diff mbox series

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

Message ID 20210730201701.3146910-2-ndesaulniers@google.com (mailing list archive)
State New, archived
Headers show
Series infer --target from SRCARCH for CC=clang | expand

Commit Message

Nick Desaulniers July 30, 2021, 8:17 p.m. UTC 
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(-)

Comments

Linus Torvalds July 30, 2021, 9:54 p.m. UTC | #1 
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
Nick Desaulniers July 30, 2021, 9:58 p.m. UTC | #2 
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
diff mbox series

Patch

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