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Re: [rfc] ``pc'' -> resume_addr?
> On Apr 11, 4:38pm, Andrew Cagney wrote:
>> GDB, in a number of places, refers to the ``pc'' - PC_REGNUM, frame->pc,
>> read_pc(), write_pc(), ...
>> I think this name choice was unfortunate. It is too easy for a
>> developer to confuse ``pc'' with the hardware ``pc''.
> Could you please explain further why you think the name choice was
I think the name ``pc'' brings with it a certain amount of baggage.
When reading a piece of code, it isn't clear if the hardware ``pc''
(possibly needing adjustment) or the program's resume address is being used.
On an x86, and m68k, for instance, the hardware PC may or may not need
to be adjusted (decr_pc_after_break()) before it becomes a frame->pc.
Within the frame, the ``pc'' designates ``resume'' address of the
function. Knowing this is important when understanding why some of the
frame code does:
if (frame->next != NULL)
return frame->pc - 1;
>> With this in mind, I'd like to propose a GDBspeak ``resume_addr''. It
>> is the address of the first instruction that will be executed when the
>> target resumes.
> So, if I understand you correctly, you're suggesting the following
> PC_REGNUM ==> RESUME_ADDR_REGNUM
This wouldn't change. If the hardware has a ``PC'' like register then
likely the maintainer will retain ``PC_REGNUM'' / $pc as an alias for it.
> frame->pc ==> frame->resume_addr
This, I think, should change. I'm 99% sure that this isn't the hardware
PC but rather the continue address for the frame (but notice I'm not
100% sure thanks to its poor definition).
> read_pc() ==> read_resume_addr()
This one is harder. Perhaphs it can be eliminated.
> write_pc() ==> write_resume_addr()
Check the default implementation. It not only modifies PC, but also NPC
and even NNPC. I think this function should be called something like -
Remember, when making an inferior function call, GDB does not set the
PC. Rather it sets the resume/continue address using the debug info.
For instance, on the sparc, it sets:
[PC] = resume_addr;
[NPC] = resume_addr + 4;
This behavour is very different to what the user is trying to achieve if
(gdb) jump *foo *bar
On a sparc, that would execute:
*(bar + 4)
*(bar + 8)
> Perhaps I've just gotten fond of ``pc'', but I don't really like any
> of these.
If someone uses PC in a typical e-mail, we'll know what they mean.
However, if someone uses PC when refering to GDB's internals, I don't
know that we'll be as sure.