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

Niclas Adlertz niclas.adlertz at oracle.com
Wed Jan 29 14:02:25 PST 2014

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)

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.

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

More information about the hotspot-compiler-dev mailing list