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@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.
