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SubjectRe: [PATCH] LoongArch: Make -mstrict-align be configurable
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On 2023/2/6 下午9:22, Arnd Bergmann wrote:
> 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
>

Got it, thank you very much, I greped
CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS and found many matched cases
including driver, lib, net and so on, it seems that it's reasonable to
use high performance way for CPUs with HAVE_EFFICIENT_UNALIGNED_ACCESS
configured.


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