[PATCH v4 0/2] infer --target from SRCARCH for CC=clang

[Nick Desaulniers]

We get constant feedback that the command line invocation of make is too
long. CROSS_COMPILE is helpful when a toolchain has a prefix of the
target triple, or is an absolute path outside of $PATH, but it's mostly
redundant for a given ARCH.

Instead, let's infer it from SRCARCH, and move some flag handling into a
new file included from the top level Makefile.

Changes v3 -> v4:
* Remove the requirement that LLVM_IAS=1 be set, as per Masahiro.
* Remove the Documentation/ change from patch 2, as per Masahiro and
Nathan.
* Add Documentation/ change as patch 3, from Masahiro.
* Reword commit message of patch 2, as per Nathan.
* Change patch 2 oneline to refer to --target and CC=clang (not
CROSS_COMPILE).
* Carry Arnd's and Nathan's AB/RB/TB tags, confirmed ok on IRC+discord.

Changes v2 -> v3:
* Remove requirement that LLVM=1 be set. Instead, if building with just
CC=clang LLVM_IAS=1 instead of LLVM=1 LLVM_IAS=1, you should use
LD=ld.lld explicitly, or LD=aarch64-linux-gnu-ld. (As per Masahiro)
Example:

$ ARCH=arm64 make CC=clang LLVM_IAS=1 LD=ld.lld OBJCOPY=llvm-objcopy \
STRIP=llvm-strip -j72 defconfig all

(It's still preferable to use LLVM=1 IMO, but this is maximally
flexible.)
* Change oneliner from LLVM=1 to CC=clang.
* Update Docs slightly.

Changes v1 -> v2:
* patch 1/2 untouched.
* Fix typos in commit message as per Geert and Masahiro.
* Use SRCARCH instead of ARCH, simplifying x86 handling, as per
Masahiro. Add his sugguested by tag.
* change commit oneline from 'drop' to 'infer.'
* Add detail about explicit host --target and relationship of ARCH to
SRCARCH, as per Masahiro.

*** BLURB HERE ***

Nick Desaulniers (2):
Makefile: infer --target from ARCH for CC=clang
Documentation/llvm: update CROSS_COMPILE inferencing

Documentation/kbuild/llvm.rst | 17 +++++++++++++++++
scripts/Makefile.clang | 30 ++++++++++++++++++++++++++++--
2 files changed, 45 insertions(+), 2 deletions(-)


base-commit: 27932b6a2088eac7a5afa5471963b926cfbb4de7
prerequisite-patch-id: 0d3072ecb5fd06ff6fd6ea81fe601f6c54c23910
--
2.32.0.554.ge1b32706d8-goog

[Nick Desaulniers]

We get constant feedback that the command line invocation of make is too

long when compiling with LLVM. CROSS_COMPILE is helpful when a toolchain

has a prefix of the target triple, or is an absolute path outside of

$PATH.

Since a Clang binary is generally multi-targeted, we can infer a given
target from SRCARCH/ARCH. If CROSS_COMPILE is not set, simply set
--target= for CLANG_FLAGS, KBUILD_CFLAGS, and KBUILD_AFLAGS based on
$SRCARCH.

Previously, we'd cross compile via:
$ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make LLVM=1 LLVM_IAS=1
Now:
$ ARCH=arm64 make LLVM=1 LLVM_IAS=1

For native builds (not involving cross compilation) we now explicitly
specify a target triple rather than rely on the implicit host triple.

Link: https://github.com/ClangBuiltLinux/linux/issues/1399
Suggested-by: Arnd Bergmann <ar...@kernel.org>
Suggested-by: Nathan Chancellor <nat...@kernel.org>
Suggested-by: Masahiro Yamada <masa...@kernel.org>
Acked-by: Arnd Bergmann <ar...@kernel.org>
Reviewed-by: Nathan Chancellor <nat...@kernel.org>
Tested-by: Nathan Chancellor <nat...@kernel.org>
Signed-off-by: Nick Desaulniers <ndesau...@google.com>
---
Changes v3 -> v4:
* Reword commit message and oneline, as per Nathan.
* Drop documentation change (it's the next patch, could be squashed in,
I guess).
* Drop LLVM_IAS=1 requirement, as per Masahiro.
* Carry Arnd's and Nathan's AB/RB/TB tags, confirmed OK on IRC+Discord.

Changes v2 -> v3:
* Drop check/requirement for LLVM=1, as per Masahiro.
* Change oneliner from LLVM=1 LLVM_IAS=1 to CC=clang LLVM_IAS=1.
* Don't carry forward Nathan's RB/TB tags. :( Sorry Nathan, but thank
you for testing+reviewing v2.
* Update wording of docs slightly.

Changes v1 -> v2:

* Fix typos in commit message as per Geert and Masahiro.
* Use SRCARCH instead of ARCH, simplifying x86 handling, as per
Masahiro. Add his sugguested by tag.
* change commit oneline from 'drop' to 'infer.'
* Add detail about explicit host --target and relationship of ARCH to
SRCARCH, as per Masahiro.

