Re: [PATCH 0/2] Support the new PPC476 processor

[Eli Zaretskii <eliz@gnu.org>]

> From: Sérgio_Durigan_Júnior <sergiodj@linux.vnet.ibm.com>
> Date: Wed, 16 Dec 2009 18:47:16 -0200
> Cc: Thiago Jung Bauermann <bauerman@br.ibm.com>,         Luis Machado <luisgpm@linux.vnet.ibm.com>,         Matt Tyrlik <tyrlik@us.ibm.com>
> 
> This series of patches implement the support for the new PowerPC 476 IBM 
> processor.  This is an embedded processor with some nice debugging 
> capabilities, and I believe you will like to see the new types of 
> breakpoints/watchpoints implemented on GDB.

Thanks!

I have a few preliminary comments and questions about the proposed UI:

>   * Simple Hardware Watchpoints: Monitors accesses to a single data address.  
> For the PPC476, 2 of them are available provided that no Range/Mask 
> watchpoints are being used.  There is also the possibility of using conditions 
> when monitoring the values.  In this case, GDB will place the condition inside 
> a register, and the evaluation will be hardware-accelerated, which will speed-
> up significantly the debugging process.  The PPC476 processor has two 
> registers that can be used to hardware-accelerate the condition evaluation.
> 
>     - Available GDB commands: [r|a]watch <variable> (rwatch/awatch/watch) or 
> [r|a]watch <variable> if <variable> == <4-bytes unsigned constant>
> 
>     - Usage example: awatch i / rwatch *0xbffff8e8 / watch i if i == 28

Will GDB decide automatically whether to use hardware-accelerated
conditions or the current implementation, whereby an unconditional
hardware watchpoint is set, and when it breaks, GDB itself evaluates
the condition?  I think this should be done automatically.

>   * Range Hardware Watchpoints: Monitors accesses to an interval of data 
> addresses.  For the 476, a single Range Hardware Watchpoint can be used 
> provided that no Simple/Mask watchpoints are being used.
> 
>     - Available GDB commands: [r|a]watch-range <address1>:<address2 | +<uint 
> length>>, where <address1> <= <address2> (rwatch-range/awatch-range/watch-
> range)
> 
>     - Usage example: watch-range &i,&k / awatch-range 0xbffff8e8,0xbffff8f8 / 
> rwatch-range 0xbffff8e8,+8

Why is there a need for a separate GDB command?  What are the
use-cases where the user would want to watch a region that is spanned
by more than one (albeit large) variable?  If such use-cases are
infrequent enough or obscure, then we could simply use the normal
watch commands, and support any corner use-cases with something like

   watch *0xbffff8e8@8

using what is a normal GDB semantics for artificial arrays.  IOW, no
need for either the comma-separated list of 2 addresses or a separate
command.

>   * Mask Hardware Watchpoints: Monitors accesses to data addresses that match 
> a specific pattern.  For the 476, a single Mask Hardware Watchpoint can be 
> used provided that no Simple/Range watchpoints are being used.  Due to the 
> processor's design, the precise data address of the watchpoint trigger is not 
> available, so the user must check the instruction that caused the trigger 
> (usually at PC - 4) to obtain such data address.  With such data address in 
> hand, it's possible to tell if its contents have changed.
> 
>     - Available GDB commands: [r|a]watch <variable | address> mask 
> <mask_value>
> 
>     - Usage example: watch i mask 0xffffff00 / awatch *0xbffff8e8 mask 
> 0xffffff00

I'm not sure about the semantics of this: what exactly does the mask
do? how does it modify the unmasked command for the same address?

>   * Range Hardware Breakpoint: Monitors an interval of instruction addresses.
> 
>     - Available GDB commands: hbreak-range <address1>:<address2 | +<unsigned 
> int length>> where <address1> <= <address2> / hbreak-range <line number 
> start>:<line number end>
> 
>     - Usage example: hbreak-range 0x10000658,0x10000660 / hbreak-range 
> 0x10000658,+8 / hbreak-range 20,30

Again, what are the use-cases where such breakpoints would be useful?