[PATCH v6 0/3] infer --target from SRCARCH for CC=clang

[Nick Desaulniers <ndesaulniers@google.com>]

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