When I was considering a similar idea, my plan was
to implement this in Kconfig instead of in Makefile
because that will pass the compiler information
in one direction (only from Kconfig to Kbuild), but
that is not so important. We can change it later
if needed.
I did not complete it because I was investigating
some issues (especially [3] below), but maybe
that is something we don't care about.
Can you address [2] below at least?
If we do not have any concern, I can merge it.
It is likely so because people are only discussing
"We want to omit omit CROSS_COMPILE".
[1] explicit target triple for native builds
The default target of my distro clang
is x86_64-pc-linux-gnu.
$ clang --version
Ubuntu clang version 11.0.0-2
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
So, previously, the kernel was built with
implied --target=x86_64-pc-linux-gnu.
With this patch, --target=x86_64-linux-gnu will be
explicitly specified.
The same applies to native-builds of other architectures.
For example, when a user builds the arm64 kernel on
an arm64 server, --target=aarch64-linux-gnu is
explicitly forced.
I guess, this is a good direction because the produced
code will be more deterministic, irrespective of the
Clang's configuration.
[2] 32/64-bit configuration is toggled in Kconfig time.
Initially, you submitted only arm64. Maybe, it was intentional
since arm64 is the simplest case.
In the kernel tree, arch/arm and arch/arm64 are very special
cases where 32-bit and 64-bit are separated by directory.
Some of the other architectures are bi-arch, and
32-bit/64-bit is specified by CONFIG_64BIT in Kconfig time.
When Makefiles are being parsed, we actually do not know
whether the user is planning to configure the kernel
for 32-bit or 64-bit because CONFIG_64BIT is not
specified at this point.
ARCH=x86 + CONFIG_64BIT=y
will build the x86_64 kernel, and
ARCH=x86 + CONFIG_64BIT=n
will build the i386 kernel.
Then, you may wonder
else ifeq ($(ARCH),x86)
CLANG_FLAGS += --target=x86_64-linux-gnu
... works?
Yes, it does fortunately.
-m32/-m64 takes precedence over the
{x86_64,i386} part of the target triple.
As far as I tested,
clang --target=x86_64-linux-gnu -m32
produced i386 code.
Interestingly,
clang --target=i386-linux-gnu -m64
produced x86_64 code.
We must rely on this behavior of Clang because
--target (which is contained in CLANG_FLAGS)
must be specified before the Kconfig time.
Then, a user can toggle CONFIG_64BIT any time
from menuconfig etc.
With this in mind, using $(ARCH) as if-else
switches is pointless.
$(SRCARCH) is the only meaningful input.
else ifeq ($(ARCH),i386)
CLANG_FLAGS += --target=i686-linux-gnu
else ifeq ($(ARCH),x86)
CLANG_FLAGS += --target=x86_64-linux-gnu
else ifeq ($(ARCH),x86_64)
CLANG_FLAGS += --target=x86_64-linux-gnu
should be replaced with:
else ifeq ($(SRCARCH),x86_64)
CLANG_FLAGS += --target=x86_64-linux-gnu
Some architectures are not only bi-arch, but also bi-endian.
You hardcoded 64bit little endian for ppc:
else ifeq ($(ARCH),powerpc)
CLANG_FLAGS += --target=powerpc64le-linux-gnu
But, we must rely on the fact that
clang --target=powerpc64le-linux-gnu -mbig-endian -m32
produces big-endian 32-bit code.
This makes the "64le" part meaningless.
This should be noted. Otherwise it is difficult
to understand why --target=x86_64-linux-gnu works fine
with building the i386 kernel.
[3] User-space compilation
This does not matter to the kernel itself, but
Kbuild compiles some userspace programs for
the target architecture.
See the samples/ directory for example.
Another example is net/bpfilter/Makefile, which
embeds the user mode helper when
CONFIG_BPFILTER_UMH=y.
For this purpose, Kconfig checks if $(CC) is
capable of linking the userspace.
(CONFIG_CC_CAN_LINK).
When cross-building with Clang, I cannot see
CONFIG_CC_CAN_LINK set.
If we care about CONFIG_CC_CAN_LINK, probably,
--sysroot or something should be set according to:
https://clang.llvm.org/docs/CrossCompilation.html
This is an existing issue, but I have no time
for looking into this.
On debian systems, sysroot for cross-compilation
are located in /usr/aarch64-linux-gnu,
/usr/arm-linux-gnueabi, /usr/arm-linux-gnueabihf,
/usr/i686-linux-gnu/ etc. but I do not know if it
is the same across distros.
[4] What is the best target if we hard-code it?
Currently, we require the correct CROSS_COMPILE
is provided by users.
The target might impact the performance
or the ABI.
It was difficult for me to define
which one is better than another.
For example for ARCH=arm, which is better
--target=arm-linux-gnueabi or
--target=arm-lnux-gnueabihf or
something we don't care about?
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