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

From: Nick Desaulniers
Date: Fri Aug 06 2021 - 13:27:10 EST


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 <ndesaulniers@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