[PATCH 5/8] s390: Represent FP without RA saved in SFrame

[Jens Remus]

On 05.05.2025 06:56, Indu Bhagat wrote:
> On 4/2/25 9:12 AM, Jens Remus wrote:
>> If an architecture uses both SFrame RA and FP tracking SFrame assumes
>> that the RA offset is the 2nd offset and the FP offset is the 3rd offset
>> following a SFrame FRE.  An architecture does not necessarily need to
>> save both on the stack (or in register) at the same time or even at all.
>> SFrame cannot represent FP without RA saved on stack (or in a register),
>> since it cannot distinguish whether the 2nd offset is the RA or FP
>> offset.
>>
>> For S390 use an invalid SFrame RA offset from CFA value of zero as
>> padding to represent the FP being saved when the RA is not saved.  This
>> aligns with the existing invalid SFrame fixed RA offset from CFA value
>> of zero.  In a stack tracer this then also naturally falls into place,
>> as it can skip restoring the RA in the topmost frame, if both the fixed
>> RA offset (from SFrame header) and the RA offset (from FDE) are zero,
>> without any need to test architecture-specific flags.

>> diff --git a/libsframe/sframe-dump.c b/libsframe/sframe-dump.c

>> @@ -200,6 +200,10 @@ dump_sframe_func_with_fres (sframe_decoder_ctx *sfd_ctx,
>>         if (sframe_decoder_get_fixed_ra_offset (sfd_ctx)
>>         != SFRAME_CFA_FIXED_RA_INVALID)
>>       strcpy (temp, "f");
>> +      /* If an ABI does track RA offset, e.g. AArch64 and S390, it can be a
>> +     dummy as padding to represent FP without RA being saved on stack.  */
>> +      else if (err[2] == 0 && ra_offset == SFRAME_FRE_RA_OFFSET_INVALID)
>> +    sprintf (temp, "u*");
>>         else if (err[2] == 0)
>>       {
>>         if (is_sframe_abi_arch_s390 (sfd_ctx)
> 
> I have slight distaste for "u*" to indicate a dummy padding (especially
> the *), how about just an "X" ? Or something else if you prefer,
> basically avoiding the '*'.

I'll be proposing "U" in V2, as the rule for the register is "undefined".
Actually "u" would also be fine.  For debugging I wanted to see it being
used.  Maybe it would be better for the end user not to get distracted by
that level of detail and use "u"?  

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

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