Re: [PATCH v2] scripts/Makefile.clang: default to LLVM_IAS=1
From: Nathan Chancellor <nathan@kernel.org>
Date: 2021-08-06 19:53:05
Also in:
linux-kbuild, linux-riscv, linux-s390, linuxppc-dev, lkml
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 <redacted>
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>
quoted hunk ↗ jump to hunk
--- 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/ (local) as per Masahiro. * Base is kbuild/for-next, plus https://lore.kernel.org/lkml/20210802183910.1802120-1-ndesaulniers@google.com/ (local) https://lore.kernel.org/lkml/20210805150102.131008-1-masahiroy@kernel.org/ (local). 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