Changes RFC -> v1:
* Rebase onto linux-kbuild/for-next
* Keep full target triples since missing the gnueabi suffix messes up
32b ARM. Drop Fangrui's sugguested by tag. Update commit message to
drop references to arm64.
* Flush out TODOS.
* Add note about -EL/-EB, -m32/-m64.
* Add note to Documentation/.

scripts/Makefile.clang | 30 ++++++++++++++++++++++++++++--
1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
index 297932e973d4..f983f65dd9f5 100644
--- a/scripts/Makefile.clang
+++ b/scripts/Makefile.clang
@@ -1,6 +1,32 @@
-ifneq ($(CROSS_COMPILE),)
+# Individual arch/{arch}/Makefiles should use -EL/-EB to set intended
+# endianness and -m32/-m64 to set word size based on Kconfigs instead of
+# relying on the target triple.
+ifeq ($(CROSS_COMPILE),)
+ifeq ($(SRCARCH),arm)
+CLANG_FLAGS += --target=arm-linux-gnueabi
+else ifeq ($(SRCARCH),arm64)
+CLANG_FLAGS += --target=aarch64-linux-gnu
+else ifeq ($(SRCARCH),hexagon)
+CLANG_FLAGS += --target=hexagon-linux-gnu
+else ifeq ($(SRCARCH),m68k)
+CLANG_FLAGS += --target=m68k-linux-gnu
+else ifeq ($(SRCARCH),mips)
+CLANG_FLAGS += --target=mipsel-linux-gnu
+else ifeq ($(SRCARCH),powerpc)
+CLANG_FLAGS += --target=powerpc64le-linux-gnu
+else ifeq ($(SRCARCH),riscv)
+CLANG_FLAGS += --target=riscv64-linux-gnu
+else ifeq ($(SRCARCH),s390)
+CLANG_FLAGS += --target=s390x-linux-gnu
+else ifeq ($(SRCARCH),x86)
+CLANG_FLAGS += --target=x86_64-linux-gnu
+else
+$(error Specify CROSS_COMPILE or add '--target=' option to scripts/Makefile.clang)
+endif # SRCARCH
+else
CLANG_FLAGS += --target=$(notdir $(CROSS_COMPILE:%-=%))
-endif
+endif # CROSS_COMPILE
+
ifeq ($(LLVM_IAS),1)
CLANG_FLAGS += -integrated-as
else
--
2.32.0.554.ge1b32706d8-goog

[Nick Desaulniers]

As noted by Masahiro, document how we can generally infer CROSS_COMPILE
(and the more specific details about --target and --prefix) based on
ARCH.

Suggested-by: Masahiro Yamada <masa...@kernel.org>

Signed-off-by: Nick Desaulniers <ndesau...@google.com>
---

Note: Masahiro, I'm sorry for putting my name on this as you mostly
wrote it. I touched it up slightly. I don't know how best to attribute
it to you. If you'd prefer to take back authorship, please do; I'd
rather not add your SOB without seeing it stated explicitly.

Documentation/kbuild/llvm.rst | 17 +++++++++++++++++
1 file changed, 17 insertions(+)

diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
index b18401d2ba82..4292f0686316 100644
--- a/Documentation/kbuild/llvm.rst
+++ b/Documentation/kbuild/llvm.rst
@@ -63,6 +63,23 @@ They can be enabled individually. The full list of the parameters: ::
Currently, the integrated assembler is disabled by default. You can pass
``LLVM_IAS=1`` to enable 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.
+
+If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` is inferred
+from ``ARCH``.
+
+That means if you use only LLVM tools, ``CROSS_COMPILE`` becomes unnecessary.
+
+For example, to cross-compile the arm64 kernel::
+
+ ARCH=arm64 make LLVM=1 LLVM_IAS=1
+
Supported Architectures
-----------------------

--
2.32.0.554.ge1b32706d8-goog

[Linus Torvalds]

On Fri, Jul 30, 2021 at 1:17 PM Nick Desaulniers
<ndesau...@google.com> wrote:
>
> +ifeq ($(SRCARCH),arm)
> +CLANG_FLAGS += --target=arm-linux-gnueabi

..,.

Ugh.

A wise man once said: "All problems in computer science can be solved
with another level of indirection".

Just do it like this:

CLANG_TARGET_FLAGS_arm = arm-linux-gnueabi
CLANG_TARGET_FLAGS_hexagon = hexagon-linux-gnu
..
CLANG_TARGET_FLAGS = $(CLANG_TARGET_FLAGS_$(ARCH))

Which is a lot denser and simpler.

And then the only if-statement can be something along the lines of

ifeq ($(CLANG_TARGET_FLAGS),)
$(error Specify clang target flags)
else
CLANG_FLAGS += --target=$(CLANG_TARGET_FLAGS)
endif

Plus add *random handwaving* about all the details for CROSS_COMPILE
and friends.

Linus

[Nick Desaulniers]

On Fri, Jul 30, 2021 at 1:17 PM Nick Desaulniers
<ndesau...@google.com> wrote:
>

Err, sorry, it looks like I did:
$ git format-patch HEAD~2 ...
when I meant to do HEAD~3 ..., will resend.

>
> Nick Desaulniers (2):
> Makefile: infer --target from ARCH for CC=clang
> Documentation/llvm: update CROSS_COMPILE inferencing
>
> Documentation/kbuild/llvm.rst | 17 +++++++++++++++++
> scripts/Makefile.clang | 30 ++++++++++++++++++++++++++++--
> 2 files changed, 45 insertions(+), 2 deletions(-)
>
>
> base-commit: 27932b6a2088eac7a5afa5471963b926cfbb4de7
> prerequisite-patch-id: 0d3072ecb5fd06ff6fd6ea81fe601f6c54c23910
> --
> 2.32.0.554.ge1b32706d8-goog
>

--
Thanks,
~Nick Desaulniers

[Nick Desaulniers]

We get constant feedback that the command line invocation of make is too
long. CROSS_COMPILE is helpful when a toolchain has a prefix of the
target triple, or is an absolute path outside of $PATH, but it's mostly
redundant for a given ARCH.

Instead, let's infer it from SRCARCH, and move some flag handling into a
new file included from the top level Makefile.

Changes v4 -> v5:
* Include previously missing first patch!

