[PATCH] gas: sframe: skip DW_CFA_GNU_args_size when safe to ignore

[Jan Beulich]

On 08.09.2025 20:14, Sam James wrote:
> Indu Bhagat via Binutils <binutils@sourceware.org> writes:
> 
>> Currently, gas warns and skips generating SFrame FDE when it sees:
>>      .cfi_escape 0x2e,XX
>>
>> From the documentation of DW_CFA_GNU_args_size:
>>  "The DW_CFA_GNU_args_size instruction takes an unsigned LEB128 operand
>>   representing an argument size. This instruction specifies the total of
>>   the size of the arguments which have been pushed onto the stack."
>>
>> With origins seemingly for VAX architecture, the usage of
>> DW_CFA_GNU_args_size seems to have evolved.  The purpose of
>> DW_CFA_GNU_args_size is to adjust SP when performing virtual stack
>> unwinding for exception handling.
>>
>> For the purpose of stack tracing using SFrame, DW_CFA_GNU_args_size is
>> safe to skip, especially when the CFA restoration is known to be FP
>> based.
> 
> I would personally include a link to that ML post/RFC from you, just for
> reference in case anyone digs in future..
> 
> Anyway, yes please, but I can't approve it. This is one of two "known
> warnings" that came up in the discussion with us and also Jan.

Btw, with this ...

>> gas/
>>         * gen-sframe.c (sframe_xlate_do_escape_gnu_args_size): New
>> 	definition.
>>         (sframe_xlate_do_cfi_escape): Handle DW_CFA_GNU_args_size.
>> gas/testsuite/
>>         * gas/cfi-sframe/cfi-sframe.exp: New test.
>>         * gas/cfi-sframe/cfi-sframe-common-12.d: New test.
>>         * gas/cfi-sframe/cfi-sframe-common-12.s: New test.
>>         * gas/cfi-sframe/cfi-sframe-x86_64-3.d: New test.
>>         * gas/cfi-sframe/cfi-sframe-x86_64-3.s: New test.
>> ---
>>  gas/gen-sframe.c                              | 62 ++++++++++++++++++-
>>  .../gas/cfi-sframe/cfi-sframe-common-12.d     | 22 +++++++
>>  .../gas/cfi-sframe/cfi-sframe-common-12.s     |  8 +++
>>  .../gas/cfi-sframe/cfi-sframe-x86_64-3.d      | 21 +++++++
>>  .../gas/cfi-sframe/cfi-sframe-x86_64-3.s      |  5 ++
>>  gas/testsuite/gas/cfi-sframe/cfi-sframe.exp   |  2 +
>>  6 files changed, 119 insertions(+), 1 deletion(-)
>>  create mode 100644 gas/testsuite/gas/cfi-sframe/cfi-sframe-common-12.d
>>  create mode 100644 gas/testsuite/gas/cfi-sframe/cfi-sframe-common-12.s
>>  create mode 100644 gas/testsuite/gas/cfi-sframe/cfi-sframe-x86_64-3.d
>>  create mode 100644 gas/testsuite/gas/cfi-sframe/cfi-sframe-x86_64-3.s

... diffstat, aiui Indu doesn't require any approval, as she's maintainer of
SFrame code.

Jan