[Bug gdb/27593] New: Scrolling up can be extremely slow.

["edd at theunixzoo dot co.uk" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27593

            Bug ID: 27593
           Summary: Scrolling up can be extremely slow.
           Product: gdb
           Version: HEAD
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: edd at theunixzoo dot co.uk
  Target Milestone: ---

It seems that Bug #26024 (Cannot scroll up in asm view) has now been fixed, but
I notice that there are scenarios where scrolling up is extremely slow.

I have an 85MB crashing Rust binary here. If I run it to the crash the
backtrace is:
```
Thread 1 "tests-014a21cc2" received signal SIGABRT, Aborted.
0x00007ffff7b367bb in raise () from /lib/x86_64-linux-gnu/libc.so.6
(gdb) bt
#0  0x00007ffff7b367bb in raise () from /lib/x86_64-linux-gnu/libc.so.6
#1  0x00007ffff7b21535 in abort () from /lib/x86_64-linux-gnu/libc.so.6
#2  0x00007ffff7b78508 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#3  0x00007ffff7b7ec1a in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#4  0x00007ffff7b7f184 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#5  0x00005555557c7151 in ?? ()
#6  0x00005555557c6cc4 in ?? ()
#7  0x00005555557c63f1 in ?? ()
#8  0x00005555557c2987 in ?? ()
#9  0x00005555557d62d3 in ?? ()
#10 0x00005555557c0f33 in ?? ()
#11 0x00005555557c1182 in ?? ()
#12 0x00005555557bee09 in tests::main () at
/home/vext01/research/yorick/yk/tests/src/lib.rs:1
```

Switching to asm layout in the top frame (#0), scroll up works well.

Moving down the stack, #1 through #4 all scroll up fine too.

In #5 however, scrolling up one line:
  - Takes about 20 seconds.
  - Causes one CPU core to be pegged at 100% during this time.
  - Blocks the GDB TUI in the meantime.

The same happens for #6 through #12 (inclusive).

I had postured that, gdb has to do some kind of search when the file for a
frame's code is unknown, but #12 being slow to scroll disproves this.

This occurs on gdb-10.1 and the current (as of today) git code.

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