[Qemu-devel] Re: Question on implementatio of GETPC()

[Jun Koi <junkoi2004@gmail.com>]

On Wed, Apr 28, 2010 at 5:12 PM, Jan Kiszka <jan.kiszka@siemens.com> wrote:
> Jun Koi wrote:
>> Hi,
>>
>> In x86, GETPC() is implemented as below:
>>
>> # define GETPC() ((void *)((unsigned long)__builtin_return_address(0) - 1))
>>
>> As I understand, it gets the returned address on the stack, then
>> subtract 1 to get back to the above address.
>>
>> Imagine we have code like this (pseudo asm code):
>>
>> ....
>> CALL <relative address>
>> <next-insn>
>> ....
>>
>> When we call GETPC, we get the address of <next-insn>, and subtract 1.
>> But the problem is that the CALL insn is more than 1 byte, so how can
>> GETPC() gives us the address of the CALL insn above?
>>
>> I guess I must misunderstood something here ....
>
> IIRC, the result of GETPC is used for a range check. So you just have to
> ensure that it points somewhere into the translated code sequence of the
> current target instruction.
>

Hmm if I am not wrong, the GETPC address is really used as jump target
of some code (such as when handling page fault), so that must be
accurate.
This is so confused to me!

Thanks,
J