[PATCH] D32563: Add LiveRangeShrink pass to shrink live range within BB.

[Andrew Trick via Phabricator via llvm-commits]

atrick added a comment.

Ideally there should be a separate pass that runs on the SSA machine code (before register coalescing) to minimize register pressure and hide latency for chains of loads or FP ops. It should work across calls and loop boundaries. You could even do a kind of poor-man's cyclic scheduling this way.

MISched works at the lower level of instructions groups and CPU pipeline hazards. It would be nice if MISched worked at the level of extended basic blocks (it would be easy to implement and has been done out of tree). I don't think it makes as much sense for it to work across call sites though. That is not hard to implement but seems it will generate large DAGS and will be bad compile-time tradeoff.

MISched is not a scheduling algorithm, it's a scheduling framework. The generic scheduler is a pile of heuristics that exercise most of the functionality and seems to be working ok for several popular targets. The strategy that it takes is:

- Make a single scheduling pass handling all heuristics at once. Don't reorder the instructions at all unless the heuristics identify a register pressure or latency problem.

- Try to determine, before scheduling a block, whether register pressure or latency is likely to become a problem. This avoids the scheduler backing itself into a corner (we don't want the scheduler to backtrack).

You'll notice that this is very conservative with respect to managing compile time and preserving the decisions made by earlier passes.

You could follow that basic strategy and simply adjust the priority of heuristics for your target. You can add better up-front analysis to detect code patterns for each block before prioritizing heuristics. Or you could implement a completely different strategy. For example, schedule for register pressure first, then reschedule for ILP.

I probably won't be able to help you must more than that. Matthias has taken over maintenance of MISched. I think it would help if you give more background on your situation (sorry if I haven't paid attention or have forgotten). Is this PowerPC? Inorder/out-of-order?


https://reviews.llvm.org/D32563