Re: [rfc] ``pc'' -> resume_addr?

[Andrew Cagney <ac131313@cygnus.com>]

> 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
> unfortunate?

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;
	else
	  return frame->pc;

>> 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
> renaming:
> 
>     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 - 
set_resume_address()?

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 
they enter:

	(gdb) jump *foo *bar

On a sparc, that would execute:

	*foo
	*bar
	*(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.

enjoy,
Andrew