[PATCH] LoongArch: Update comment about bottom bit usage in TLS GOT construction

[mengqinggang]

These comments mainly explain the issue of another implementation method.


If the code like below, TLS IE and GD processed separately, we need two 
bits as flags.

case  R_LARCH_TLS_IE_PC_HI20:
     if ((got_off & 1) == 0)
         {
             ...
         }

case R_LARCH_TLS_GD_PC_HI20:
     if ((got_off & 2) == 0)
         {
             ...
         }


在 2023/12/29 下午5:11, Tatsuyuki Ishi 写道:
>> On Dec 29, 2023, at 17:50, mengqinggang <mengqinggang@loongson.cn> wrote:
>>
>> At the beginning of implementation, we try to IE and GD only generate their own GOT entry,
>> so we need two bits to do the flags.
> I checked the tree back at the date the comment was introduced (6d13722a: "bfd: Add supported for LoongArch new relocations.”). Even at that time, there was no |= 2 accesses, so if my understanding is correct, this comment was wrong from the beginning.
>
> Could you clarify what you mean here, in particular what “At the beginning of implementation” refer to?
>
>> 在 2023/12/28 下午10:58, Tatsuyuki Ishi 写道:
>>> The GOT assignment logic, likely copied from another backend, does not in
>>> fact require multiple flags to construct multiple type of slots. Instead,
>>> all slots are initialized on the first relocation encountered. Update
>>> comment to avoid confusion.
>>> ---
>>>   bfd/elfnn-loongarch.c | 17 +++++++----------
>>>   1 file changed, 7 insertions(+), 10 deletions(-)
>>>
>>> diff --git a/bfd/elfnn-loongarch.c b/bfd/elfnn-loongarch.c
>>> index 64c34e99261..d66dcee1100 100644
>>> --- a/bfd/elfnn-loongarch.c
>>> +++ b/bfd/elfnn-loongarch.c
>>> @@ -3656,16 +3656,13 @@ loongarch_elf_relocate_section (bfd *output_bfd, struct bfd_link_info *info,
>>>   	  relocation -= elf_hash_table (info)->tls_sec->vma;
>>>   	  break;
>>>   -	/* TLS IE LD/GD process separately is troublesome.
>>> -	   When a symbol is both ie and LD/GD, h->got.off |= 1
>>> -	   make only one type be relocated.  We must use
>>> -	   h->got.offset |= 1 and h->got.offset |= 2
>>> -	   diff IE and LD/GD.  And all (got_off & (~(bfd_vma)1))
>>> -	   (IE LD/GD and reusable GOT reloc) must change to
>>> -	   (got_off & (~(bfd_vma)3)), beause we use lowest 2 bits
>>> -	   as a tag.
>>> -	   Now, LD and GD is both GOT_TLS_GD type, LD seems to
>>> -	   can be omitted.  */
>>> +	/* The bottom bit of h->got.offset is (ab)used as a flag.
>>> +	   Upon encountering the first TLS relocation, we initialize all GOT
>>> +	   slots for the corresponding symbol and set the bottom bit to 1.
>>> +
>>> +	   The second and subsequent relocations will check the flag, see that
>>> +	   the slot is already initialized, and move on to just relocating the
>>> +	   entry.  */
>>>   	case R_LARCH_TLS_IE_PC_HI20:
>>>   	case R_LARCH_TLS_IE_HI20:
>>>   	case R_LARCH_TLS_LD_PC_HI20: