Re: [PATCH v2] scripts/Makefile.clang: default to LLVM_IAS=1

[Nathan Chancellor <nathan@kernel.org>]

On Fri, Aug 06, 2021 at 10:27:01AM -0700, Nick Desaulniers wrote:
> LLVM_IAS=1 controls enabling clang's integrated assembler via
> -integrated-as. This was an explicit opt in until we could enable
> assembler support in Clang for more architecures. Now we have support
> and CI coverage of LLVM_IAS=1 for all architecures except a few more
> bugs affecting s390 and powerpc.

The powerpc and s390 folks have been testing with clang, I think they
should have been on CC for this change (done now).

> This commit flips the default from opt in via LLVM_IAS=1 to opt out via
> LLVM_IAS=0.  CI systems or developers that were previously doing builds
> with CC=clang or LLVM=1 without explicitly setting LLVM_IAS must now
> explicitly opt out via LLVM_IAS=0, otherwise they will be implicitly
> opted-in.
> 
> This finally shortens the command line invocation when cross compiling
> with LLVM to simply:
> 
> $ make ARCH=arm64 LLVM=1
> 
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

I am still not really sure how I feel about this. I would prefer not to
break people's builds but I suppose this is inevitabile eventually.

A little support matrix that I drafted up where based on ARCH and clang
version for LLVM_IAS=1 support:

             | 10.x | 11.x | 12.x | 13.x | 14.x |
ARCH=arm     |  NO  |  NO  |  NO  |  YES |  YES |
ARCH=arm64   |  NO  |  YES |  YES |  YES |  YES |
ARCH=i386    |  YES |  YES |  YES |  YES |  YES |
ARCH=mips*   |  YES |  YES |  YES |  YES |  YES |
ARCH=powerpc |  NO  |  NO  |  NO  |  NO  |  NO  |
ARCH=s390    |  NO  |  NO  |  NO  |  NO  |  NO  |
ARCH=x86_64  |  NO  |  YES |  YES |  YES |  YES |

The main issue that I have with this change is that all of these
architectures work fine with CC=clang and their build commands that used
to work fine will not with this change, as they will have to specify
LLVM_IAS=0. I think that making this change for LLVM=1 makes sense but
changing the default for just CC=clang feels like a bit much at this
point in time. I would love to hear from others on this though, I am not
going to object much further than this.

Regardless of that concern, this patch does what it says so:

Reviewed-by: Nathan Chancellor <nathan@kernel.org>

> ---
> Changes v1 -> v2:
> * Drop "Currently" from Documentation/, as per Matthew.
> * Drop Makefile and riscv Makefile, rebase on
>   https://lore.kernel.org/lkml/20210805150102.131008-1-masahiroy@kernel.org/
>   as per Masahiro.
> * Base is kbuild/for-next, plus
>   https://lore.kernel.org/lkml/20210802183910.1802120-1-ndesaulniers@google.com/
>   https://lore.kernel.org/lkml/20210805150102.131008-1-masahiroy@kernel.org/.
> 
>  Documentation/kbuild/llvm.rst | 14 ++++++++------
>  scripts/Makefile.clang        |  6 +++---
>  2 files changed, 11 insertions(+), 9 deletions(-)
> 
> diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
> index f8a360958f4c..e87ed5479963 100644
> --- a/Documentation/kbuild/llvm.rst
> +++ b/Documentation/kbuild/llvm.rst
> @@ -60,17 +60,14 @@ They can be enabled individually. The full list of the parameters: ::
>  	  OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump READELF=llvm-readelf \
>  	  HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar HOSTLD=ld.lld
>  
> -Currently, the integrated assembler is disabled by default. You can pass
> -``LLVM_IAS=1`` to enable it.
> +The integrated assembler is enabled by default. You can pass ``LLVM_IAS=0`` to
> +disable it.
>  
>  Omitting CROSS_COMPILE
>  ----------------------
>  
>  As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``.
>  
> -Unless ``LLVM_IAS=1`` is specified, ``CROSS_COMPILE`` is also used to derive
> -``--prefix=<path>`` to search for the GNU assembler and linker.
> -
>  If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` is inferred
>  from ``ARCH``.
>  
> @@ -78,7 +75,12 @@ That means if you use only LLVM tools, ``CROSS_COMPILE`` becomes unnecessary.
>  
>  For example, to cross-compile the arm64 kernel::
>  
> -	make ARCH=arm64 LLVM=1 LLVM_IAS=1
> +	make ARCH=arm64 LLVM=1
> +
> +If ``LLVM_IAS=0`` is specified, ``CROSS_COMPILE`` is also used to derive
> +``--prefix=<path>`` to search for the GNU assembler and linker. ::
> +
> +	make ARCH=arm64 LLVM=1 LLVM_IAS=0 CROSS_COMPILE=aarch64-linux-gnu-
>  
>  Supported Architectures
>  -----------------------
> diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
> index 1f4e3eb70f88..3ae63bd35582 100644
> --- a/scripts/Makefile.clang
> +++ b/scripts/Makefile.clang
> @@ -22,12 +22,12 @@ else
>  CLANG_FLAGS	+= --target=$(notdir $(CROSS_COMPILE:%-=%))
>  endif # CROSS_COMPILE
>  
> -ifeq ($(LLVM_IAS),1)
> -CLANG_FLAGS	+= -integrated-as
> -else
> +ifeq ($(LLVM_IAS),0)
>  CLANG_FLAGS	+= -no-integrated-as
>  GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))
>  CLANG_FLAGS	+= --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
> +else
> +CLANG_FLAGS	+= -integrated-as
>  endif
>  CLANG_FLAGS	+= -Werror=unknown-warning-option
>  KBUILD_CFLAGS	+= $(CLANG_FLAGS)
> 
> base-commit: d7a86429dbc691bf540688fcc8542cc20246a85b
> prerequisite-patch-id: 0d3072ecb5fd06ff6fd6ea81fe601f6c54c23910
> prerequisite-patch-id: 2654829756eb8a094a0ffad1679caa75a4d86619
> prerequisite-patch-id: a51e7885ca2376d008bbf146a5589da247806f7b
> prerequisite-patch-id: 6a0342755115ec459610657edac1075f069faa3d
> -- 
> 2.32.0.605.g8dce9f2422-goog
>