Nick Desaulniers (3):
Makefile: move initial clang flag handling into scripts/Makefile.clang

Makefile: infer --target from ARCH for CC=clang
Documentation/llvm: update CROSS_COMPILE inferencing

Documentation/kbuild/llvm.rst | 17 +++++++++++++++

MAINTAINERS | 1 +
Makefile | 15 +------------
scripts/Makefile.clang | 40 +++++++++++++++++++++++++++++++++++
4 files changed, 59 insertions(+), 14 deletions(-)
create mode 100644 scripts/Makefile.clang


base-commit: 27932b6a2088eac7a5afa5471963b926cfbb4de7
--
2.32.0.554.ge1b32706d8-goog

[Nick Desaulniers]

With some of the changes we'd like to make to CROSS_COMPILE, the initial
block of clang flag handling which controls things like the target triple,
whether or not to use the integrated assembler and how to find GAS,
and erroring on unknown warnings is becoming unwieldy. Move it into its
own file under scripts/.

Reviewed-by: Nathan Chancellor <nat...@kernel.org>

Signed-off-by: Nick Desaulniers <ndesau...@google.com>
---

MAINTAINERS | 1 +
Makefile | 15 +--------------
scripts/Makefile.clang | 14 ++++++++++++++
3 files changed, 16 insertions(+), 14 deletions(-)
create mode 100644 scripts/Makefile.clang

diff --git a/MAINTAINERS b/MAINTAINERS
index 81e1edeceae4..9c1205c258c7 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -4433,6 +4433,7 @@ B: https://github.com/ClangBuiltLinux/linux/issues
C: irc://chat.freenode.net/clangbuiltlinux
F: Documentation/kbuild/llvm.rst
F: include/linux/compiler-clang.h
+F: scripts/Makefile.clang
F: scripts/clang-tools/
K: \b(?i:clang|llvm)\b

diff --git a/Makefile b/Makefile
index cbab0dc53065..010e3a4e770b 100644
--- a/Makefile
+++ b/Makefile
@@ -586,20 +586,7 @@ endif
CC_VERSION_TEXT = $(subst $(pound),,$(shell $(CC) --version 2>/dev/null | head -n 1))

ifneq ($(findstring clang,$(CC_VERSION_TEXT)),)
-ifneq ($(CROSS_COMPILE),)
-CLANG_FLAGS += --target=$(notdir $(CROSS_COMPILE:%-=%))
-endif
-ifeq ($(LLVM_IAS),1)
-CLANG_FLAGS += -integrated-as
-else
-CLANG_FLAGS += -no-integrated-as
-GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))
-CLANG_FLAGS += --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
-endif
-CLANG_FLAGS += -Werror=unknown-warning-option
-KBUILD_CFLAGS += $(CLANG_FLAGS)
-KBUILD_AFLAGS += $(CLANG_FLAGS)
-export CLANG_FLAGS
+include $(srctree)/scripts/Makefile.clang
endif

# Include this also for config targets because some architectures need
diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
new file mode 100644
index 000000000000..297932e973d4
--- /dev/null
+++ b/scripts/Makefile.clang
@@ -0,0 +1,14 @@
+ifneq ($(CROSS_COMPILE),)
+CLANG_FLAGS += --target=$(notdir $(CROSS_COMPILE:%-=%))
+endif
+ifeq ($(LLVM_IAS),1)
+CLANG_FLAGS += -integrated-as
+else
+CLANG_FLAGS += -no-integrated-as
+GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))
+CLANG_FLAGS += --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
+endif
+CLANG_FLAGS += -Werror=unknown-warning-option
+KBUILD_CFLAGS += $(CLANG_FLAGS)
+KBUILD_AFLAGS += $(CLANG_FLAGS)
+export CLANG_FLAGS
--
2.32.0.554.ge1b32706d8-goog

[Nick Desaulniers]

We get constant feedback that the command line invocation of make is too

long when compiling with LLVM. CROSS_COMPILE is helpful when a toolchain

has a prefix of the target triple, or is an absolute path outside of

$PATH.

Since a Clang binary is generally multi-targeted, we can infer a given
target from SRCARCH/ARCH. If CROSS_COMPILE is not set, simply set
--target= for CLANG_FLAGS, KBUILD_CFLAGS, and KBUILD_AFLAGS based on
$SRCARCH.

Previously, we'd cross compile via:
$ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make LLVM=1 LLVM_IAS=1
Now:
$ ARCH=arm64 make LLVM=1 LLVM_IAS=1

For native builds (not involving cross compilation) we now explicitly
specify a target triple rather than rely on the implicit host triple.

Link: https://github.com/ClangBuiltLinux/linux/issues/1399
Suggested-by: Arnd Bergmann <ar...@kernel.org>
Suggested-by: Nathan Chancellor <nat...@kernel.org>
Suggested-by: Masahiro Yamada <masa...@kernel.org>
Acked-by: Arnd Bergmann <ar...@kernel.org>

Reviewed-by: Nathan Chancellor <nat...@kernel.org>
Signed-off-by: Nick Desaulniers <ndesau...@google.com>
---

Changes v3 -> v4:

* Reword commit message and oneline, as per Nathan.
* Drop documentation change (it's the next patch, could be squashed in,
I guess).
* Drop LLVM_IAS=1 requirement, as per Masahiro.

* Carry Arnd's and Nathan's AB/RB/TB tags, confirmed OK on IRC+Discord.

Changes v2 -> v3:

* Drop check/requirement for LLVM=1, as per Masahiro.
* Change oneliner from LLVM=1 LLVM_IAS=1 to CC=clang LLVM_IAS=1.
* Don't carry forward Nathan's RB/TB tags. :( Sorry Nathan, but thank
you for testing+reviewing v2.

* Update wording of docs slightly.

