Re: [PATCH 3/5] range stepping: gdb

[Pedro Alves <palves at redhat dot com>]

On 05/15/2013 12:21 PM, Eli Zaretskii wrote:
>> Date: Wed, 15 May 2013 11:23:24 +0100
>> From: Pedro Alves <palves@redhat.com>
>> CC: gdb-patches@sourceware.org
>>
>>>> +@var{end} is the address of the first instruction beyond the step
>>>> +range, and @strong{not} the address of the last instruction within it.
>>>> +(This has the property that @var{start} == @var{end} single-steps
>>>> +once, and only once, even if the instruction at @var{start} jumps to
>>>> +@var{end}.)
>>>
>>> This sentence in parentheses got me completely confused.  Before
>>> reading it, I thought I understood what is this about; now I don't.
>>> In particular, if START is equal to END, then how in the world could
>>> the instruction at START jump to END?
>>
>> Sorry, I had that typo in the gdbserver code as well, fixed it
>> there, but missed this one.
>>
>> It should read, even if the instruction at @var{start} jumps to @var{start}.
>>
>> vCont;r first steps, then checks.  IOW:
>>
>>  vCont ;r ADDR1,ADDR1
>>
>> is equivalent to (and could be thought to supersede):
>>
>>  vCont ;s
>>
>>> And if END is excluded from the
>>> range, then why when START equals END do we step at all?  Please
>>> explain.
>>
>> It's just a design decision.  I recall at least one target I saw I worked
>> with that supported range stepping, and it didn't even a distinction
>> between range vs no-range step commands.  The way to do a single step
>> was to pass both addresses the same.  I find it a better design than
>> requiring the target do one current-address check _before_ stepping,
>> and another _after_ single-stepping.
> 
> Doesn't this mean that these two use cases are explicit exceptions
> from the rule that END is excluded? 

Nope.  There's no exception.

With:

  vCont ;r START,END

 #1 - The stub single-steps the thread.
 #2 - Once the thread stops, the stub checks whether the thread
      stopped in the [START,END) range.  If so, goto #1.
      It not, goto #3.
 #3 - The stub reports to gdb that the thread stopped stepping.

If it happens that START and END are the same, then #2 always
goes to #3.

When I said:

 "(This has the property that @var{start} == @var{end} single-steps
  once, and only once, even if the instruction at @var{start} jumps to
  @var{start}.)"

I was trying to clarify the case of the instruction at START being:

   jump START

Then,

  vCont ;r START,START

always single-steps once, and only once, instead of
continuously single-stepping that instruction without
reporting to GDB.

> If so, we should describe them as
> exceptions, not use them as evidence for the rule (which they
> evidently violate).
> 
> Or did I misunderstand again?

-- 
Pedro Alves