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

[Lulu Cheng <chenglulu@loongson.cn>]

在 2022/8/5 上午9:28, Xi Ruoyao 写道:
> On Fri, 2022-08-05 at 09:05 +0800, Lulu Cheng wrote:
>> I'm working on the implementation of specifing attributes of variables for other architectures.
>> If the address is obtained through the GOT table and 4 instructions, there is not much difference in performance.
> In this case I still prefer a GOT table entry because for 4-instruction
> absolute addressing sequence we'll need to implement 4 new relocation
> types in the kernel module loader.

If it is accessed through the GOT table, dynamic relocation is required 
when the module is loaded. And accessing the got table may have a cache 
miss.

>> Is it more reasonable for us to refer to the implementation of the model attribute under the IA64 architecture?
> Maybe we can use "model(got)", "model(abs)", "model(pcrel)" etc.

We have a set of instruction implementations that can get a relative pc 
64-bit offset:

   "pcalau12i %1,%%pc_hi20(%3);"

   "addi.d %2,$r0,%%pc_lo12(%3);"
   "lu32i.d %2,%%pc64_lo20(%3);"
   "lu52i.d %2,%2,%%pc64_hi12(%3);"

   "add.d %1,%1,%2;",

This set of instructions can be selected according to the size of the 
offset:

   "pcalau12i %1,%%pc_hi20(%3);"

   "addi.d %2,$r0,%%pc_lo12(%3);"

   "lu32i.d %2,%%pc64_lo20(%3);"

   "add.d %1,%1,%2;",

for offset within signed 52 bits.

or

   "pcalau12i %1,%%pc_hi20(%3);"

   "addi.d %2,$r0,%%pc_lo12(%3);"
   "lu32i.d %2,%%pc64_lo20(%3);"
   "lu52i.d %2,%2,%%pc64_hi12(%3);"

   "add.d %1,%1,%2;"

for offset within signed 64 bits.

So my idea is "model(normal)","model (large)" etc.

>> I will compare the performance of the two soon. Do you know the approximate release date of GCC 12.2?
>> I also want to fix this before 12.2 is released.
> GCC 12.2 rc1 will be frozen on Aug 12th.
>