[llvm] [NFCI][AMDGPU] Try to use PressureDiff to Calculate RegPressure. (PR #94221)

[Matt Arsenault via llvm-commits]

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@@ -116,31 +116,85 @@ void GCNSchedStrategy::initialize(ScheduleDAGMI *DAG) {
                     << ", SGPRExcessLimit = " << SGPRExcessLimit << "\n\n");
 }
 
+static bool canUsePressureDiffs(const SUnit &SU) {
+  if (!SU.isInstr())
+    return false;
+
+  // Cannot use pressure diffs for subregister defs or with physregs, it's
+  // imprecise in both cases.
+  for (const auto &Op : SU.getInstr()->operands()) {
+    if (!Op.isReg() || Op.isImplicit())
+      continue;
+    if (Op.getReg().isPhysical() ||
+        (Op.isDef() && Op.getSubReg() != AMDGPU::NoSubRegister))
+      return false;
+  }
+  return true;
+}
+
+static void getRegisterPressures(bool AtTop,
+                                 const RegPressureTracker &RPTracker, SUnit *SU,
+                                 std::vector<unsigned> &Pressure,
+                                 std::vector<unsigned> &MaxPressure) {
+  // getDownwardPressure() and getUpwardPressure() make temporary changes to
+  // the tracker, so we need to pass those function a non-const copy.
+  RegPressureTracker &TempTracker = const_cast<RegPressureTracker &>(RPTracker);
+  if (AtTop)
+    TempTracker.getDownwardPressure(SU->getInstr(), Pressure, MaxPressure);
+  else
+    TempTracker.getUpwardPressure(SU->getInstr(), Pressure, MaxPressure);
+}
+
 void GCNSchedStrategy::initCandidate(SchedCandidate &Cand, SUnit *SU,
                                      bool AtTop,
                                      const RegPressureTracker &RPTracker,
                                      const SIRegisterInfo *SRI,
                                      unsigned SGPRPressure,
-                                     unsigned VGPRPressure) {
+                                     unsigned VGPRPressure, bool IsBottomUp) {
   Cand.SU = SU;
   Cand.AtTop = AtTop;
 
   if (!DAG->isTrackingPressure())
     return;
 
-  // getDownwardPressure() and getUpwardPressure() make temporary changes to
-  // the tracker, so we need to pass those function a non-const copy.
-  RegPressureTracker &TempTracker = const_cast<RegPressureTracker&>(RPTracker);
-
   Pressure.clear();
   MaxPressure.clear();
 
-  if (AtTop)
-    TempTracker.getDownwardPressure(SU->getInstr(), Pressure, MaxPressure);
-  else {
-    // FIXME: I think for bottom up scheduling, the register pressure is cached
-    // and can be retrieved by DAG->getPressureDif(SU).
-    TempTracker.getUpwardPressure(SU->getInstr(), Pressure, MaxPressure);
+  if (AtTop || !canUsePressureDiffs(*SU)) {
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arsenm wrote:

Document this is fast vs. slow path? 

https://github.com/llvm/llvm-project/pull/94221