linux-kernel - [PATCH v2] scripts/Makefile.clang: default to LLVM

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Date:   Fri,  6 Aug 2021 10:27:01 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>
Cc:     Khem Raj <raj.khem@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Jonathan Corbet <corbet@....net>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        clang-built-linux@...glegroups.com, linux-kbuild@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org
Subject: [PATCH v2] 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 <ndesaulniers@...gle.com>
---
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