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    SubjectRe: [PATCH 00/15] Add support for clang LTO
    Hi Mark,

    El Fri, Nov 03, 2017 at 05:51:56PM +0000 Mark Rutland ha dit:

    > 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?

    In general upstream should be a good starting point.

    > 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

    This should be fixed by https://patchwork.kernel.org/patch/10039329/
    and https://patchwork.kernel.org/patch/10030583/

    Unfortunately we didn't encounter these earlier, in Chrome OS this
    was/is hidden by invoking an architecture aware wrapper instead of
    bare clang :(

    > 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

    This is fixed in newer clang versions, otherwise you need

    [PATCH 07/15] arm64: use -mno-implicit-float instead of
    -mgeneral-regs-only

    of this series.

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