[v3,0/2] infer CROSS_COMPILE from SRCARCH for CC=clang LLVM_IAS=1

Message

Nick Desaulniers July 29, 2021, 4:50 p.m. UTC

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 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 (2):
  Makefile: move initial clang flag handling into scripts/Makefile.clang
  Makefile: infer CROSS_COMPILE from SRCARCH for CC=clang LLVM_IAS=1

 Documentation/kbuild/llvm.rst |  6 +++++
 MAINTAINERS                   |  1 +
 Makefile                      | 15 +------------
 scripts/Makefile.clang        | 42 +++++++++++++++++++++++++++++++++++
 4 files changed, 50 insertions(+), 14 deletions(-)
 create mode 100644 scripts/Makefile.clang


base-commit: 27932b6a2088eac7a5afa5471963b926cfbb4de7