiler upgrade.
> +CC_VERSION_TEXT = $(shell $(CC) --version 2>/dev/null | head -n 1 | sed 's/\#//g')
> +
> +ifneq ($(findstring clang,$(CC_VERSION_TEXT)),)
> +CLANG_FLAGS	+= $(TENTATIVE_CLANG_FLAGS)
>  KBUILD_CFLAGS	+= $(CLANG_FLAGS)
>  KBUILD_AFLAGS	+= $(CLANG_FLAGS)
>  export CLANG_FLAGS
> -- 
> 2.17.1
>