[Bug tdep/31507] [gdb, aarch64] FAIL: gdb.arch/disp-step-insn-reloc.exp: can_relocate_adr_forward: relocated instruction

["vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31507

Tom de Vries <vries at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |luis.machado at arm dot com
          Component|gdb                         |tdep

--- Comment #7 from Tom de Vries <vries at gcc dot gnu.org> ---
This may be a regression due to:
...
commit 0c27188999bfc5bf03536bf44593c4ed8df296c3
Author: Luis Machado <luis.machado@linaro.org>
Date:   Thu Jan 9 16:04:36 2020 -0300

    Fix step-over-syscall.exp failure

    In particular, this one:

    FAIL: gdb.base/step-over-syscall.exp: fork: displaced=on:
check_pc_after_cross_syscall: single step over fork final pc

    When ptrace fork event reporting is enabled, GDB gets a PTRACE_EVENT_FORK
    event whenever the inferior executes the fork syscall.

    Then the logic is that GDB needs to step the inferior yet again in order to
    receive a predetermined SIGTRAP, but no execution takes place because the
    signal was already queued for delivery. That means the PC should stay the
same.

    I noticed the aarch64 code is currently adjusting the PC in this situation,
    making the inferior skip an instruction without executing it.

    The following change checks if we did not execute the instruction
    (pc - to == 0), making proper adjustments for such case.

    Regression tested on aarch64-linux-gnu on the tryserver.

    gdb/ChangeLog:

    2020-01-21  Luis Machado  <luis.machado@linaro.org>
...

Luis, could you take a look?

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