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

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Message-Id: <b9f274ad-591b-40b5-9441-a45fe67b5b8d@app.fastmail.com>
Date:   Mon, 06 Feb 2023 14:22:17 +0100
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Jianmin Lv" <lvjianmin@...ngson.cn>,
        "Xi Ruoyao" <xry111@...111.site>,
        "WANG Xuerui" <kernel@...0n.name>,
        "Huacai Chen" <chenhuacai@...ngson.cn>,
        "Huacai Chen" <chenhuacai@...nel.org>
Cc:     loongarch@...ts.linux.dev, Linux-Arch <linux-arch@...r.kernel.org>,
        "Xuefeng Li" <lixuefeng@...ngson.cn>, guoren <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, Feb 6, 2023, at 14:13, Jianmin Lv wrote:
> On 2023/2/6 下午7:18, Xi Ruoyao wrote:
>> 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.
>> 
> AFAIK, GCC builds to produce unaligned-access-enabled target binary by 
> default (without -mstrict-align) for improving user space performance 
> (small size and runtime high performance), which is also based the fact 
> that the vast majority of LoongArch CPUs support unaligned-access.
>
>> (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.
>> 
> Frankly, I'm not sure if there is this kind of hot code in kernel, I 
> just see the difference from different kernel size and different 
> assemble code slice. And I'm afraid that it may be difficult to judge 
> whether it is reasonable hot code or not if exists.

Just look for CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS, this will
show you code locations that use different implementations based on
whether the kernel should run on CPUs without unaligned access or
not.

      Arnd