[PATCH v2 08/11] s390: Store SFrame CFA offset adjusted

[Jens Remus]

Hello Indu,

thank you for your review feedback!

On 02.06.2025 22:46, Indu Bhagat via Binutils wrote:
> On 5/27/25 4:07 AM, Jens Remus wrote:
>> In SFrame V2 the size of the offsets following an SFrame FRE can be
>> either signed 8-bit, 16-bit, or 32-bit integer, which the largest offset
>> determining their size:
>>    1. CFA offset from CFA base register
>>    2. RA (stack save slot) offset from CFA, usually -48 on s390x if saved
>>    3. FP (stack save slot) offset from CFA, usually -72 on s390x if saved
>> The FP and RA offsets from CFA, when FP/RA saved on the stack, usually
>> have fixed values that fit into signed 8-bit SFrame offsets.  Likewise
>> the DWARF register numbers on s390x of general registers (GR; 0-15) and
>> floating-point registers (FPR; 16-31), when FP/RA saved in registers.
>> With that the CFA offset from CFA base register has the greatest impact
>> on the signed SFrame offset size.
>>
>> The s390x ELF ABI [1] defines the CFA as stack pointer (SP) at call
>> site +160. [2]  Therefore the minimum CFA offset from CFA base register
>> on s390x is 160.  This does not fit into a signed 8-bit integer and
>> therefore effectively prevents any use of signed 8-bit SFrame offsets
>> on s390x.
>>
>> For s390x store the CFA offset from CFA base register adjusted by -160
>> to enable the use of signed 8-bit SFrame offsets.
>>
>> [1]: s390x ELF ABI, https://github.com/IBM/s390x-abi/releases
>> [2]: s390x ELF ABI, commit 4e38ad9c8a88 ("Document the CFA"),
>>       https://github.com/IBM/s390x-abi/commit/4e38ad9c8a88

> SFrame specific implementation looks good to me.  One question below.
> It will be good to double check in case we have overlooked something.

Will do.

> 
>> Suggested-by: Indu Bhagat <indu.bhagat@oracle.com>
>> Signed-off-by: Jens Remus <jremus@linux.ibm.com>
> 
> 
> Reviewed-by: Indu Bhagat <indu.bhagat@oracle.com>

Thanks!

>> Notes (jremus):
>>      Changes in V2:
>>      - Reword commit message (s390 to s390x, offsets, FDE to FRE). (Indu)
>>      - Rename s390x-specific macros from *_S390_* to *_S390X_*. (Indu)
>>      - Use "s390x" instead of "S390" in comments and SFrame spec. (Indu)
>>      - Change order of CFA restore instructions in SFrame spec. (Indu)
>>      - Update new libsframe test plt-findfre-2.
>>           A test build of Glibc tag 2.41 on s390x libc.so shows a ~8.8% reduction in
>>      .sframe section size due to storing the SFrame CFA offsets adjusted by
>>      -160.
>>           Statistics for libc.so - base (no CFA offset adjustment):
>>             .sframe size:  169,749 bytes
>>             VALUE        TOTAL      MIN        MAX        AVG
>>        FDEs:        3652       -          -          -
>>        FREs/FDE:    15236      1          20         4
>>        Offsets/FDE: 29792      1          38         8
>>           8-bit:    0          0          0          0
>>          16-bit:    29792      1          38         8
>>          32-bit:    0          0          0          0
>>        Offsets/FRE: 29792      1          3          1
>>           8-bit:    -          0          0          0
>>          16-bit:    -          1          3          1
>>          32-bit:    -          0          0          0
>>        O_Padd/FDE:  342        -          -          0
>>           8-bit:    0
>>          16-bit:    342
>>          32-bit:    0
>>           Statistics for libc.so - CFA offset adjustment:
>>             .sframe size: 154,757 bytes
>>             VALUE        TOTAL      MIN        MAX        AVG
>>        FDEs:        3654       -          -          -
> 
> Hmm, why would the number of FDEs shift (3652 above vs 3654 now) with
> this sort of experiment.

Good catch!  I guess I erroneously mixed the output of some old and new
Glibc test builds.  I will repeat them to make sure there is nothing
else going on.

> 
>>        FREs/FDE:    15238      1          20         4
>>        Offsets/FDE: 29794      1          38         8
>>           8-bit:    14992      1          38         4
>>          16-bit:    14802      0          0          4
>>          32-bit:    0          0          0          0
>>        Offsets/FRE: 29794      2          6          1
>>           8-bit:    -          1          3          0
>>          16-bit:    -          1          3          0
>>          32-bit:    -          0          0          0
>>        O_Padd/FDE:  342        -          -          0
>>           8-bit:    283
>>          16-bit:    59
>>          32-bit:    0
>>           Changes in V2:

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

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