RegisterPressureTracker: reword stale comments.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237544 91177308-0d34-0410-b5e6-96231b3b80d8

Author: atrick

lib/CodeGen/MachineScheduler.cpp

@@ -2611,8 +2611,7 @@ void GenericScheduler::tryCandidate(SchedCandidate &Cand,
                  TryCand, Cand, PhysRegCopy))
     return;
 
-  // Avoid exceeding the target's limit. If signed PSetID is negative, it is
-  // invalid; convert it to INT_MAX to give it lowest priority.
+  // Avoid exceeding the target's limit.
   if (DAG->isTrackingPressure() && tryPressure(TryCand.RPDelta.Excess,
                                                Cand.RPDelta.Excess,
                                                TryCand, Cand, RegExcess))

lib/CodeGen/RegisterPressure.cpp

@@ -750,9 +750,11 @@ void RegPressureTracker::bumpUpwardPressure(const MachineInstr *MI) {
 ///
 /// This assumes that the current LiveOut set is sufficient.
 ///
-/// FIXME: This is expensive for an on-the-fly query. We need to cache the
-/// result per-SUnit with enough information to adjust for the current
-/// scheduling position. But this works as a proof of concept.
+/// This is expensive for an on-the-fly query because it calls
+/// bumpUpwardPressure to recompute the pressure sets based on current
+/// liveness. This mainly exists to verify correctness, e.g. with
+/// -verify-misched. getUpwardPressureDelta is the fast version of this query
+/// that uses the per-SUnit cache of the PressureDiff.
 void RegPressureTracker::
 getMaxUpwardPressureDelta(const MachineInstr *MI, PressureDiff *PDiff,
                           RegPressureDelta &Delta,
@@ -809,10 +811,8 @@ getMaxUpwardPressureDelta(const MachineInstr *MI, PressureDiff *PDiff,
 #endif
 }
 
-/// This is a prototype of the fast version of querying register pressure that
-/// does not directly depend on current liveness. It's still slow because we
-/// recompute pressure change on-the-fly. This implementation only exists to
-/// prove correctness.
+/// This is the fast version of querying register pressure that does not
+/// directly depend on current liveness.
 ///
 /// @param Delta captures information needed for heuristics.
 ///
@@ -950,6 +950,11 @@ void RegPressureTracker::bumpDownwardPressure(const MachineInstr *MI) {
 /// register units of that pressure set introduced by this instruction.
 ///
 /// This assumes that the current LiveIn set is sufficient.
+///
+/// This is expensive for an on-the-fly query because it calls
+/// bumpDownwardPressure to recompute the pressure sets based on current
+/// liveness. We don't yet have a fast version of downward pressure tracking
+/// analagous to getUpwardPressureDelta.
 void RegPressureTracker::
 getMaxDownwardPressureDelta(const MachineInstr *MI, RegPressureDelta &Delta,
                             ArrayRef<PressureChange> CriticalPSets,