[llvm-dev] Register pressure calculation in the machine scheduler and live-through registers

[Jonas Paulsson via llvm-dev]

On 2017-08-30 23:14, Andrew Trick wrote:
>
>> On Aug 30, 2017, at 1:43 PM, Matthias Braun <matze at braunis.de 
>> <mailto:matze at braunis.de>> wrote:
>>
>> That means you cannot use the code from RegisterPressure.{cpp|h} to 
>> compute this. The other liveness analysis we have in llvm codegen is 
>> LiveIntervals (LiveItnervalAnalysis) which gives you a list of 
>> liveness segments of a given vreg (the same representation is used in 
>> most linear scan allocators even though LLVM is not using a linear 
>> scan approach any more). This representation is not optimized for 
>> register pressure queries though: If you want to know how many 
>> variables are alive at a certain point in the program you have to 
>> check all virtual registers to see whether that point is contained in 
>> the liverange of that variable.
>> To make this efficient you probably need some form of precomputation 
>> over the whole function.
>
> The code in RegisterPressure.cpp is meant to work with LiveIntervals. 
> Those queries only see within a block but are meant to be “seeded” 
> with live-through information. That could be done be directly calling 
> `addLiveRegs`. Alternately you can record live-through pressure 
> separately via `initLiveThru`. It’s just that the MachineScheduler 
> does not bother initializing the live-through information.
>
> As Matthias said, actually determining live-through information 
> requires a separate global liveness analysis, because LiveIntervals 
> doesn’t tell you “what’s live at this point”.
>
> -Andy
Ghassan, have you managed to try this, yet? This seems interesting to me 
on SystemZ, as I am still seeing increased spilling when activating 
mischeduler for SystemZ and switching to isel source-order scheduling.

/Jonas
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