Re: RFA: Breakpoint infrastructure cleanups [1/8] - define impl_breakpoint

[Andrew Cagney <ac131313@redhat.com>]

> Daniel Jacobowitz <drow@mvista.com> writes:
> 
>> +/* GDB maintains two groups of breakpoints and related events.  One
>> +   group are the "implementation breakpoints"; these are minimal
>> +   structures used to manage stopping the program.  They map to a specific
>> +   stop reason (trap at a particular PC, for instance).  The other group
>> +   are "user breakpoints"; these carry higher-level information including
>> +   source locations and breakpoint conditions.  */

GDB users are going to need to know that breakpoints are implemented at 
two levels, and both of those levels will need to be visible.  That's 
why people are suggesting pairs such as logical/physical, 
virtual/actual, ...  Calling one "user" and the other "impl" or 
"machine" does not convey that implicit relationship.  In fact calling 
one "user" is saying "hey users, just you stick to this user stuff".

``specific machine-level mechanisms used to stop the program: trap 
instructions patched into the code ("software breakpoints"), hardware 
breakpoints, hardware watchpoint registers, and so on.''

This isn't correct.  The low-level breakpoints correspond to 
target-specific and not machine-specific mechanisms.   For instance, the 
target can directly implement relatively abstract breakpoint/watchpoint 
mechanisms.  Eg: per-thread breakpoints where the the underlying 
machine-specific mechanisms are hidden from GDB.  This abstract model is 
extreemly common in 'debug agent' code, I've seen at least three people 
being forced to hack around GDB's bogus machine-level assumptions.

Andrew