gdb: more debug output for displaced stepping

gdb: more debug output for displaced stepping

While investigating a displaced stepping issue I wanted an easy way to
see what GDB thought the original instruction was, and what
instruction GDB replaced that with when performing the displaced step.

We do print out the address that is being stepped, so I can track down
the original instruction, I just need to go find the information
myself.

And we do print out the bytes of the new instruction, so I can figure
out what the replacement instruction was, but it's not really easy.

Also, the code that prints the bytes of the replacement instruction
only prints 4 bytes, which clearly isn't always going to be correct.

In this commit I remove the existing code that prints the bytes of the
replacement instruction, and add two new blocks of code to
displaced_step_prepare_throw.  This new code prints the original
instruction, and the replacement instruction.  In each case we print
both the bytes that make up the instruction and the completely
disassembled instruction.

Here's an example of what the output looks like on x86-64 (this is
with 'set debug displaced on').  The two interesting lines contain the
strings 'original insn' and 'replacement insn':

  (gdb) step
  [displaced] displaced_step_prepare_throw: displaced-stepping 2892655.2892655.0 now
  [displaced] displaced_step_prepare_throw: original insn 0x401030: ff 25 e2 2f 00 00	jmp    *0x2fe2(%rip)        # 0x404018 <puts@got.plt>
  [displaced] prepare: selected buffer at 0x401052
  [displaced] prepare: saved 0x401052: 1e fa 31 ed 49 89 d1 5e 48 89 e2 48 83 e4 f0 50
  [displaced] fixup_riprel: %rip-relative addressing used.
  [displaced] fixup_riprel: using temp reg 2, old value 0x7ffff7f8a578, new value 0x401036
  [displaced] amd64_displaced_step_copy_insn: copy 0x401030->0x401052: ff a1 e2 2f 00 00 68 00 00 00 00 e9 e0 ff ff ff
  [displaced] displaced_step_prepare_throw: prepared successfully thread=2892655.2892655.0, original_pc=0x401030, displaced_pc=0x401052
  [displaced] displaced_step_prepare_throw: replacement insn 0x401052: ff a1 e2 2f 00 00	jmp    *0x2fe2(%rcx)
  [displaced] finish: restored 2892655.2892655.0 0x401052
  [displaced] amd64_displaced_step_fixup: fixup (0x401030, 0x401052), insn = 0xff 0xa1 ...
  [displaced] amd64_displaced_step_fixup: restoring reg 2 to 0x7ffff7f8a578
  0x00007ffff7e402c0 in puts () from /lib64/libc.so.6
  (gdb)

One final note.  For many targets that support displaced stepping (in
fact all targets except ARM) the replacement instruction is always a
single instruction.  But on ARM the replacement could actually be a
series of instructions.

The debug code tries to handle this by disassembling the entire
displaced stepping buffer.  Obviously this might actually print more
than is necessary, but there's (currently) no easy way to know how
many instructions to disassemble; that knowledge is all locked in the
architecture specific code.  Still I don't think it really hurts, if
someone is looking at this debug then hopefully they known what to
expect.

Obviously we can imagine schemes where the architecture specific
displaced stepping code could communicate back how many bytes its
replacement sequence was, and then our debug print code could use this
to limit the disassembly.  But this seems like a lot of effort just to
save printing a few additional instructions in some debug output.

I'm not proposing to do anything about this issue for now.

Approved-By: Simon Marchi <simon.marchi@efficios.com>