Changes v1 -> v2:

* Fix typos in commit message as per Geert and Masahiro.
* Use SRCARCH instead of ARCH, simplifying x86 handling, as per
Masahiro. Add his sugguested by tag.
* change commit oneline from 'drop' to 'infer.'
* Add detail about explicit host --target and relationship of ARCH to
SRCARCH, as per Masahiro.

Changes RFC -> v1:
* Rebase onto linux-kbuild/for-next
* Keep full target triples since missing the gnueabi suffix messes up
32b ARM. Drop Fangrui's sugguested by tag. Update commit message to
drop references to arm64.
* Flush out TODOS.
* Add note about -EL/-EB, -m32/-m64.
* Add note to Documentation/.

scripts/Makefile.clang | 30 ++++++++++++++++++++++++++++--
1 file changed, 28 insertions(+), 2 deletions(-)

diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
index 297932e973d4..f983f65dd9f5 100644
--- a/scripts/Makefile.clang
+++ b/scripts/Makefile.clang
@@ -1,6 +1,32 @@
-ifneq ($(CROSS_COMPILE),)
+# Individual arch/{arch}/Makefiles should use -EL/-EB to set intended
+# endianness and -m32/-m64 to set word size based on Kconfigs instead of
+# relying on the target triple.
+ifeq ($(CROSS_COMPILE),)

+ifeq ($(SRCARCH),arm)
+CLANG_FLAGS += --target=arm-linux-gnueabi

+else ifeq ($(SRCARCH),arm64)
+CLANG_FLAGS += --target=aarch64-linux-gnu
+else ifeq ($(SRCARCH),hexagon)
+CLANG_FLAGS += --target=hexagon-linux-gnu
+else ifeq ($(SRCARCH),m68k)
+CLANG_FLAGS += --target=m68k-linux-gnu
+else ifeq ($(SRCARCH),mips)
+CLANG_FLAGS += --target=mipsel-linux-gnu
+else ifeq ($(SRCARCH),powerpc)
+CLANG_FLAGS += --target=powerpc64le-linux-gnu
+else ifeq ($(SRCARCH),riscv)
+CLANG_FLAGS += --target=riscv64-linux-gnu
+else ifeq ($(SRCARCH),s390)
+CLANG_FLAGS += --target=s390x-linux-gnu
+else ifeq ($(SRCARCH),x86)
+CLANG_FLAGS += --target=x86_64-linux-gnu
+else
+$(error Specify CROSS_COMPILE or add '--target=' option to scripts/Makefile.clang)
+endif # SRCARCH
+else

CLANG_FLAGS += --target=$(notdir $(CROSS_COMPILE:%-=%))
-endif

[Nick Desaulniers]

As noted by Masahiro, document how we can generally infer CROSS_COMPILE
(and the more specific details about --target and --prefix) based on
ARCH.

Suggested-by: Masahiro Yamada <masa...@kernel.org>

Signed-off-by: Nick Desaulniers <ndesau...@google.com>
---

[Nick Desaulniers]

On Fri, Jul 30, 2021 at 2:54 PM Linus Torvalds
<torv...@linux-foundation.org> wrote:
>
> On Fri, Jul 30, 2021 at 1:17 PM Nick Desaulniers
> <ndesau...@google.com> wrote:
> >
> > +ifeq ($(SRCARCH),arm)
> > +CLANG_FLAGS += --target=arm-linux-gnueabi
> ..,.
>
> Ugh.
>
> A wise man once said: "All problems in computer science can be solved
> with another level of indirection".
>
> Just do it like this:
>
> CLANG_TARGET_FLAGS_arm = arm-linux-gnueabi
> CLANG_TARGET_FLAGS_hexagon = hexagon-linux-gnu
> ..
> CLANG_TARGET_FLAGS = $(CLANG_TARGET_FLAGS_$(ARCH))
>
> Which is a lot denser and simpler.
>
> And then the only if-statement can be something along the lines of
>
> ifeq ($(CLANG_TARGET_FLAGS),)
> $(error Specify clang target flags)
> else
> CLANG_FLAGS += --target=$(CLANG_TARGET_FLAGS)
> endif

D'oh, just sent v5 before seeing this. SGTM will work on v6.

>
> Plus add *random handwaving* about all the details for CROSS_COMPILE
> and friends.
>
> Linus

--
Thanks,
~Nick Desaulniers

[Fangrui Song]

On 2021-07-30, Nick Desaulniers wrote:
>As noted by Masahiro, document how we can generally infer CROSS_COMPILE
>(and the more specific details about --target and --prefix) based on
>ARCH.
>
>Suggested-by: Masahiro Yamada <masa...@kernel.org>
>Signed-off-by: Nick Desaulniers <ndesau...@google.com>
>---
> Documentation/kbuild/llvm.rst | 17 +++++++++++++++++
> 1 file changed, 17 insertions(+)
>
>diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
>index b18401d2ba82..4292f0686316 100644
>--- a/Documentation/kbuild/llvm.rst
>+++ b/Documentation/kbuild/llvm.rst
>@@ -63,6 +63,23 @@ They can be enabled individually. The full list of the parameters: ::
> Currently, the integrated assembler is disabled by default. You can pass
> ``LLVM_IAS=1`` to enable 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 the GNU linker.

Reviewed-by: Fangrui Song <mas...@google.com>

>+If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` is inferred
>+from ``ARCH``.
>+
>+That means if you use only LLVM tools, ``CROSS_COMPILE`` becomes unnecessary.
>+
>+For example, to cross-compile the arm64 kernel::
>+
>+ ARCH=arm64 make LLVM=1 LLVM_IAS=1

Is ARCH=arm64 make LLVM=1 LLVM_IAS=1

preferred over

make ARCH=arm64 LLVM=1 LLVM_IAS=1

?

[Masahiro Yamada]

I do not think so.

