Re: [PATCH v2 2/2] LoongArch: When the code model is extreme, the symbol address is obtained through macro instructions regardless of the value of -mexplicit-relocs.

[chenglulu <chenglulu@loongson.cn>]

在 2024/1/5 下午7:55, Xi Ruoyao 写道:
> On Fri, 2024-01-05 at 18:25 +0800, Xi Ruoyao wrote:
>> On Fri, 2024-01-05 at 17:57 +0800, chenglulu wrote:
>>> 在 2024/1/5 下午4:37, Xi Ruoyao 写道:
>>>> On Fri, 2024-01-05 at 11:40 +0800, Lulu Cheng wrote:
>>>>>    bool
>>>>>    loongarch_explicit_relocs_p (enum loongarch_symbol_type type)
>>>>>    {
>>>>> +  /* Instructions pcalau12i, addi.d, lu32i.d and lu52i.d must be adjancent
>>>>> +     so that the linker can infer the PC of pcalau12i to apply relocations
>>>>> +     to lu32i.d and lu52i.d.  Otherwise, the results would be incorrect if
>>>>> +     these four instructions are not in the same 4KiB page.
>>>>> +     Therefore, macro instructions are used when cmodel=extreme.  */
>>>>> +  if (loongarch_symbol_extreme_p (type))
>>>>> +    return false;
>>>> I think this is a bit of strange.  With -mexplicit-relocs={auto,always}
>>>> we should still use explicit relocs, but coding all 4 instructions
>>>> altogether as
>>>>
>>>> "pcalau12i.d\t%1,%pc64_hi12(%2)\n\taddi.d\t%0,$r0,%pclo12(%2)\n\tlu32i.d\t%0,%pc64_lo20(%2)\n\tlu52i.d\t%0,%0,%pc64_hi12(%2)"
>>>>
>>>> Give me several hours trying to implement this...
>>>>
>>> I think there is no difference between macros and these instructions put
>>> together. If implement it in a split form, I think I can try it through
>>> TARGET_SCHED_MACRO_FUSION_PAIR_P
> We don't need to split the insn.  We can just add a "large insn"
> containing the assembly output we want.
>
> See the attached patch.  Note that TLS LE/LD/GD needs a fix too because
> they are basically an variation of GOT addressing.
>
> I've ran some small tests and now trying to bootstrap GCC with -
> mcmodel=extreme in BOOT_CFLAGS...
>
>> There is a difference:
>>
>> int x;
>> int t() { return x; }
>>
>> pcalau12i.d t0, %pc_hi20(x)
>> addi.d t1, r0, %pc_lo12(x)
>> lu32i.d t1, %pc64_lo20(x)
>> lu52i.d t1, t1, %pc64_hi12(x)
>> ldx.w a0, t0, t1
>>
>> is slightly better than
>>
>> pcalau12i.d t0, %pc_hi20(x)
>> addi.d t1, r0, %pc_lo12(x)
>> lu32i.d t1, %pc64_lo20(x)
>> lu52i.d t1, t1, %pc64_hi12(x)
>> addi.d t0, t0, t1
>> ld.w a0, t0, 0
>>
>> And generating macros when -mexplicit-relocs=always can puzzle people
>> (it says "always" :-\ ).
>>
Thumbs up! This method is much better than my method, I learned 
something! grateful!
But I still have to test the accuracy.