linux-kernel - Re: [PATCH] LoongArch: Make -mstrict-align be configurable

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Date:   Mon, 06 Feb 2023 19:18:47 +0800
From:   Xi Ruoyao <xry111@...111.site>
To:     Jianmin Lv <lvjianmin@...ngson.cn>,
        WANG Xuerui <kernel@...0n.name>,
        Huacai Chen <chenhuacai@...ngson.cn>,
        Arnd Bergmann <arnd@...db.de>,
        Huacai Chen <chenhuacai@...nel.org>
Cc:     loongarch@...ts.linux.dev, linux-arch@...r.kernel.org,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Guo Ren <guoren@...nel.org>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] LoongArch: Make -mstrict-align be configurable

On Mon, 2023-02-06 at 18:24 +0800, Jianmin Lv wrote:
> Hi, Xuerui
> 
> I think the kernels produced with and without -mstrict-align have mainly 
> following differences:
> - Diffirent size. I build two kernls (vmlinux), size of kernel with 
> -mstrict-align is 26533376 bytes and size of kernel without 
> -mstrict-align is 26123280 bytes.
> - Diffirent performance. For example, in kernel function jhash(), the 
> assemble code slices with and without -mstrict-align are following:

But there are still questions remaining:

(1) Is the difference contributed by a bad code generation of GCC?  If
true, it's better to improve GCC before someone starts to build a distro
for LA264 as it would benefit the user space as well.

(2) Is there some "big bad unaligned access loop" on a hot spot in the
kernel code?  If true, it may be better to just refactor the C code
because doing so will benefit all ports, not only LoongArch.  Otherwise,
it may be unworthy to optimize for some cold paths.

-- 
Xi Ruoyao <xry111@...111.site>
School of Aerospace Science and Technology, Xidian University