BACKPORT: kbuild: support LLVM=1 to switch the default tools to Clang/LLVM

commit a0d1c951ef08ed24f35129267e3595d86f57f5d3 upstream.

As Documentation/kbuild/llvm.rst implies, building the kernel with a
full set of LLVM tools gets very verbose and unwieldy.

Provide a single switch LLVM=1 to use Clang and LLVM tools instead
of GCC and Binutils. You can pass it from the command line or as an
environment variable.

Please note LLVM=1 does not turn on the integrated assembler. You need
to pass LLVM_IAS=1 to use it. When the upstream kernel is ready for the
integrated assembler, I think we can make it default.

We discussed what we need, and we agreed to go with a simple boolean
flag that switches both target and host tools:

  https://lkml.org/lkml/2020/3/28/494
  https://lkml.org/lkml/2020/4/3/43

Some items discussed, but not adopted:

- LLVM_DIR

  When multiple versions of LLVM are installed, I just thought supporting
  LLVM_DIR=/path/to/my/llvm/bin/ might be useful.

  CC      = $(LLVM_DIR)clang
  LD      = $(LLVM_DIR)ld.lld
    ...

  However, we can handle this by modifying PATH. So, we decided to not do
  this.

- LLVM_SUFFIX

  Some distributions (e.g. Debian) package specific versions of LLVM with
  naming conventions that use the version as a suffix.

  CC      = clang$(LLVM_SUFFIX)
  LD      = ld.lld(LLVM_SUFFIX)
    ...

  will allow a user to pass LLVM_SUFFIX=-11 to use clang-11 etc.,
  but the suffixed versions in /usr/bin/ are symlinks to binaries in
  /usr/lib/llvm-#/bin/, so this can also be handled by PATH.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com> # build
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
[nd: conflict in exported vars list from not backporting commit
 e83b9f55448a ("kbuild: add ability to generate BTF type info for vmlinux")]
[nd: hunk against Documentation/kbuild/kbuild.rst dropped due to not backporting
 commit cd238effefa2 ("docs: kbuild: convert docs to ReST and rename to *.rst")]
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry-picked from commit 62353048e2d4e398cfc22dbee3e1a7f6f535dcc1)
Change-Id: Ib62c20d4d01b0b95bdcd5e73fa18f32cdb8f585c

Documentation/kbuild/kbuild.txt

@@ -238,3 +238,8 @@ KBUILD_VMLINUX_MAIN
 All object files for the main part of vmlinux.
 KBUILD_VMLINUX_INIT and KBUILD_VMLINUX_MAIN together specify
 all the object files used to link vmlinux.
+
+LLVM
+----
+If this variable is set to 1, Kbuild will use Clang and LLVM utilities instead
+of GCC and GNU binutils to build the kernel.

Makefile

@@ -301,8 +301,14 @@ CONFIG_SHELL := $(shell if [ -x "$$BASH" ]; then echo $$BASH; \
 	  else if [ -x /bin/bash ]; then echo /bin/bash; \
 	  else echo sh; fi ; fi)
 
+ifneq ($(LLVM),)
+HOSTCC       = clang
+HOSTCXX      = clang++
+else
 HOSTCC       = gcc
 HOSTCXX      = g++
+endif
+
 HOSTCFLAGS   := -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 -fomit-frame-pointer -std=gnu89
 HOSTCXXFLAGS = -O2
 
@@ -337,16 +343,28 @@ scripts/Kbuild.include: ;
 include scripts/Kbuild.include
 
 # Make variables (CC, etc...)
+LDGOLD		= $(CROSS_COMPILE)ld.gold
+CPP		= $(CC) -E
+ifneq ($(LLVM),)
+CC		= clang
+LD		= ld.lld
+AR		= llvm-ar
+NM		= llvm-nm
+OBJCOPY	= llvm-objcopy
+OBJDUMP	= llvm-objdump
+READELF	= llvm-readelf
+OBJSIZE	= llvm-size
+STRIP		= llvm-strip
+else
 AS		= $(CROSS_COMPILE)as
 LD		= $(CROSS_COMPILE)ld
 CC		= $(CROSS_COMPILE)gcc
-LDGOLD		= $(CROSS_COMPILE)ld.gold
-CPP		= $(CC) -E
 AR		= $(CROSS_COMPILE)ar
 NM		= $(CROSS_COMPILE)nm
 STRIP		= $(CROSS_COMPILE)strip
 OBJCOPY		= $(CROSS_COMPILE)objcopy
 OBJDUMP		= $(CROSS_COMPILE)objdump
+endif
 DTC		= scripts/dtc/dtc
 AWK		= awk
 GENKSYMS	= scripts/genksyms/genksyms