From 2856f6bf24e36c5fb1e7b5906efb69ceb8dd2a3f Mon Sep 17 00:00:00 2001 From: Nick Desaulniers Date: Fri, 6 Aug 2021 10:27:01 -0700 Subject: [PATCH] BACKPORT: scripts/Makefile.clang: default to LLVM_IAS=1 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. 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 Reviewed-by: Nathan Chancellor Signed-off-by: Masahiro Yamada Bug: 209655537 [nd: conflict in Documentation/kbuild/llvm.rst] (cherry picked from commit f12b034afeb3a977bbb1c6584dedc0f3dc666f14) Change-Id: I026d42aa40c83eb149c8e39fe91a0b0164ff71b0 --- Documentation/kbuild/llvm.rst | 67 +++++++++++++++++++++++++++++++++-- Makefile | 2 +- 2 files changed, 66 insertions(+), 3 deletions(-) diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst index bf02347d1eb2..310b9fbee38b 100644 --- a/Documentation/kbuild/llvm.rst +++ b/Documentation/kbuild/llvm.rst @@ -61,8 +61,71 @@ They can be enabled individually. The full list of the parameters: :: 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=``. + +If ``CROSS_COMPILE`` is not specified, the ``--target=`` is inferred +from ``ARCH``. + +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 + +If ``LLVM_IAS=0`` is specified, ``CROSS_COMPILE`` is also used to derive +``--prefix=`` to search for the GNU assembler and linker. :: + + make ARCH=arm64 LLVM=1 LLVM_IAS=0 CROSS_COMPILE=aarch64-linux-gnu- + +Supported Architectures +----------------------- + +LLVM does not target all of the architectures that Linux supports and +just because a target is supported in LLVM does not mean that the kernel +will build or work without any issues. Below is a general summary of +architectures that currently work with ``CC=clang`` or ``LLVM=1``. Level +of support corresponds to "S" values in the MAINTAINERS files. If an +architecture is not present, it either means that LLVM does not target +it or there are known issues. Using the latest stable version of LLVM or +even the development tree will generally yield the best results. +An architecture's ``defconfig`` is generally expected to work well, +certain configurations may have problems that have not been uncovered +yet. Bug reports are always welcome at the issue tracker below! + +.. list-table:: + :widths: 10 10 10 + :header-rows: 1 + + * - Architecture + - Level of support + - ``make`` command + * - arm + - Supported + - ``LLVM=1`` + * - arm64 + - Supported + - ``LLVM=1`` + * - mips + - Maintained + - ``CC=clang`` + * - powerpc + - Maintained + - ``CC=clang`` + * - riscv + - Maintained + - ``CC=clang`` + * - s390 + - Maintained + - ``CC=clang`` + * - x86 + - Supported + - ``LLVM=1`` Getting Help ------------ diff --git a/Makefile b/Makefile index 41821aa6c947..df40004dd283 100644 --- a/Makefile +++ b/Makefile @@ -584,7 +584,7 @@ else CLANG_FLAGS += --target=$(notdir $(CROSS_COMPILE:%-=%)) endif # CROSS_COMPILE -ifneq ($(LLVM_IAS),1) +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))