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

[Tatsuyuki Ishi <ishitatsuyuki@gmail.com>]

> 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:
>