Re: read target register to decide breakpoint size

[Tim Newsome <tim at sifive dot com>]

Actually, this seems to work inside breakpoint_kind_from_pc():
```
    struct frame_info *frame = get_current_frame ();
    uint32_t misa = get_frame_register_unsigned (frame, RISCV_CSR_MISA_REGNUM);
```

Is that kosher? If so, is there any reason for me to implement
breakpoint_kind_from_current_state?

Thanks,
Tim

On Tue, Dec 13, 2016 at 12:58 PM, Tim Newsome <tim@sifive.com> wrote:
> I finally got around to implementing this, but gdb still insists on
> calling breakpoint_kind_from_pc, where I don't have access to a
> regcache object (I think). Eg.
> ```
> #0  riscv_breakpoint_kind_from_pc (gdbarch=0xcc8a90,
> pcptr=0x7fffffffde68) at riscv-tdep.c:184
> #1  0x0000000000541301 in default_breakpoint_from_pc
> (gdbarch=0xcc8a90, pcptr=<optimized out>, lenptr=0x7fffffffde64)
>     at arch-utils.c:847
> #2  0x00000000004b40b3 in program_breakpoint_here_p
> (gdbarch=<optimized out>, address=2147550320) at breakpoint.c:9113
> #3  0x00000000004b424c in bp_loc_is_permanent (loc=0xdd57b0) at
> breakpoint.c:9156
> #4  add_location_to_breakpoint (b=b@entry=0xda9aa0,
> sal=sal@entry=0x7fffffffdf70) at breakpoint.c:9093
> #5  0x00000000004b5169 in init_raw_breakpoint (ops=0xb4d200
> <bkpt_breakpoint_ops>, bptype=bp_breakpoint, sal=...,
>     gdbarch=0xcc8a90, b=0xda9aa0) at breakpoint.c:7586
> #6  init_breakpoint_sal (b=b@entry=0xda9aa0,
> gdbarch=gdbarch@entry=0xcc8a90, location=location@entry=0xda8f30,
>     filter=filter@entry=0x0, cond_string=0x0, extra_string=0x0,
> type=bp_breakpoint, disposition=disp_donttouch,
>     thread=-1, task=0, ignore_count=0, ops=0xb4d200
> <bkpt_breakpoint_ops>, from_tty=1, enabled=1, flags=0,
>     display_canonical=0, internal=<optimized out>, sals=...) at
> breakpoint.c:9300
> #7  0x00000000004bb490 in create_breakpoint_sal (display_canonical=0,
> flags=0, internal=0, enabled=1, from_tty=1,
>     ops=0xb4d200 <bkpt_breakpoint_ops>, ignore_count=0, task=0,
> thread=-1, disposition=disp_donttouch,
>     type=bp_breakpoint, extra_string=0x0, cond_string=0x0, filter=0x0,
> location=0xda8f30, gdbarch=0xcc8a90, sals=...)
>     at breakpoint.c:9436
> #8  create_breakpoints_sal (gdbarch=0xcc8a90,
> canonical=0x7fffffffe170, cond_string=0x0, extra_string=0x0,
>     type=bp_breakpoint, disposition=disp_donttouch, thread=-1, task=0,
> ignore_count=0,
>     ops=0xb4d200 <bkpt_breakpoint_ops>, from_tty=1, enabled=1,
> internal=0, flags=0) at breakpoint.c:9490
> #9  0x00000000004bbbfc in create_breakpoint (gdbarch=0xcc8a90,
> location=location@entry=0xda3af0,
>     cond_string=cond_string@entry=0x0, thread=-1, thread@entry=0,
> extra_string=<optimized out>,
>     extra_string@entry=0xda3ad6 "", parse_extra=parse_extra@entry=1,
> tempflag=0, type_wanted=bp_breakpoint,
>     ignore_count=0, pending_break_support=AUTO_BOOLEAN_AUTO,
> ops=0xb4d200 <bkpt_breakpoint_ops>, from_tty=1, enabled=1,
>     internal=0, flags=0) at breakpoint.c:9912
> #10 0x00000000004bc099 in break_command_1 (arg=0xda3ad6 "",
> flag=<optimized out>, from_tty=1) at breakpoint.c:10020
> ...
> ```
>
> Is there some way I can check the misa register of my target from
> default_breakpoint_from_pc, or have gdb not call
> default_breakpoint_from_pc and use breakpoint_kind_from_current_state
> instead?
>
> Thank you,
> Tim
>
> On Mon, Nov 21, 2016 at 10:00 AM, Tim Newsome <tim@sifive.com> wrote:
>> Thanks, Antoine! That's exactly what I was looking for.
>>
>> Tim
>>
>> On Mon, Nov 21, 2016 at 8:36 AM, Antoine Tremblay
>> <antoine.tremblay@ericsson.com> wrote:
>>>
>>>
>>> Tim Newsome writes:
>>>
>>> > I'm still working on RISC-V support for gdb. Any given RISC-V core may
>>> > support a compressed instruction set (2 bytes per instruction as
>>> > opposed to 4). There are corresponding 2-byte and 4-byte breakpoint
>>> > instructions. On cores that support the compressed instruction set it
>>> > is safe to just always use the 2-byte version, and there is a register
>>> > I can read to tell me whether the compressed instruction set is
>>> > supported. What I would like to do is read (and cache) that register
>>> > when breakpoint size is determined. That seems more robust than making
>>> > a decision based on ELF info, which may not reflect what is actually
>>> > being executed.
>>> >
>>> > Is that a good idea? Are there examples of operations that read target
>>> > registers to complete?
>>>
>>> Yes actually you can check how ARM does it, it has the same kind of
>>> problem with 3 breakpoints you can set for thumb, thumb2 and arm
>>> instruction sets.
>>>
>>> See arm-tdep.c:arm_sw_breakpoint_from_kind and
>>> arm_breakpoint_kind_from_current_state
>>>
>>> This is called in breakpoint.c:breakpoint_kind and it can use a register
>>> to make the decision from the current state of that register.
>>>
>>> So possibly just implementing the sw_breakpoint_from_kind and
>>> breakpoint_kind_from_current state would be ok your you.
>>>
>>> Regards,
>>> Antoine Tremblay