[PATCH] D68338: [AMDGPU] Remove dubious logic in bidirectional list scheduler

[Jay Foad via Phabricator via llvm-commits]

foad created this revision.
foad added reviewers: arsenm, tstellar, rampitec, kzhuravl, vpykhtin, dstuttard, tpr, atrick, MatzeB.
Herald added subscribers: hiraditya, t-tye, yaxunl, nhaehnle, wdng, jvesely.
Herald added a project: LLVM.

pickNodeBidirectional tried to compare the best top candidate and the
best bottom candidate by examining TopCand.Reason and BotCand.Reason.
This is unsound because, after calling pickNodeFromQueue, Cand.Reason
does not reflect the most important reason why Cand was chosen. Rather
it reflects the most recent reason why it beat some other potential
candidate, which could have been for some low priority tie breaker
reason.

I have seen this cause problems where TopCand is a good candidate, but
because TopCand.Reason is ORDER (which is very low priority) it is
repeatedly ignored in favour of a mediocre BotCand. This is not how
bidirectional scheduling is supposed to work.

To fix this I changed the code to always compare TopCand and BotCand
directly, like the generic implementation of pickNodeBidirectional does.
This removes some uncommented AMDGPU-specific logic; if this logic turns
out to be important then perhaps it could be moved into an override of
tryCandidate instead.

Graphics shader benchmarking on gfx10 shows a lot more positive than
negative effects from this change.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D68338

Files:
  llvm/lib/Target/AMDGPU/GCNSchedStrategy.cpp


Index: llvm/lib/Target/AMDGPU/GCNSchedStrategy.cpp
===================================================================
--- llvm/lib/Target/AMDGPU/GCNSchedStrategy.cpp
+++ llvm/lib/Target/AMDGPU/GCNSchedStrategy.cpp
@@ -230,33 +230,11 @@
   // Pick best from BotCand and TopCand.
   LLVM_DEBUG(dbgs() << "Top Cand: "; traceCandidate(TopCand);
              dbgs() << "Bot Cand: "; traceCandidate(BotCand););
-  SchedCandidate Cand;
-  if (TopCand.Reason == BotCand.Reason) {
-    Cand = BotCand;
-    GenericSchedulerBase::CandReason TopReason = TopCand.Reason;
-    TopCand.Reason = NoCand;
-    GenericScheduler::tryCandidate(Cand, TopCand, nullptr);
-    if (TopCand.Reason != NoCand) {
-      Cand.setBest(TopCand);
-    } else {
-      TopCand.Reason = TopReason;
-    }
-  } else {
-    if (TopCand.Reason == RegExcess && TopCand.RPDelta.Excess.getUnitInc() <= 0) {
-      Cand = TopCand;
-    } else if (BotCand.Reason == RegExcess && BotCand.RPDelta.Excess.getUnitInc() <= 0) {
-      Cand = BotCand;
-    } else if (TopCand.Reason == RegCritical && TopCand.RPDelta.CriticalMax.getUnitInc() <= 0) {
-      Cand = TopCand;
-    } else if (BotCand.Reason == RegCritical && BotCand.RPDelta.CriticalMax.getUnitInc() <= 0) {
-      Cand = BotCand;
-    } else {
-      if (BotCand.Reason > TopCand.Reason) {
-        Cand = TopCand;
-      } else {
-        Cand = BotCand;
-      }
-    }
+  SchedCandidate Cand = BotCand;
+  TopCand.Reason = NoCand;
+  GenericScheduler::tryCandidate(Cand, TopCand, nullptr);
+  if (TopCand.Reason != NoCand) {
+    Cand.setBest(TopCand);
   }
   LLVM_DEBUG(dbgs() << "Picking: "; traceCandidate(Cand););
 


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