For consistency,

make ARCH=arm64 LLVM=1 LLVM_IAS=1

or

ARCH=arm64 LLVM=1 LLVM_IAS=1 make

might be preferred.



But, in the same doc, I see an example, which
mixes up the env variables and a make variable.

ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang




--
Best Regards
Masahiro Yamada

[Nick Desaulniers]

Sure, I'll add this, but wanted to note that generally the compiler is
not used as the driver for kernel builds; the linker is invoked
directly. IIRC, there were a few spots where the compiler is used as
the driver; I think some of the various VDSO's are still built that
way, but it's not common.

>
> Reviewed-by: Fangrui Song <mas...@google.com>
>
> >+If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` is inferred
> >+from ``ARCH``.
> >+
> >+That means if you use only LLVM tools, ``CROSS_COMPILE`` becomes unnecessary.
> >+
> >+For example, to cross-compile the arm64 kernel::
> >+
> >+ ARCH=arm64 make LLVM=1 LLVM_IAS=1
>
> Is ARCH=arm64 make LLVM=1 LLVM_IAS=1
>
> preferred over
>
> make ARCH=arm64 LLVM=1 LLVM_IAS=1
>
> ?

Will respond to this against Masahiro's reply.

--
Thanks,
~Nick Desaulniers

[Nick Desaulniers]

Perhaps it's just muscle memory from when I started working on Nexus
phone kernels ~5 years ago; that's how our build scripts invoked make
(with ARCH and CROSS_COMPILE as env vars).

IIRC, GNU make has the notion of `origin` for variables.
https://www.gnu.org/software/make/manual/html_node/Origin-Function.html
So "before `make`" would be "environment" and "after `make`" would be
"command line."

I'm rereading the section on assignment to see if one of the
assignment operators was affected by origin.
https://www.gnu.org/software/make/manual/html_node/Setting.html
https://www.gnu.org/software/make/manual/html_node/Flavors.html#Flavors

I could have sworn that due to one of the assignment operators
(perhaps `?=`) that the orderings weren't commutative.
$ make ARCH=arm64 LLVM=1 LLVM_IAS=1 -j72
works
$ ARCH=arm64 LLVM=1 LLVM_IAS=1 make -j72
also works. Hmm...maybe I misremembered the issue then.

Ok, I will reorder all instances to use the command line rather than
env vars in v6.
--
Thanks,
~Nick Desaulniers

[Nick Desaulniers]

We get constant feedback that the command line invocation of make is too

long. CROSS_COMPILE is helpful when a toolchain has a prefix of the
target triple, or is an absolute path outside of $PATH, but it's mostly
redundant for a given ARCH.

Instead, let's infer --target from SRCARCH, and move some flag handling into a

new file included from the top level Makefile.

Changes v5 -> v6:
* Use indirection as per Linus.
* Change hexagon triple to use -musl rather than -gnu. glibc doesn't
have support for hexagon, and hexagon-linux-musl is the triple we use
in CI.
https://github.com/ClangBuiltLinux/continuous-integration2/blob/d659897d1700894d67feb64fe28e298da399a287/generator.yml#L53
* Pick up Fangrui's RB.
* Reorder use of Env vars in documentation to use command line
parameters instead, for consistency.

Changes v4 -> v5:
* Include previously missing first patch!

Changes v3 -> v4:

* Remove the requirement that LLVM_IAS=1 be set, as per Masahiro.
* Remove the Documentation/ change from patch 2, as per Masahiro and
Nathan.
* Add Documentation/ change as patch 3, from Masahiro.
* Reword commit message of patch 2, as per Nathan.
* Change patch 2 oneline to refer to --target and CC=clang (not
CROSS_COMPILE).

* Carry Arnd's and Nathan's AB/RB/TB tags, confirmed ok on IRC+discord.

Changes v2 -> v3:

* Remove requirement that LLVM=1 be set. Instead, if building with just
CC=clang LLVM_IAS=1 instead of LLVM=1 LLVM_IAS=1, you should use
LD=ld.lld explicitly, or LD=aarch64-linux-gnu-ld. (As per Masahiro)
Example:

$ ARCH=arm64 make CC=clang LLVM_IAS=1 LD=ld.lld OBJCOPY=llvm-objcopy \
STRIP=llvm-strip -j72 defconfig all

(It's still preferable to use LLVM=1 IMO, but this is maximally
flexible.)
* Change oneliner from LLVM=1 to CC=clang.

* Update Docs slightly.

Changes v1 -> v2:
* patch 1/2 untouched.

* Fix typos in commit message as per Geert and Masahiro.
* Use SRCARCH instead of ARCH, simplifying x86 handling, as per
Masahiro. Add his sugguested by tag.
* change commit oneline from 'drop' to 'infer.'
* Add detail about explicit host --target and relationship of ARCH to
SRCARCH, as per Masahiro.

Nick Desaulniers (3):
Makefile: move initial clang flag handling into scripts/Makefile.clang
Makefile: infer --target from ARCH for CC=clang
Documentation/llvm: update CROSS_COMPILE inferencing

Documentation/kbuild/llvm.rst | 19 ++++++++++++++++++-

MAINTAINERS | 1 +
Makefile | 15 +--------------

scripts/Makefile.clang | 35 +++++++++++++++++++++++++++++++++++
4 files changed, 55 insertions(+), 15 deletions(-)
create mode 100644 scripts/Makefile.clang


base-commit: d7a86429dbc691bf540688fcc8542cc20246a85b
--
2.32.0.554.ge1b32706d8-goog

[Nick Desaulniers]

With some of the changes we'd like to make to CROSS_COMPILE, the initial
block of clang flag handling which controls things like the target triple,
whether or not to use the integrated assembler and how to find GAS,
and erroring on unknown warnings is becoming unwieldy. Move it into its
own file under scripts/.

