Hi,

On Fri, Nov 03, 2017 at 10:11:44AM -0700, Sami Tolvanen wrote:

This series adds build system support for compiling the kernel with clang
Link Time Optimization (LTO), using GNU gold with the LLVMgold plug-in
for linking. Some background for clang's LTO support is available here:

  https://llvm.org/docs/LinkTimeOptimization.html

With -flto, clang produces LLVM bitcode instead of object files, and
the compilation to native code happens at link time. In addition, clang
cannot use an external assembler for inline assembly when LTO is enabled,
which causes further compatibility issues.

The patches in this series remove intermediate linking steps when LTO is
used, postpone processing done on object files until after the LTO link
step, add workarounds for GNU gold incompatibilities, and address inline
assembly incompatibilities for arm64.

These changes allow arm64 defconfig to be compiled with LTO, but other
architectures are not enabled until compatibility issues have been
addressed. In particular, x86 inline assembly doesn't currently compile
with clang's integrated assembler due to this LLVM bug:

  https://bugs.llvm.org/show_bug.cgi?id=24487

Due to recent bug fixes in the toolchain, it's recommended to use clang
5.0 or later, and GNU gold from binutils 2.27 or later

Which tree are you testing with?

I had a go building the arm64 for/next/core branch defconfig using clang
5.0.0 from llvm.org, with the Linaro 17.05 toolchain's binutils
(2.27.0.20161019), and I hit what appears to be a compiler bug quite
quickly.

I had to create an aarch64-linux-gnu-clang wrapper, too. I'm not sure if
there's build system help to avoid needing that?

----
[mark at lakrids:~/src/linux]% uselinaro 17.05 usellvm 5.0.0 make CROSS_COMPILE=aarch64-linux-gnu- CC=clang  ARCH=arm64 
Segmentation fault
arch/arm64/Makefile:27: ld does not support --fix-cortex-a53-843419; kernel may be susceptible to erratum
arch/arm64/Makefile:48: Detected assembler with broken .inst; disassembly will be unreliable
  CHK     include/config/kernel.release
  CHK     include/generated/uapi/linux/version.h
  CHK     include/generated/utsrelease.h
  CHK     include/generated/bounds.h
  CHK     include/generated/timeconst.h
  CHK     include/generated/asm-offsets.h
  CALL    scripts/checksyscalls.sh
  CHK     scripts/mod/devicetable-offsets.h
  CHK     include/generated/compile.h
  CC      arch/arm64/crypto/aes-ce-cipher.o
fatal error: error in backend: Do not know how to split the result of this operator!

clang-5.0: error: clang frontend command failed with exit code 70 (use -v to see invocation)
clang version 5.0.0 (tags/RELEASE_500/final)
Target: aarch64--linux-gnu
Thread model: posix
InstalledDir: /mnt/data/opt/toolchain/llvm/5.0.0/bin
clang-5.0: note: diagnostic msg: PLEASE submit a bug report to http://llvm.org/bugs/ and include the crash backtrace, preprocessed source, and associated run script.
clang-5.0: note: diagnostic msg: 
********************

PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
Preprocessed source(s) and associated run script(s) are located at:
clang-5.0: note: diagnostic msg: /tmp/aes-ce-cipher-7850ed.c
clang-5.0: note: diagnostic msg: /tmp/aes-ce-cipher-7850ed.sh
clang-5.0: note: diagnostic msg: 

********************
scripts/Makefile.build:313: recipe for target 'arch/arm64/crypto/aes-ce-cipher.o' failed
make[1]: *** [arch/arm64/crypto/aes-ce-cipher.o] Error 70
Makefile:1019: recipe for target 'arch/arm64/crypto' failed
make: *** [arch/arm64/crypto] Error 2
----

Thanks,
Mark.