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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Mon, 25 Jul 2022 07:42:07 +0200
Message-ID: <CAMj1kXHTf4f+onREnWhYk-vYXkPaQtPpZz+SqFCP+Ey7+bOXkQ@mail.gmail.com>
Subject: Re: [linux-stable-rc:linux-5.15.y 6874/8723] arch/arm/lib/xor-neon.c:30:2:
 warning: This code requires at least version 4.6 of GCC
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     kernel test robot <lkp@intel.com>,
        "Justin M. Forbes" <jforbes@fedoraproject.org>,
        llvm@lists.linux.dev, kbuild-all@lists.01.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Jason A. Donenfeld" <zx2c4@kernel.org>
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On Mon, 25 Jul 2022 at 00:40, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> Hi Ard,
>
> On Mon, Jul 25, 2022 at 04:14:48AM +0800, kernel test robot wrote:
> > All warnings (new ones prefixed by >>):
> >
> > >> arch/arm/lib/xor-neon.c:30:2: warning: This code requires at least v=
ersion 4.6 of GCC [-W#warnings]
> >    #warning This code requires at least version 4.6 of GCC
> >     ^
> >    1 warning generated.
> >
> >
> > vim +30 arch/arm/lib/xor-neon.c
> >
> > 01956597cbc46d Ard Biesheuvel 2013-05-17  16
> > 01956597cbc46d Ard Biesheuvel 2013-05-17  17  /*
> > 01956597cbc46d Ard Biesheuvel 2013-05-17  18   * Pull in the reference =
implementations while instructing GCC (through
> > 01956597cbc46d Ard Biesheuvel 2013-05-17  19   * -ftree-vectorize) to a=
ttempt to exploit implicit parallelism and emit
> > 01956597cbc46d Ard Biesheuvel 2013-05-17  20   * NEON instructions.
> > 01956597cbc46d Ard Biesheuvel 2013-05-17  21   */
> > 01956597cbc46d Ard Biesheuvel 2013-05-17  22  #if __GNUC__ > 4 || (__GN=
UC__ =3D=3D 4 && __GNUC_MINOR__ >=3D 6)
> > 01956597cbc46d Ard Biesheuvel 2013-05-17  23  #pragma GCC optimize "tre=
e-vectorize"
> > 01956597cbc46d Ard Biesheuvel 2013-05-17  24  #else
> > 01956597cbc46d Ard Biesheuvel 2013-05-17  25  /*
> > 01956597cbc46d Ard Biesheuvel 2013-05-17  26   * While older versions o=
f GCC do not generate incorrect code, they fail to
> > 01956597cbc46d Ard Biesheuvel 2013-05-17  27   * recognize the parallel=
 nature of these functions, and emit plain ARM code,
> > 01956597cbc46d Ard Biesheuvel 2013-05-17  28   * which is known to be s=
lower than the optimized ARM code in asm-arm/xor.h.
> > 01956597cbc46d Ard Biesheuvel 2013-05-17  29   */
> > 01956597cbc46d Ard Biesheuvel 2013-05-17 @30  #warning This code requir=
es at least version 4.6 of GCC
> > 01956597cbc46d Ard Biesheuvel 2013-05-17  31  #endif
> > 01956597cbc46d Ard Biesheuvel 2013-05-17  32
>
> Does this file need a depends on =E2=89=A54.6 thing in Kconfig? Or is som=
ething
> else happening here since 4.6 isn't even supported by the kernel these
> days?
>

Hi Jason,

This is explained/fixed in

commit a69cb445f7d129abf7c50d48c8a8eca7c8d5df15
Author: Ard Biesheuvel <ardb@kernel.org>
Date:   Sat Feb 5 16:23:46 2022 +0100

    crypto: arm/xor - make vectorized C code Clang-friendly

    The ARM version of the accelerated XOR routines are simply the 8-way C
    routines passed through the auto-vectorizer with SIMD codegen enabled.
    This used to require GCC version 4.6 at least, but given that 5.1 is no=
w
    the baseline, this check is no longer necessary, and actually
    misidentifies Clang as GCC < 4.6 as Clang defines the GCC major/minor a=
s
    well, but makes no attempt at doing this in a way that conveys feature
    parity with a certain version of GCC (which would not be a great idea i=
n
    the first place).

which landed in v5.18. Note that the patch depends on its parent,
which touches many different architectures, so it may not be
backportable cleanly.