Reviewed-by: Nathan Chancellor <nat...@kernel.org>

Signed-off-by: Nick Desaulniers <ndesau...@google.com>
---

MAINTAINERS | 1 +
Makefile | 15 +--------------

scripts/Makefile.clang | 14 ++++++++++++++
3 files changed, 16 insertions(+), 14 deletions(-)
create mode 100644 scripts/Makefile.clang

diff --git a/MAINTAINERS b/MAINTAINERS
index 19135a9d778e..3af8d39f43ef 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -4501,6 +4501,7 @@ B: https://github.com/ClangBuiltLinux/linux/issues

C: irc://chat.freenode.net/clangbuiltlinux
F: Documentation/kbuild/llvm.rst
F: include/linux/compiler-clang.h
+F: scripts/Makefile.clang
F: scripts/clang-tools/
K: \b(?i:clang|llvm)\b

diff --git a/Makefile b/Makefile

index 6b555f64df06..444558e62cbc 100644

--- a/Makefile
+++ b/Makefile
@@ -586,20 +586,7 @@ endif
CC_VERSION_TEXT = $(subst $(pound),,$(shell $(CC) --version 2>/dev/null | head -n 1))

ifneq ($(findstring clang,$(CC_VERSION_TEXT)),)
-ifneq ($(CROSS_COMPILE),)

-CLANG_FLAGS += --target=$(notdir $(CROSS_COMPILE:%-=%))
-endif

[Nick Desaulniers]

We get constant feedback that the command line invocation of make is too

long when compiling with LLVM. CROSS_COMPILE is helpful when a toolchain

has a prefix of the target triple, or is an absolute path outside of

$PATH.

Since a Clang binary is generally multi-targeted, we can infer a given
target from SRCARCH/ARCH. If CROSS_COMPILE is not set, simply set
--target= for CLANG_FLAGS, KBUILD_CFLAGS, and KBUILD_AFLAGS based on
$SRCARCH.

Previously, we'd cross compile via:
$ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make LLVM=1 LLVM_IAS=1
Now:
$ ARCH=arm64 make LLVM=1 LLVM_IAS=1

For native builds (not involving cross compilation) we now explicitly
specify a target triple rather than rely on the implicit host triple.

Link: https://github.com/ClangBuiltLinux/linux/issues/1399
Suggested-by: Arnd Bergmann <ar...@kernel.org>

Suggested-by: Linus Torvalds <torv...@linux-foundation.org>
Suggested-by: Masahiro Yamada <masa...@kernel.org>
Suggested-by: Nathan Chancellor <nat...@kernel.org>
Acked-by: Arnd Bergmann <ar...@kernel.org>

Reviewed-by: Nathan Chancellor <nat...@kernel.org>
Signed-off-by: Nick Desaulniers <ndesau...@google.com>
---

Changes v5 -> v6:
* Use indirection as per Linus. We now have CLANG_TARGET_FLAGS and a
few variables suffixed with $SRCARCH.
* Add Linus' SB tag.
* Change hexagon target triple from -gnu to -musl.

Changes v3 -> v4:

* Reword commit message and oneline, as per Nathan.
* Drop documentation change (it's the next patch, could be squashed in,
I guess).
* Drop LLVM_IAS=1 requirement, as per Masahiro.

* Carry Arnd's and Nathan's AB/RB/TB tags, confirmed OK on IRC+Discord.

Changes v2 -> v3:

* Drop check/requirement for LLVM=1, as per Masahiro.
* Change oneliner from LLVM=1 LLVM_IAS=1 to CC=clang LLVM_IAS=1.
* Don't carry forward Nathan's RB/TB tags. :( Sorry Nathan, but thank
you for testing+reviewing v2.

* Update wording of docs slightly.

Changes v1 -> v2:

* Fix typos in commit message as per Geert and Masahiro.
* Use SRCARCH instead of ARCH, simplifying x86 handling, as per
Masahiro. Add his sugguested by tag.
* change commit oneline from 'drop' to 'infer.'
* Add detail about explicit host --target and relationship of ARCH to
SRCARCH, as per Masahiro.

Changes RFC -> v1:
* Rebase onto linux-kbuild/for-next
* Keep full target triples since missing the gnueabi suffix messes up
32b ARM. Drop Fangrui's sugguested by tag. Update commit message to
drop references to arm64.
* Flush out TODOS.
* Add note about -EL/-EB, -m32/-m64.
* Add note to Documentation/.

scripts/Makefile.clang | 25 +++++++++++++++++++++++--
1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
index 297932e973d4..1f4e3eb70f88 100644
--- a/scripts/Makefile.clang
+++ b/scripts/Makefile.clang
@@ -1,6 +1,27 @@

-ifneq ($(CROSS_COMPILE),)
+# Individual arch/{arch}/Makefiles should use -EL/-EB to set intended
+# endianness and -m32/-m64 to set word size based on Kconfigs instead of
+# relying on the target triple.

