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

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Message-Id: <20210802234304.3519577-1-ndesaulniers@google.com>
Date:   Mon,  2 Aug 2021 16:43:03 -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>,
        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] 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

Link: https://github.com/ClangBuiltLinux/linux/issues/1434
Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
---
Note: base is:
https://lore.kernel.org/lkml/20210802183910.1802120-1-ndesaulniers@google.com/

 Documentation/kbuild/llvm.rst | 14 ++++++++------
 Makefile                      |  2 +-
 arch/riscv/Makefile           |  2 +-
 scripts/Makefile.clang        |  6 +++---
 4 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
index f8a360958f4c..16712fab4d3a 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.
+Currently, 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/Makefile b/Makefile
index 444558e62cbc..b24b48c9ebb7 100644
--- a/Makefile
+++ b/Makefile
@@ -845,7 +845,7 @@ else
 DEBUG_CFLAGS	+= -g
 endif
 
-ifneq ($(LLVM_IAS),1)
+ifeq ($(LLVM_IAS),0)
 KBUILD_AFLAGS	+= -Wa,-gdwarf-2
 endif
 
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index bc74afdbf31e..807f7c94bc6f 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -41,7 +41,7 @@ endif
 ifeq ($(CONFIG_LD_IS_LLD),y)
 	KBUILD_CFLAGS += -mno-relax
 	KBUILD_AFLAGS += -mno-relax
-ifneq ($(LLVM_IAS),1)
+ifeq ($(LLVM_IAS),0)
 	KBUILD_CFLAGS += -Wa,-mno-relax
 	KBUILD_AFLAGS += -Wa,-mno-relax
 endif
diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
index 1f4e3eb70f88..3ae63bd35582 100644
--- a/scripts/Makefile.clang
+++ b/scripts/Makefile.clang
@@ -22,12 +22,12 @@ else
 CLANG_FLAGS	+= --target=$(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
-- 
2.32.0.554.ge1b32706d8-goog