Re: 回复：[PATCH v5] LoongArch: add movable attribute

[Lulu Cheng <chenglulu@loongson.cn>]

在 2022/8/5 下午3:41, WANG Xuerui 写道:
> On 2022/8/5 15:19, Lulu Cheng wrote:
>>
>>
>> 在 2022/8/5 下午2:03, Xi Ruoyao 写道:
>>> On Fri, 2022-08-05 at 12:01 +0800, Lulu Cheng wrote:
>>>> 在 2022/8/5 上午11:45, Xi Ruoyao 写道:
>>>>
>>>>> On Fri, 2022-08-05 at 11:34 +0800, Xi Ruoyao via Gcc-patches wrote:
>>>>>
>>>>>> Or maybe we should just use a PC-relative addressing with 4 instructions
>>>>>> instead of GOT for -fno-PIC?
>>>>> Not possible, Glibc does not support R_LARCH_PCALA* relocations in
>>>>> ld.so.  So we still need a -mno-got (or something) option to disable GOT
>>>>> for special cases like the kernel.
>>>>>
>>>>>> Both way consumes 16 bytes (4 instructions
>>>>>> for PC-relative, 2 instructions and a 64-bit GOT entry for GOT) and PC-
>>>>>> relative may be more cache friendly.   But such a major change cannot be
>>>>>> backported for 12.2 IMO.
>>>> I'm very sorry, my understanding of the precpu variable is wrong,
>>>> I just read the code of the kernel you submitted, this precpu variable
>>>> not only has a large offset but also has an uncertain address when compiling,
>>>> so no matter whether it is addressed with pcrel Still got addressing needs
>>>> dynamic relocation when loading. It seems that accessing through the got table
>>>> is a better choice.
>>>>
>>>> The name movable is also very vivid to describe this function in the kernel,
>>>> indicating that the address of the variable can be changed at will.
>>>>
>>>> But this name is more difficult to understand in gcc, I have no opinion on other,
>>>> can this name be changed?
>>> Yes, we don't need to be compatible with old vendor compiler IMO.
>>>
>>>
>>> "force_got_access" as Xuerui suggested?
>> Compared with these names, I think addr_global is better.
>
> Actually if "model(...)" can be implemented I'd prefer a descriptive 
> word/phrase inside model(). Because it may well be the case that more 
> peculiar ways of accessing some special data will have to be supported 
> in the future, and all of them are kind of "data models" so we'd be 
> able to nicely group them with model(...).
>
> Otherwise I actually don't have a particularly strong opinion, aside 
> from "movable" which IMO should definitely not be taken.
>
>
I think the model of precpu is not very easy to describe. 
model(got)?model(global)? I also want to use attribute model and 
-mcmodel together, but this is just an initial idea, what do you think?