[GDB] Add the "disassembler-option" hook

[shahab-vahedi]

ARC GDB should allow users to override the disassembling options. For instance:

(gdb) set disassembler-options cpu=hs38_linux
'set disassembler-options ...' is not supported on this architecture.

This will alleviate the problem described here.

What needs to be done?

Look at this patch for inspiration. Probably we need to define the "set_disassembler_options_arc()" in opcode/arc-dis.c.

[claziss]

Can we use arc object attributes here? And/or the CPU configuration found in the AUX regs?

[shahab-vahedi]

This issue is not about determining the CPU type automatically (see #43 for that). This is a mere task to allow the user to specify the CPU manually (as a workaround to #43).

[shahab-vahedi]

With all the recurring issues (e.g. #59 ), it is time to work on this promptly.

[vineetgarc]

But what is so special about gdb that it needs this hook registetred for an mpu ? while the assembler/disassembler can do this. Its not like we have opcode reuse - LDD is supposed to be a unique opcode. What am I missing here - guess I need a bit of gdb for dummies primer.

[shahab-vahedi]

But what is so special about gdb that it needs this hook registetred for an mpu ?

MPU?

Anyhow, it is about the disassembler and not GDB per se. What GDB is doing wrong, is setting the incorrect disassembler option. Take a look at the example below:

$ arc-elf32-objdump -dr test.exe
  ...
  0000012c <main>:
    12c:  1100 0180             ldd r0r1,[r1]

$ arc-elf32-objdump -dr --disassembler-options=cpu=em4_fpuda test.exe
  ...
  0000012c <main>:
    12c: 1100 0180             .word 0x11000180

What this issue (#50) suggests, is that user should be able to override GDB's mistake:

gdb> set disassembler-option cpu=hs38_linux

The fundamental solution is discussed at #43.

Last but not least, one may ask why this happens with QEMU and not nSIM. It's because QEMU's gdbstub supports more exchange of information with the debugger. Unfortunately in this case, the debugger takes a wrong turn on making a decision on such abundance of data.

[shahab-vahedi]

The following patches take care of this issue:

1. arc: Construct disassembler options dynamically
2. arc: Add 'set disassembler-options' support

Update

These changes have also landed in the following branches:
arc-2021.09-gdb
dev
shahab-gdb

This is an illustration of usage of the hook by the end user:

(gdb) disas $pc,$pc+0x10
Dump of assembler code from 0x100 to 0x110:
=> 0x00000100: mov     r0,0x12345678
   0x00000108: sr      r0,[1120]              # aux reg name not recognized
   0x0000010c: lr      r1,[1120]              # aux reg name not recognized
End of assembler dump.

(gdb) show disassembler-options
The current disassembler options are 'cpu=em4_fpuda'

(gdb) set disassembler-options cpu=archs      # the new added hook

(gdb) disas $pc,$pc+0x10
Dump of assembler code from 0x100 to 0x110:
=> 0x00000100: mov     r0,0x12345678
   0x00000108: sr      r0,[tlbpd0]
   0x0000010c: lr      r1,[tlbpd0]
End of assembler dump.