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@xxxxxxxxxx>
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@xxxxxxxxxx>
> ---
> 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@xxxxxxxxxx/
> as per Masahiro.
> * Base is kbuild/for-next, plus
> https://lore.kernel.org/lkml/20210802183910.1802120-1-ndesaulniers@xxxxxxxxxx/
> https://lore.kernel.org/lkml/20210805150102.131008-1-masahiroy@xxxxxxxxxx/.
>
> 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
>
