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

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

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

--- Comment #5 from tomas.vanek at fbl dot cz ---
(In reply to Luis Machado from comment #1)
> My understanding is that there is only one active SP at any point, ...
> 
> We should probably have code to figure out what is the expected stack
> pointer register before we go about fetching values. That should bring the
> number of requests down. Does that sound reasonable?

Let's have M-profile without sec ext.
To figure out the active stack pointer we need xPSR and CONTROL registers.
So we need to fetch 2 registers and the 3rd fetch will get the active stack
pointer. Provided that fetching xPSR and CONTROL does not generate additional
requests we need approx n^2 + 2n calls get_frame_register_unsigned() where n is
frame level. It's much better than exponential but not a great performance...
And the code would be kind of balancing on the edge: if somebody adds fetching
one more register we get into an exponential again.

Why not use a cache to ensure linear dependency to the frame level?

The prologue cache is occupied by dwarf2 unwinder.
We need small changes in dwarf2/frame.c code to allow
arm_dwarf2_prev_register()
to allocate struct arm_prologue_cache and store the this_cache pointer to
struct dwarf2_frame_cache as a secondary, cascaded cache. This would allow
re-using the stack pointer related code in arm_make_prologue_cache(),
arm_cache_switch_prev_sp() and arm_cache_set_active_sp_value()

Or alternatively modify dwarf2/frame.c to allow changing struct
dwarf2_frame_state_reg on the fly from arm_dwarf2_prev_register().
arm_dwarf2_frame_init_reg() initializes
  reg->how = DWARF2_FRAME_REG_FN;
  reg->loc.fn = arm_dwarf2_prev_register;
The first arm_dwarf2_prev_register() call makes the decision which sp is active
and sets
  reg->how = DWARF2_FRAME_REG_CFA;
for the active sp and
  reg->how = DWARF2_FRAME_REG_SAME_VALUE;
for an inactive sp.
All following get_frame_register_unsigned() will reuse the decision cached in
reg->how and fetch the selected register directly.

What do you thing?

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