[RFC] "single step" atomic instruction sequences as a whole on PPC

[Luis Machado]

> I'd just do "opcode = insn >> 26" same as in rs6000_software_single_step.
> (In fact I'm wondering why branch_dest doesn't just that for itself ...).

Follows the updated patch. The "opcode" variable is now being assigned
the correct instruction's opcode value (only the corresponding bits).

As a consequence of this change, i've noticed problems with branch
instructions next to the stwcx/stdcx instructions (the end of the
sequence). Getting the destination address of this type of branch could
potentially (upon a failing branch condition) lead to the function
placing a breakpoint right at the stwcx/stdcx instruction, thus leading
us back to the same locking problem.

The variable "closing_insn" was created to check if the breakpoint at
the branch instruction's destination is right at the stwcx/stdcx
instruction. If so, ignore this breakpoint and consider only the
breakpoint after the closing of the atomic sequence.

Regards,
Luis
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