[PATCH] Make sure DW_CFA_advance_loc4 is in the same frag

[Jan Beulich]

On 11.08.2023 01:02, Alan Modra wrote:
> On Thu, Aug 10, 2023 at 02:55:20PM +0200, Jan Beulich wrote:
>> On 10.08.2023 14:36, Jinyang He wrote:
>>> On /Thu Aug 10 07:55:58 GMT 2023 /Jan Beulich wrote:
>>>
>>>> On 10.08.2023 04:21, Jinyang He wrote:
>>>>> /The DW_CFA_advance_loc4 may be in different frags. Then fr_fix />/may caused something wrong. Referenced by commit b9d8f5601bcf />/("Re: Optimise away eh_frame advance_loc 0"). /
>>>> I'm afraid I don't understand that earlier fix: frag_more(1) there
>>>> ought to guarantee fr_fix (once the frag is closed) to be >= 1.
>>>> It would then seem to me that ...
>>>
>>> I'm not familiar with it. Please point out my mistakes. Thanks in advance.
>>
>> Well, Alan has approved your change. Maybe it's me who is wrong here.
> 
> The idea behind commit b9d8f5601bcf was that when a
> DW_CFA_advance_loc4 of zero is seen in eh_frame_relax_frag and
> eh_frame_convert_frag we want to remove the opcode entirely, not just
> convert to a nop.  If the opcode was split over two frags then the
> size adjustment would need to be done to the first frag, not the
> second as is correct for other cases with split frags.  This would
> complicate the eh relaxation.  It's easier to ensure the frag is not
> split.

Oh, right, I can see that this makes things easier. But for the change
here, does frag_grow() actually help preventing the split? Wouldn't it
need to be yet beyond what I suggested and require frag_more(5), such
that intermediate section switches wouldn't close off the first frag?
Of course this would collide with what you say regarding size
adjustments needing to happen on the 2nd (variable) frag. (Aiui just
frag_grow() is enough in output_cfi_insn(), as there we aren't at risk
of intermediate section changes.)

Jan