Re: [PATCH] btf: do not use the CHAR `encoding' bit for BTF

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

> On 8/2/22 08:42, Jose E. Marchesi wrote:
>> 
>>> On 7/26/22 14:58, Indu Bhagat wrote:
>>>> On 7/22/22 4:23 AM, Jose E. Marchesi via Gcc-patches wrote:
>>>>>
>>>>> Contrary to CTF and our previous expectations, as per [1], turns out
>>>>> that in BTF:
>>>>>
>>>>> 1) The `encoding' field in integer types shall not be treated as a
>>>>>     bitmap, but as an enumerated, i.e. these bits are exclusive to each
>>>>>     other.
>>>>>
>>>>> 2) The CHAR bit in `encoding' shall _not_ be set when emitting types
>>>>>     for char nor `unsigned char'.
>>>>>
>>>>
>>>> Hmm...well.  At this time, I suggest we make a note of this in the btf.h 
>>>> for posterity that BTF_INT_CHAR is to not be used (i.e., BTF_INT_CHAR 
>>>> should not be set for char / unsigned char).
>>>
>>> Agreed it would be good to add this note.
>> 
>> Hmm, I am not sure such a comment actually belongs to include/btf.h,
>> which is not specific to the compiler and is supposed to reflect the BTF
>> format per-se.  The CHAR bit is documented in the kernel documentation
>> and it may be used at some point by bpflib, or who knows what.
>
> OK you make a good point.
>
> In that case the patch LGTM to commit. Thanks!

Pushed to master.
Thanks!

>> 
>> That's why I put the comment in btfout.cc instead, to make it clear that
>> BTF_INT_CHAR is indeed not to be set for char / unsigned char by the
>> compiler:
>> 
>>>>> +      /* In BTF the CHAR `encoding' seems to not be used, so clear it
>>>>> +         here.  */
>>>>> +      dtd->dtd_u.dtu_enc.cte_format &= ~BTF_INT_CHAR;