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

Jonas Paulsson via llvm-dev llvm-dev at lists.llvm.org
Mon Sep 11 03:15:29 PDT 2017

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 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170911/01d85805/attachment.html>

More information about the llvm-dev mailing list