[PATCH 2/2] Makefile: drop CROSS_COMPILE for LLVM=1 LLVM_IAS=1

[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.

If CROSS_COMPILE is not set, simply set --target= for CLANG_FLAGS,
KBUILD_CFLAGS, and KBUILD_AFLAGS based on $ARCH.

Previously, we'd cross compile via:
$ ARCH=arm64 CROSS_COMPILE=aarch64-linxu-gnu make LLVM=1 LLVM_IAS=1
Now:
$ ARCH=arm64 make LLVM=1 LLVM_IAS=1

Link: https://github.com/ClangBuiltLinux/linux/issues/1399
Suggested-by: Arnd Bergmann <arnd@kernel.org>
Suggested-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
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/.

 Documentation/kbuild/llvm.rst |  5 +++++
 scripts/Makefile.clang        | 38 +++++++++++++++++++++++++++++++++--
 2 files changed, 41 insertions(+), 2 deletions(-)

diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
index b18401d2ba82..80c63dd9a6d1 100644
--- a/Documentation/kbuild/llvm.rst
+++ b/Documentation/kbuild/llvm.rst
@@ -46,6 +46,11 @@ example: ::
 
 	clang --target=aarch64-linux-gnu foo.c
 
+When both ``LLVM=1`` and ``LLVM_IAS=1`` are used, ``CROSS_COMPILE`` becomes
+unnecessary and can be inferred from ``ARCH``. Example: ::
+
+	ARCH=arm64 make LLVM=1 LLVM_IAS=1
+
 LLVM Utilities
 --------------
 
diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
index 297932e973d4..a79088797a50 100644
--- a/scripts/Makefile.clang
+++ b/scripts/Makefile.clang
@@ -1,6 +1,40 @@
-ifneq ($(CROSS_COMPILE),)
+# Individual arch/{arch}/Makfiles should use -EL/-EB to set intended endianness
+# and -m32/-m64 to set word size based on Kconfigs instead of relying on the
+# target triple.
+ifeq ($(CROSS_COMPILE),)
+ifneq ($(LLVM),)
+ifeq ($(LLVM_IAS),1)
+ifeq ($(ARCH),arm)
+CLANG_FLAGS	+= --target=arm-linux-gnueabi
+else ifeq ($(ARCH),arm64)
+CLANG_FLAGS	+= --target=aarch64-linux-gnu
+else ifeq ($(ARCH),hexagon)
+CLANG_FLAGS	+= --target=hexagon-linux-gnu
+else ifeq ($(ARCH),i386)
+CLANG_FLAGS	+= --target=i686-linux-gnu
+else ifeq ($(ARCH),m68k)
+CLANG_FLAGS	+= --target=m68k-linux-gnu
+else ifeq ($(ARCH),mips)
+CLANG_FLAGS	+= --target=mipsel-linux-gnu
+else ifeq ($(ARCH),powerpc)
+CLANG_FLAGS	+= --target=powerpc64le-linux-gnu
+else ifeq ($(ARCH),riscv)
+CLANG_FLAGS	+= --target=riscv64-linux-gnu
+else ifeq ($(ARCH),s390)
+CLANG_FLAGS	+= --target=s390x-linux-gnu
+else ifeq ($(ARCH),x86)
+CLANG_FLAGS	+= --target=x86_64-linux-gnu
+else ifeq ($(ARCH),x86_64)
+CLANG_FLAGS	+= --target=x86_64-linux-gnu
+else
+$(error Specify CROSS_COMPILE or add '--target=' option to scripts/Makefile.clang)
+endif # ARCH
+endif # LLVM_IAS
+endif # LLVM
+else
 CLANG_FLAGS	+= --target=$(notdir $(CROSS_COMPILE:%-=%))
-endif
+endif # CROSS_COMPILE
+
 ifeq ($(LLVM_IAS),1)
 CLANG_FLAGS	+= -integrated-as
 else
-- 
2.32.0.93.g670b81a890-goog