+CLANG_TARGET_FLAGS_arm := arm-linux-gnueabi
+CLANG_TARGET_FLAGS_arm64 := aarch64-linux-gnu
+CLANG_TARGET_FLAGS_hexagon := hexagon-linux-musl
+CLANG_TARGET_FLAGS_m68k := m68k-linux-gnu
+CLANG_TARGET_FLAGS_mips := mipsel-linux-gnu
+CLANG_TARGET_FLAGS_powerpc := powerpc64le-linux-gnu
+CLANG_TARGET_FLAGS_riscv := riscv64-linux-gnu
+CLANG_TARGET_FLAGS_s390 := s390x-linux-gnu
+CLANG_TARGET_FLAGS_x86 := x86_64-linux-gnu
+CLANG_TARGET_FLAGS := $(CLANG_TARGET_FLAGS_$(SRCARCH))
+
+ifeq ($(CROSS_COMPILE),)
+ifeq ($(CLANG_TARGET_FLAGS),)

+$(error Specify CROSS_COMPILE or add '--target=' option to scripts/Makefile.clang)

+else
+CLANG_FLAGS += --target=$(CLANG_TARGET_FLAGS)
+endif # CLANG_TARGET_FLAGS
+else

CLANG_FLAGS += --target=$(notdir $(CROSS_COMPILE:%-=%))
-endif

[Nick Desaulniers]

As noted by Masahiro, document how we can generally infer CROSS_COMPILE
(and the more specific details about --target and --prefix) based on
ARCH.

Change use of env vars to command line parameters.

Suggested-by: Masahiro Yamada <masa...@kernel.org>
Reviewed-by: Fangrui Song <mas...@google.com>

Signed-off-by: Nick Desaulniers <ndesau...@google.com>
---

Changes v5 -> v6:
* Pick up Fangrui's RB tag.
* Change use of env vars to command line parameters for consistency.

Documentation/kbuild/llvm.rst | 19 ++++++++++++++++++-
1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
index b18401d2ba82..f8a360958f4c 100644
--- a/Documentation/kbuild/llvm.rst
+++ b/Documentation/kbuild/llvm.rst
@@ -38,7 +38,7 @@ Cross Compiling
A single Clang compiler binary will typically contain all supported backends,
which can help simplify cross compiling. ::

- ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang
+ make ARCH=arm64 CC=clang CROSS_COMPILE=aarch64-linux-gnu-

``CROSS_COMPILE`` is not used to prefix the Clang compiler binary, instead
``CROSS_COMPILE`` is used to set a command line flag: ``--target=<triple>``. For

@@ -63,6 +63,23 @@ They can be enabled individually. The full list of the parameters: ::
Currently, the integrated assembler is disabled by default. You can pass
``LLVM_IAS=1`` to enable 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``.
+
+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
+

[Nathan Chancellor]

On 8/2/2021 11:39 AM, 'Nick Desaulniers' via Clang Built Linux wrote:
> As noted by Masahiro, document how we can generally infer CROSS_COMPILE
> (and the more specific details about --target and --prefix) based on
> ARCH.
>
> Change use of env vars to command line parameters.
>
> Suggested-by: Masahiro Yamada <masa...@kernel.org>
> Reviewed-by: Fangrui Song <mas...@google.com>
> Signed-off-by: Nick Desaulniers <ndesau...@google.com>

Thanks for the update, I think this is much easier for non-CBL folks to
understand.

Reviewed-by: Nathan Chancellor <nat...@kernel.org>

[Fāng-ruì Sòng]

-i* options are for includes. -fintegrated-as is the canonical spelling.
Since -fintegrated-as is the default (for most llvm/lib/Target/
targets and the ones clang builds actually support),
it can even be deleted.

[Nick Desaulniers]

On Mon, Aug 2, 2021 at 2:06 PM Fāng-ruì Sòng <mas...@google.com> wrote:
>
> On Mon, Aug 2, 2021 at 11:39 AM Nick Desaulniers
> <ndesau...@google.com> wrote:
> > diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
> > new file mode 100644
> > index 000000000000..297932e973d4
> > --- /dev/null
> > +++ b/scripts/Makefile.clang
> > @@ -0,0 +1,14 @@
> > +ifneq ($(CROSS_COMPILE),)
> > +CLANG_FLAGS += --target=$(notdir $(CROSS_COMPILE:%-=%))
> > +endif
> > +ifeq ($(LLVM_IAS),1)
> > +CLANG_FLAGS += -integrated-as
>
> -i* options are for includes. -fintegrated-as is the canonical spelling.
> Since -fintegrated-as is the default (for most llvm/lib/Target/
> targets and the ones clang builds actually support),
> it can even be deleted.

It was made explicit by Masahiro in
git.kernel.org/linus/ba64beb17493a4bfec563100c86a462a15926f24
So I don't think we need to go back to the implicit default.

It's definitely nicer to use groupings rather than these raw prefixed
flags IMO. If you sent a patch for that I would approve of it.
Otherwise we don't really need to change this as this is how it's
worked in LLVM for as long as we've been able to build the kernel with
LLVM.
--
Thanks,
~Nick Desaulniers

[Masahiro Yamada]

If -fintegrated-as is preferred to -integrated-as,
please send a patch.
(on top of this series)


Thanks.

> Since -fintegrated-as is the default (for most llvm/lib/Target/
> targets and the ones clang builds actually support),
> it can even be deleted.
>
> > +else
> > +CLANG_FLAGS += -no-integrated-as
> > +GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))
> > +CLANG_FLAGS += --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
> > +endif
> > +CLANG_FLAGS += -Werror=unknown-warning-option
> > +KBUILD_CFLAGS += $(CLANG_FLAGS)
> > +KBUILD_AFLAGS += $(CLANG_FLAGS)
> > +export CLANG_FLAGS
> > --
> > 2.32.0.554.ge1b32706d8-goog
> >
>

> --
> You received this message because you are subscribed to the Google Groups "Clang Built Linux" group.
> To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-li...@googlegroups.com.
> To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/CAFP8O3Jc%3DiwzAQojgBZZzdT8iVBY9TO6GLTq%2B0vkXoo6L5JJ-A%40mail.gmail.com.

[Masahiro Yamada]

Is there any place where we rely on --prefix
to search for the linker?

In general, the compiler stops after generating an object
since it is passed with the -c option.
The linking stage is separated.

