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

[Matthias Braun via llvm-dev]

> On Sep 12, 2017, at 6:44 AM, Jonas Paulsson via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi Ghassan,
>> As for live-through information, we found that the machine scheduler does call initLiveThru() and here is a pointer to the code:
>> 
>> 
>> 
>> https://gitlab.com/CSUS_LLVM/LLVM_DRAGONEGG/blob/master/Generic/llvmTip/llvm-master/lib/CodeGen/MachineScheduler.cpp#L921 <https://gitlab.com/CSUS_LLVM/LLVM_DRAGONEGG/blob/master/Generic/llvmTip/llvm-master/lib/CodeGen/MachineScheduler.cpp#L921>
>> 
> The first part of the comment above initLiveThru() says "The register tracker is unaware of global liveness so ignores normal live-thru ranges...". It is then of course confusing to see these methods like initLiveThru()...
> 
> My understanding is that (please correct me if I'm wrong)
> 1. All instructions are traversed bottom-up during DAG building. While doing this reg pressure is tracked based on just looking at those instructions. So if a def has no use in an mbb it is a "live-out" reg, and if there is a use with no def, it would become "live-in". This is then a kind of local live-through concept, in contrast to a true live-through analysis which would be aware of registers not used/defed in the region as well.
Yes, the first pass during DAG construction determines the maximum register pressure and the list of live-out values. I think the code consults liveintervals to differentiate dead-defs from true live-outs or detect killing uses that aren't marked as such.

> 2. We should ideally have an analysis of global liveness so that the regpressure trackers could be properly initialized, but this is currently missing. Of course, one might hope that it wouldn't be too hard to extend LiveIntervals to also provide this information... It would be interesting to merely try this and see how valuable it would be...
Again a global minimum amount of spills/reloads is not necessarily better than a good schedule inside a loop with extra spills/reloads outside the loop. But would certainly be worth exploring.
Writing a naive function to determine all live values at a certain point is easy: Just iterate over all vregs and check for each whether the point in question is covered by a live interval. However to do this efficiently so it can be used in production would probably require a concept such as keeping live-in/live-out lists on basic blocks up-to-date. To put that into production we should first demonstrate that it is worth it.

- Matthias

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