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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Fri, 4 Aug 2023 09:15:54 -0700
Message-ID: <CAKwvOdnk8ofgW+4D8HnFBqgoi49vkLHqsAMCtM=SpuKyJHVkaQ@mail.gmail.com>
Subject: Re: [PATCH] LoongArch: Drop -ffreestanding from CFLAGS
To:     Xi Ruoyao <xry111@xry111.site>
Cc:     Huacai Chen <chenhuacai@kernel.org>,
        WANG Xuerui <kernel@xen0n.name>,
        Nathan Chancellor <nathan@kernel.org>,
        loongarch@lists.linux.dev, llvm@lists.linux.dev,
        linux-kernel@vger.kernel.org, WANG Xuerui <git@xen0n.name>
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On Fri, Aug 4, 2023 at 8:56=E2=80=AFAM Xi Ruoyao <xry111@xry111.site> wrote=
:
>
> On Fri, 2023-08-04 at 08:46 -0700, Nick Desaulniers wrote:
> > > Not all processors support unaligned access, so we need the
> > > alternative mechanism to select memset/memcpy/memmove implementations=
.
> > > If remove -ffreestanding, the builtin implementation cannot be used o=
n
> > > all hardware.
> >
> > That sounds like a compiler bug in that compiler's implementation of
> > string.h builtins then; it should default to the safest implementation
> > (aligned accesses) until instructed otherwise.  Have you filed a bug
> > against your compiler vendor for which compiler you observe that
> > behavior from?
>
> AFAIK there is no such bug in GCC.
>
> But GCC indeed has a bug about builtin expansion: it generates really
> stupid code for __builtin_memcpy and friends.  See
> https://gcc.gnu.org/PR109465.  The bug is fixed for GCC 14, but GCC 14
> won't be released soon.

Perhaps -fno-builtin-* flags may be of help here, and more precise an
incision than the blunt -ffreestanding.

>
> > At the very least, there should be a comment above the addition of
> > -ffreestanding justifying why it's being used, probably with a link to
> > the above bug report.
> >
> > I would expect either -mcpu or perhaps some other -m flag to guide the
> > compiler when it is safe to emit memcpy (and friends) in terms of
> > unaligned access or not.
>
> It's controlled by -m{no-,}strict-align.  LoongArch kernel defaults to -
> mstrict-align, with this the compiler should not generate unaligned
> access.  -mno-strict-align is hidden behind CONFIG_EXPERT (FWIW I
> personally dislike the decision to hide it).

If GCC generates unaligned accesses in its implementation of
__builtin_memcpy when -mstrict-align is set, then that is a bug in GCC
and should be reported.

Clang users should not have to pay for such mistakes.

>
> --
> Xi Ruoyao <xry111@xry111.site>
> School of Aerospace Science and Technology, Xidian University


--=20
Thanks,
~Nick Desaulniers