In the old days, VDSO was an exceptional case
where $(CC) was used as the linker driver, but
commit fe00e50b2db8c60e4ec90befad1f5bab8ca2c800 fixed it.

> +
> +If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` 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 LLVM_IAS=1
> +
> Supported Architectures
> -----------------------
>
> --
> 2.32.0.554.ge1b32706d8-goog
>

> --
> You received this message because you are subscribed to the Google Groups "Clang Built Linux" group.
> To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-li...@googlegroups.com.

> To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/20210802183910.1802120-4-ndesaulniers%40google.com.

[Masahiro Yamada]

When I was browsing the code, I noticed a nit.

I believe non-build targets such as
'make help', 'make clean' etc. should succeed
without any correct CROSS_COMPILE set
since they do not use the compiler at all.


The remaining architectures may fail to clean.

$ export LLVM=1
$ make ARCH=csky clean
scripts/Makefile.clang:17: *** Specify CROSS_COMPILE or add
'--target=' option to scripts/Makefile.clang. Stop.



I decided to not be worried about it because it
is fixable later.
I will pick it up unless a significant issue is found.

> +else
> +CLANG_FLAGS += --target=$(CLANG_TARGET_FLAGS)
> +endif # CLANG_TARGET_FLAGS
> +else
> CLANG_FLAGS += --target=$(notdir $(CROSS_COMPILE:%-=%))
> -endif
> +endif # CROSS_COMPILE
> +
> ifeq ($(LLVM_IAS),1)
> CLANG_FLAGS += -integrated-as
> else
> --
> 2.32.0.554.ge1b32706d8-goog
>

[Miguel Ojeda]

On Mon, Aug 2, 2021 at 8:39 PM 'Nick Desaulniers' via Clang Built
Linux <clang-bu...@googlegroups.com> wrote:
>
> We get constant feedback that the command line invocation of make is too
> long when compiling with LLVM. CROSS_COMPILE is helpful when a toolchain
> has a prefix of the target triple, or is an absolute path outside of
> $PATH.
>
> Since a Clang binary is generally multi-targeted, we can infer a given
> target from SRCARCH/ARCH. If CROSS_COMPILE is not set, simply set
> --target= for CLANG_FLAGS, KBUILD_CFLAGS, and KBUILD_AFLAGS based on
> $SRCARCH.
>
> Previously, we'd cross compile via:
> $ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make LLVM=1 LLVM_IAS=1
> Now:
> $ ARCH=arm64 make LLVM=1 LLVM_IAS=1
>
> For native builds (not involving cross compilation) we now explicitly
> specify a target triple rather than rely on the implicit host triple.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1399
> Suggested-by: Arnd Bergmann <ar...@kernel.org>
> Suggested-by: Linus Torvalds <torv...@linux-foundation.org>
> Suggested-by: Masahiro Yamada <masa...@kernel.org>
> Suggested-by: Nathan Chancellor <nat...@kernel.org>
> Acked-by: Arnd Bergmann <ar...@kernel.org>
> Reviewed-by: Nathan Chancellor <nat...@kernel.org>
> Signed-off-by: Nick Desaulniers <ndesau...@google.com>

Acked-by: Miguel Ojeda <oj...@kernel.org>

Cheers,
Miguel

[Nick Desaulniers]

On Thu, Aug 5, 2021 at 6:58 AM Masahiro Yamada <masa...@kernel.org> wrote:
>
> On Tue, Aug 3, 2021 at 3:39 AM 'Nick Desaulniers' via Clang Built
> Linux <clang-bu...@googlegroups.com> wrote:
> > diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
> > index b18401d2ba82..f8a360958f4c 100644
> > --- a/Documentation/kbuild/llvm.rst
> > +++ b/Documentation/kbuild/llvm.rst

> > @@ -63,6 +63,23 @@ They can be enabled individually. The full list of the parameters: ::
> > Currently, the integrated assembler is disabled by default. You can pass
> > ``LLVM_IAS=1`` to enable 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.
>
>
> Is there any place where we rely on --prefix
> to search for the linker?
>
> In general, the compiler stops after generating an object
> since it is passed with the -c option.
> The linking stage is separated.
>
> In the old days, VDSO was an exceptional case
> where $(CC) was used as the linker driver, but
> commit fe00e50b2db8c60e4ec90befad1f5bab8ca2c800 fixed it.

See my previous reply to Fangrui.
https://lore.kernel.org/lkml/CAKwvOdnK=SUm1_--tcLRO3LVeXd_2...@mail.gmail.com/

To be more specific, I believe this is still a problem for ppc vdso.
https://github.com/ClangBuiltLinux/linux/issues/774

I had sent patches for that, but binutils 2.26 would crash (IIUC,
newer GNU binutils are ok). See this thread:
https://lore.kernel.org/lkml/b2066ccd-2b81-6032...@csgroup.eu/

So "we'd prefer the linker was used as the driver, but there's at
least one place I know of in the tree where that's not currently the
case."
--
Thanks,
~Nick Desaulniers

[Nick Desaulniers]

Also, I think the CC_CAN_LINK functionality also fits the bill.
https://github.com/ClangBuiltLinux/linux/issues/1290
--
Thanks,
~Nick Desaulniers

[Masahiro Yamada]

Ah, this is it.

Thanks.

[Masahiro Yamada]

On Tue, Aug 3, 2021 at 3:39 AM Nick Desaulniers <ndesau...@google.com> wrote:
>
> We get constant feedback that the command line invocation of make is too
> long. CROSS_COMPILE is helpful when a toolchain has a prefix of the
> target triple, or is an absolute path outside of $PATH, but it's mostly
> redundant for a given ARCH.
>
> Instead, let's infer --target from SRCARCH, and move some flag handling into a
> new file included from the top level Makefile.

Series, applied to linux-kbuild.
Thanks.