[PATCH v4 3/4] gas: sframe: Represent .cfi_undefined RA as FRE without offsets

[Jens Remus]

On 7/30/2025 10:30 AM, Indu Bhagat wrote:
> On 7/29/25 4:19 AM, Jens Remus wrote:

>> My intend was to set as much of the FRE info word zero as possible,
>> to make it easier for humans to parse SFrame data for debugging
>> purposes.  I did not touch mangled_ra_p, as I was unsure whether
>> that can also be unconditionally set to zero.  But I guess that
>> would be fine?
>>
>> I used zero, to denote that this is about setting the field to zero
>> instead of a specific value with an assigned meaning, like
>> SFRAME_BASE_REG_SP.
>>
>> What if I reverted all the changes to the SFrame helpers and did the
>> following change to output_sframe_row_entry instead, to make that
>> more explicit?
>>
>>     /* Create the fre_info using the CFA base register, number of offsets and max
>> -     size of offset in this frame row entry.  */
>> -  fre_base_reg = get_fre_base_reg_id (sframe_fre);
>> -  fre_num_offsets = get_fre_num_offsets (sframe_fre);
>> -  fre_offset_size = sframe_get_fre_offset_size (sframe_fre);
>> +     size of offset in this frame row entry.  RA undefined is represented
>> +     as FRE without any offsets and all FRE info word fields zeroed.  */
>> +  if (sframe_fre->ra_undefined)
>> +    {
>> +      fre_base_reg = 0;
>> +      fre_num_offsets = 0;
>> +      fre_offset_size = 0;
>> +      fre_mangled_ra_p = 0;
>> +    }
>> +  else
>> +    {
>> +      fre_base_reg = get_fre_base_reg_id (sframe_fre);
>> +      fre_num_offsets = get_fre_num_offsets (sframe_fre);
>> +      fre_offset_size = sframe_get_fre_offset_size (sframe_fre);
>> +      fre_mangled_ra_p = sframe_fre->mangled_ra_p;
>> +    }
>>     fre_info = sframe_set_fre_info (fre_base_reg, fre_num_offsets,
>> -                                 fre_offset_size, sframe_fre->mangled_ra_p);
>> +                                 fre_offset_size, fre_mangled_ra_p);
>>     out_one (fre_info);
>>
>>     /* Represent RA undefined as FRE without any offsets.  */
>>
>> Alternatively:
>>
>>    fre_info = 0;
> 
> How about we reset the state of cur_fre when handling the case in
> sframe_xlate_do_cfi_undefined () itself. Then this handling in
> output_sframe_row_entry will not be necessary.

Do you happen to know how this is handled in objdump -WF for DWARF?
I guess it only affects the RA register (return column register).
So we should not alter any other register rules, such as CFA base
register, CFA offset, and FP offset.  In theory the next FRE could
define a valid rule for the RA register, no?

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@de.ibm.com

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