linux-kernel - Re: [PATCH 00/15] Add support for clang LTO

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Message-ID: <20171103180739.GG96615@google.com>
Date:   Fri, 3 Nov 2017 11:07:39 -0700
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Hackmann <ghackmann@...gle.com>
Subject: Re: [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@...rids:~/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.