[PATCH 3/4] gas: sframe: handle .cfi_undefined

[Jan Beulich]

On 15.05.2025 00:55, Indu Bhagat wrote:
> On 5/13/25 11:28 PM, Jan Beulich wrote:
>> On 13.05.2025 22:27, Indu Bhagat wrote:
>>> On 5/13/25 6:06 AM, Jan Beulich wrote:
>>>> On 11.05.2025 09:35, Indu Bhagat wrote:
>>>>> --- a/gas/gen-sframe.c
>>>>> +++ b/gas/gen-sframe.c
>>>>> @@ -1507,6 +1507,32 @@ sframe_xlate_do_cfi_escape (const struct sframe_xlate_ctx *xlate_ctx,
>>>>>      return err;
>>>>>    }
>>>>>    
>>>>> +/* Translate DW_CFA_undefined into SFrame context.
>>>>> +
>>>>> +   DW_CFA_undefined op implies that from now on the previous value of register
>>>>> +   can’t be restored anymore.  In SFrame stack trace, we cannot represent such
>>>>> +   a semantic.  So, we skip generating an SFrame FDE for this, when a register
>>>>> +   of interest is used with DW_CFA_undefined.
>>>>> +
>>>>> +   Return SFRAME_XLATE_OK if success.  */
>>>>> +
>>>>> +static int
>>>>> +sframe_xlate_do_cfi_undefined (const struct sframe_xlate_ctx *xlate_ctx ATTRIBUTE_UNUSED,
>>>>> +			       const struct cfi_insn_data *cfi_insn)
>>>>> +{
>>>>> +  if (cfi_insn->u.ri.reg == SFRAME_CFA_FP_REG
>>>>> +      || cfi_insn->u.ri.reg == SFRAME_CFA_RA_REG
>>>>> +      || cfi_insn->u.ri.reg == SFRAME_CFA_SP_REG)
>>>>> +    {
>>>>> +      as_warn (_("skipping SFrame FDE; %s reg %u in .cfi_undefined"),
>>>> As the comment says, what you skip is generating an FDE; you don't skip any
>>>> FDE itself afaict. I think this needs reflecting in the diagnostic, for it
>>>> to not be confusing.
>>> Can you elaborate - it is not clear what you mean. Yes, what we skip is
>>> generating an SFrame FDE.  The diagnostic "skipping SFrame FDE" is meant
>>> to indicate that there will be no SFrame FDE for that function/block.
>> The message as it is tells me that there is an FDE (somewhere) which is
>> being skipped. It does not tell me that my expectation of an FDE being
>> generated is not met. "not emitting ..." or "no ... emitted: ..." or
>> some such would be what I think makes things more clear.
> 
> I have slight preference to not change the warning messages now.  I am 
> afraid users may have gotten used to grepping for "skipping SFrame FDE" 
> over time.

Well, SFrame is still pretty new and perhaps even still kind of experimental.
When, if not now, would you suggest to make such an adjustment? It's bad
enough that the ambiguity made it in originally. I, for one, will certainly
not approve new code with unclear diagnostics.

Jan