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

[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

[PATCH v4 1/2] Makefile: infer --target from ARCH for CC=clang

[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 <arnd@kernel.org>
Suggested-by: Nathan Chancellor <nathan@kernel.org>
Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Arnd Bergmann <arnd@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@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

[PATCH v4 2/2] Documentation/llvm: update CROSS_COMPILE inferencing

[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 <masahiroy@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@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

Re: [PATCH v4 1/2] Makefile: infer --target from ARCH for CC=clang

[Linus Torvalds]

On Fri, Jul 30, 2021 at 1:17 PM Nick Desaulniers
<ndesaulniers@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

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

[Nick Desaulniers]

On Fri, Jul 30, 2021 at 1:17 PM Nick Desaulniers
<ndesaulniers@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 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 ***

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

Re: [PATCH v4 1/2] Makefile: infer --target from ARCH for CC=clang

[Nick Desaulniers]

On Fri, Jul 30, 2021 at 2:54 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Fri, Jul 30, 2021 at 1:17 PM Nick Desaulniers
> <ndesaulniers@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