[Bug tdep/29738] Arm M-profile dwarf2 unwinder performance suffers from exponential grows

["tomas.vanek at fbl dot cz" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29738

tomas.vanek at fbl dot cz changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Arm v8 M-profile secure     |Arm M-profile dwarf2
                   |extension dwarf2 unwinder   |unwinder performance
                   |performance                 |suffers from exponential
                   |                            |grows

--- Comment #4 from tomas.vanek at fbl dot cz ---
(In reply to Luis Machado from comment #3)
> I suppose we'd have the same issue with non-secure m-profiles, as they'd
> need to do 3 ^ number_of_frame calls as well, based on this code:
> 
>       else if (tdep->is_m)
>         {
>           CORE_ADDR sp
>             = get_frame_register_unsigned (this_frame, ARM_SP_REGNUM);
>           CORE_ADDR msp
>             = get_frame_register_unsigned (this_frame,
>                                            tdep->m_profile_msp_regnum);
>           CORE_ADDR psp
>             = get_frame_register_unsigned (this_frame,
>                                            tdep->m_profile_psp_regnum);

Yes, the problem exists on all m-profiles. I edited the bug summary.

As I wrote in the reply to Torbjorn email from 31th Oct (you should have
received a CC, didn't you?), the exponential growth has the base "only" 2 for
m-profile w/o the security extension, because ARM_SP_REGNUM is read from CFA in
dwarf2 cache and does not generate new get_frame_register_unsigned() request.

Unfortunately each exponential base greater than 1 is strong enough to spoil
the performance. If the debugged app has 16 consecutive dwarf2 frames the 'bt'
command is for very very patient users only.

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