[RFC 13/28] [SFrame-V3] include: sframe: s390x: keep unused bits when possible

[Jens Remus]

Hello Indu!

On 12/12/2025 5:28 PM, Jens Remus wrote:
> On 12/9/2025 10:07 AM, Indu Bhagat wrote:
>> TBD:
>>   - This is a backwards incompatible change.  But its better to keep unused
>>     bits for future extensibility.  Discuss with s390x SFrame maintainer.
> 
> s390x may save FP/RA in registers when in the topmost frame, e.g. when
> in leaf function.  Currently it does so in floating-point registers.
> These have the DWARF register numbers 16-31 on s390x.  With the left
> shift by 3 these do no longer fit into a signed 8-bit offset:
> 
>   16 << 3 = 128
>   31 << 3 = 248
> 
> When considering this I would also evaluate whether it wouldn't be the
> better choice to use the new FLEX_TOPMOST_FRAME and get rid of this
> s390x-particularity.
> 
> I am working on a prototype to get some numbers for Glibc:
> 
> git://sourceware.org/git/binutils-gdb.git users/jremus/sframe-v3-c1-s390x
> https://sourceware.org/git/?p=binutils-gdb.git;a=shortlog;h=refs/heads/users/jremus/sframe-v3-c1-s390x
Some numbers from a Glibc build for libc.so:


SFrame V3 (vanilla)

Section Headers:
  [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
  [16] .sframe           GNU_SFRAME      00000000001dab10 1dab10 023553 00   A  0   0  8

Num FDEs: 3650
Num FREs: 15196


SFrame V3 with my hacked s390x prototype:

Section Headers:
  [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
  [16] .sframe           GNU_SFRAME      00000000001dab10 1dab10 023ac7 00   A  0   0  8 SFrame V3 + s390x
                                                                 (+1396 bytes)
Num FDEs: 3655 (+5)
Num FREs: 15225 (+29)


I compared the objdump --sframe dumps. They are identical (ignoring the
+0 offsets), except for the following additional FDEs using my s390x
prototype:

    func idx [243]: pc = 0x4bb80, size = 84 bytes <__longjmp>
    STARTPC         CFA       FP        RA
    000000000004bb80  sp+160    u         U
    000000000004bba8  r1+0      r0+40     r4+0    <-- r1-based CFA, FP=r0, RA=r4

    func idx [483]: pc = 0x5f110, size = 13138 bytes <__GI_____strtold_l_internal>, <____strtold_l_internal>
    STARTPC         CFA       FP        RA
    000000000005f110  sp+160    u         U
    000000000005f116  sp+160    r0-72     r0-48
    000000000005f122  r14+12448 r0-72     r0-48   <-- r14-based CFA
    000000000005f13a  sp+12448  r0-72     r0-48
    000000000005f140  sp+14360  r0-72     r0-48
    000000000005f32c  sp+160    u         U
    000000000005f32e  sp+14360  r0-72     r0-48

    func idx [1567]: pc = 0xcfdf0, size = 11644 bytes <____wcstold_l_internal>
    STARTPC         CFA       FP        RA
    00000000000cfdf0  sp+160    u         U
    00000000000cfdf6  sp+160    r0-72     r0-48
    00000000000cfe02  r14+12448 r0-72     r0-48   <-- r14-based CFA
    00000000000cfe1a  sp+12448  r0-72     r0-48
    00000000000cfe20  sp+14352  r0-72     r0-48
    00000000000d00cc  sp+160    u         U
    00000000000d00ce  sp+14352  r0-72     r0-48

    func idx [1831]: pc = 0xfdd10, size = 3948 bytes <build_trtable>
    STARTPC         CFA       FP        RA
    00000000000fdd10  sp+160    u         U
    00000000000fdd16  sp+160    r0-72     r0-48
    00000000000fdd1c  r14+16544 r0-72     r0-48   <-- r14-based CFA
    00000000000fdd34  sp+16544  r0-72     r0-48
    00000000000fdd3a  sp+19040  r0-72     r0-48
    00000000000fe87a  sp+160    u         U
    00000000000fe87c  sp+19040  r0-72     r0-48

    func idx [2503]: pc = 0x12d360, size = 200 bytes <____longjmp_chk>
    STARTPC         CFA       FP        RA
    000000000012d360  sp+160    u         U
    000000000012d394  sp+160    u         r0-48
    000000000012d39c  sp+344    u         r0-48
    000000000012d3e8  sp+160    u         r0-48
    000000000012d3f4  sp+160    u         U
    000000000012d3fc  r1+0      r0+40     r4+0   <-- r1-based CFA, FP=r0, RA=r4

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

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