Messages in this thread | | | Date | Fri, 3 Nov 2017 11:07:39 -0700 | From | Matthias Kaehlcke <> | Subject | Re: [PATCH 00/15] Add support for clang LTO |
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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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