RFR(XS) 8032894: Remove dead code in Pressure::lower

[Vladimir Kozlov]

 > Do you still want me to add the assert even though we only find a new
 > _final_pressure when raising?

No, but I want you to look on my suggestion:

 > May be all _*_pressure fields should be private and code which updates
 > them is located in one place. Then it would be more obvious that code
 > you are removing is useless.

Thanks,
Vladimir

On 1/29/14 3:05 PM, Niclas Adlertz wrote:
> Hi Vladimir,
>
> On 2014-01-29 23:31, Vladimir Kozlov wrote:> Hi Niclas
>>
>> On 1/29/14 2:02 PM, Niclas Adlertz wrote:
>>> Hi Vladimir,
>>>
>>> The initial states of _current_pressure and _final_pressure are computed
>>> in PhaseChaitin::compute_initial_block_pressure().
>>> We loop through all live ranges that are in the LIVE_OUT of the block,
>>> and raise the _current_pressure (for either int or float) for each live
>>> range. If the _current_pressure is bigger than the _final_pressure
>>> (which is initially 0), we set the _final_pressure to be
>>> _current_pressure. We check this each time we increase the
>>> _current_pressure. This is done in Pressure::raise(). (I.e. the
>>> _final_pressure is the highest value of _current_pressure that we ever
>>> encounter in the block)
>>
>> It is not accurate, it could be higher. You overwrite it in
>> check_for_high_pressure_transition_at_fatproj().
> In a way it is accurate, because at a fat_proj the _current_pressure
> will be raised and then lowered right after (stepping backwards) since
> the fat_proj is only "live" at its definition.
> (That's why we never change the _current_pressure at a fat_proj but
> still look for a _final_pressure change).
>
>>
>> May be all _*_pressure fields should be private and code which updates
>> them is located in one place. Then it would be more obvious that code
>> you are removing is useless.
>>
>>>
>>> The _high_pressure_limit is passed into the constructor of the Pressure
>>> class. This is done in the beginning of the build_ifg_physical():
>>>
>>> Pressure int_pressure(last_inst + 1, INTPRESSURE);
>>> Pressure float_pressure(last_inst + 1, FLOATPRESSURE);
>>>
>>>> How we can get case (_current_pressure == _high_pressure_limit)?
>>> When we lower the pressure, it is assumed that we lower each time by 1
>>> (this happens every time we remove a definition from the LIVE_OUT when
>>> stepping backwards in the block). So if _current_pressure is greater
>>> than _high_pressure_limit, and we lower the _current_pressure, we might
>>> hit the case when _current_pressure == _high_pressure_limit. If so, we
>>> have found a Low to High pressure transition in the block. (Low to High
>>> when starting from the top of the block)
>>> However, there is one problem with this case, since the pressure could
>>> be lowered by more than 1. (Long or Double will lower it by 2). So we
>>> might miss a transition when lowering. There is already a bug
>>> (JDK-8032886) filed for this which will be out soon. I didn't fix this
>>> bug in the clean up since I wanted them to be separate commits.
>>
>> Okay, sounds good.
>>
>> Thanks,
>> Vladimir
>>
>
> Do you still want me to add the assert even though we only find a new
> _final_pressure when raising?
>
> Kind Regards,
> Niclas Adlertz
>
>>>
>>> Kind Regards,
>>> Niclas Adlertz
>>>
>>>
>>> On 2014-01-29 19:53, Vladimir Kozlov wrote:
>>>> What is initial state of _current_pressure, _high_pressure_limit,
>>>> _final_pressure? How we can get case (_current_pressure ==
>>>> _high_pressure_limit)? May be we should replace the code with assert:
>>>>
>>>> assert(_current_pressure <= _final_pressure
>>>>
>>>> Thanks,
>>>> Vladimir
>>>>
>>>> On 1/29/14 4:09 AM, Niclas Adlertz wrote:
>>>>> Hi all,
>>>>>
>>>>> When building the physical IFG, we step backwards in each block, and
>>>>> remove things that are defined from the LIVE_OUT, (and lower the
>>>>> current
>>>>> pressure) and add input to the LIVE_OUT (and raising the current
>>>>> pressure). Each time we lower or raise the current pressure, we
>>>>> check if
>>>>> it's bigger than the current maximum pressure, known as
>>>>> final_pressure.
>>>>> However the final_pressure can never increase when removing
>>>>> definitions
>>>>> (i.e. lower the current pressure) so the check for a new
>>>>> final_pressure
>>>>> in Pressure::lower is useless.
>>>>>
>>>>> webrev: http://cr.openjdk.java.net/~adlertz/JDK-8032894/webrev00/
>>>>> bug: https://bugs.openjdk.java.net/browse/JDK-8032894
>>>>>
>>>>> Kind Regards,
>>>>> Niclas Adlertz