Re: RFC : Handling breakpoints on archs. with imprecise exceptions.

[Ramana Radhakrishnan <ramana.radhakrishnan@codito.com>]

Hi,


>>While looking at a GDB port to a processor that has 
>>imprecise exceptions/ interrupts i.e. the equivalent of a 
>>software breakpoint would require 4 instructions to stop. 
>>With my research I was unable to find any GDB port that 
>>needed to handle such a case.
>>
>>The mechanism that is in mind is the following for setting 
>>breakpoints.
> 
> 
> It sounds plausible, although messy.  Does a single-instruction
> branch always give you enough range to reach a breakpoint table?

At the moment yes, since the branch would give a 16MB range 
per app. The case is for uClinux apps on the ARC600 
platform. ( though one could possibly dream up a solution 
that cascaded branches maybe if the footprint so demanded 
this sic :-)  or KevinB's suggestion later on in the thread . )

> 
> I suspect you could handle this by wrapping gdbarch_read_pc, so that
> a "breakpoint" at a particular "pc" would appear to stop there rather
> than in the table.  Be sure to restore the correct pc at that point.

This is where one does the reverse mapping.

> That and breakpoint_from_pc may be all the hooks you need.  And maybe
> hooks in target_insert_breakpoint/target_remove_breakpoint to reference
> count.
> 
> 
>>a. Define gdbarch_adjust_breakpoint_address in the backend 
>>to store the mapping in the backend for the PC at which 
>>breakpoint has been set to the actual value for the PC where 
>>the breakpoint would be reported to have been hit.
>>
>>b. Define deprecated_target_wait_hook in the backend to 
>>restore the actual value of the PC for GDB to continue with 
>>its work.However as this is a deprecated hook I would not 
>>like to use this in a new port.
>>
>>c. Add a new notify_backend_breakpoint_deleted_hook since 
>>the backend needs notification for the breakpoint being 
>>deleted and hence free an entry in the breakpoint table.
> 
> 
> You should be hooking insert/remove breakpoint, not add/delete user
> breakpoint.

Aha! that would be because for one, breakpoints for single 
stepping need not necessarily be in the breakpoint table .

> 
> Does gdbarch_read_pc do everything you need for the wait_hook?  You can
> update the PC from there if necessary.
> 

Sounds equivalent, though not yet sure. Will give it a shot 
in the next couple of days and post the results.

cheers
Ramana

-- 
Ramana Radhakrishnan
GNU Tools
codito ergo sum (www.codito.com)