Message-ID: <74ffc2c05475c6af391b87a06df477ae390cc45c.camel@xry111.site>
Subject: Re: [PATCH] LoongArch: Make -mstrict-align be configurable
From:   Xi Ruoyao <xry111@xry111.site>
To:     Jianmin Lv <lvjianmin@loongson.cn>,
        WANG Xuerui <kernel@xen0n.name>,
        Huacai Chen <chenhuacai@loongson.cn>,
        Arnd Bergmann <arnd@arndb.de>,
        Huacai Chen <chenhuacai@kernel.org>
Cc:     loongarch@lists.linux.dev, linux-arch@vger.kernel.org,
        Xuefeng Li <lixuefeng@loongson.cn>,
        Guo Ren <guoren@kernel.org>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        linux-kernel@vger.kernel.org
Date:   Mon, 06 Feb 2023 21:30:55 +0800
In-Reply-To: <3b17d229-bad4-e6a0-9055-c585dd5a62e4@loongson.cn>
References: <20230202084238.2408516-1-chenhuacai@loongson.cn>
         <5fc85453-1e2c-1f00-7879-1b5fa318c78a@xen0n.name>
         <5303aeda-5c66-ede6-b3ac-7d8ebd73ec70@loongson.cn>
         <b1809500e4d55564a1084a3014fb9603ba3d1438.camel@xry111.site>
         <3b17d229-bad4-e6a0-9055-c585dd5a62e4@loongson.cn>
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On Mon, 2023-02-06 at 21:13 +0800, Jianmin Lv wrote:
> > (1) Is the difference contributed by a bad code generation of GCC?=C2=
=A0 If
> > true, it's better to improve GCC before someone starts to build a distr=
o
> > for LA264 as it would benefit the user space as well.
> >=20
> AFAIK, GCC builds to produce unaligned-access-enabled target binary by
> default (without -mstrict-align) for improving user space performance=20
> (small size and runtime high performance), which is also based the fact=
=20
> that the vast majority of LoongArch CPUs support unaligned-access.

I mean: if someone starts to build a distro for a less-capable LoongArch
processor, (s)he will need an entire user space compiled with -mstrict-
align.  So it would be better to start preparation now.

And it's likely (s)he will either submit a GCC patch to make GCC
enable/disable -mstrict-align based on the -march=3D (--with-arch at
configure time) value, or hack GCC to enable -mstrict-align by default
for the distro.  So I think we'll also need:

> +ifdef CONFIG_ARCH_STRICT_ALIGN may enable strict align by default.
>  # Don't emit unaligned accesses.
>  # Not all LoongArch cores support unaligned access, and as kernel we can=
't
>  # rely on others to provide emulation for these accesses.
>  KBUILD_CFLAGS +=3D $(call cc-option,-mstrict-align)
  +else
  +# Distros designed for running on both kind of processors may disable
  +# strict align by default, but the user may want a no-strict-align=C2=A0
  +# kernel for his/her specific hardware.
   KBUILD_CFLAGS +=3D $(call cc-option,-mno-strict-align)
> +endif

--=20
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University