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
Subject: Re: [PATCH] LoongArch: Make -mstrict-align be configurable
Date: Mon, 06 Feb 2023 19:18:47 +0800	[thread overview]
Message-ID: <b1809500e4d55564a1084a3014fb9603ba3d1438.camel@xry111.site> (raw)
In-Reply-To: <5303aeda-5c66-ede6-b3ac-7d8ebd73ec70@loongson.cn>

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@xry111.site>
School of Aerospace Science and Technology, Xidian University