[llvm] [AMDGPU] Add and optionally use GCNIterativeRPTrackers (PR #88797)

[Matt Arsenault via llvm-commits]

================
@@ -570,6 +612,127 @@ bool GCNUpwardRPTracker::isValid() const {
   return true;
 }
 
+////////////////////////////////////////////////////////////////////////////////
+// GCNIterativeRPTrackers
+
+void GCNIterativeRPTracker::reset(const MachineRegisterInfo *MRI_,
+                                  const LiveRegSet *LiveRegsCopy) {
+
+  MRI = MRI_;
+  if (LiveRegsCopy && &LiveRegs != LiveRegsCopy)
+    LiveRegs = *LiveRegsCopy;
+  if (!LiveRegsCopy)
+    LiveRegs.clear();
+  MaxPressure = CurPressure = getRegPressure(*MRI, LiveRegs);
+}
+
+// Mostly copy+paste from GCNUpwardRPTracker::recede
+void GCNIterativeUpwardRPTracker::recede(const MachineInstr &MI,
+                                         LiveIntervals *LIS) {
+  assert(MRI && "call reset first");
+
+  if (MI.isDebugInstr())
+    return;
+
+  SmallVector<RegisterMaskPair, 8> RegUses;
+  collectVirtualRegUses(RegUses, MI, *LIS, *MRI);
+
+  // calc pressure at the MI (defs + uses)
+  auto AtMIPressure = CurPressure;
+  for (const auto &U : RegUses) {
+    auto LiveMask = LiveRegs[U.RegUnit];
+    AtMIPressure.inc(U.RegUnit, LiveMask, LiveMask | U.LaneMask, *MRI);
+  }
+  // update max pressure
+  MaxPressure = max(AtMIPressure, MaxPressure);
+
+  for (const auto &MO : MI.all_defs()) {
+    if (!MO.getReg().isVirtual() || MO.isDead())
+      continue;
+
+    auto Reg = MO.getReg();
+    auto I = LiveRegs.find(Reg);
+    if (I == LiveRegs.end())
+      continue;
+    auto &LiveMask = I->second;
+    auto PrevMask = LiveMask;
+    LiveMask &= ~getDefRegMask(MO, *MRI);
+    CurPressure.inc(Reg, PrevMask, LiveMask, *MRI);
+    if (LiveMask.none())
+      LiveRegs.erase(I);
+  }
+  for (const auto &U : RegUses) {
+    auto &LiveMask = LiveRegs[U.RegUnit];
+    auto PrevMask = LiveMask;
+    LiveMask |= U.LaneMask;
+    CurPressure.inc(U.RegUnit, PrevMask, LiveMask, *MRI);
+  }
+  assert(CurPressure == getRegPressure(*MRI, LiveRegs));
+}
+
+// Mostly copy+paste from GCNDownwardRPTracker::(advanceBeforeNext +
+// advanceToNext)
+void GCNIterativeDownwardRPTracker::advance(const MachineInstr &MI,
+                                            LiveIntervals *LIS) {
+  assert(MRI && "call reset first");
+  // Add new registers or mask bits.
+  for (const auto &MO : MI.all_defs()) {
+    Register Reg = MO.getReg();
+    if (!Reg.isVirtual())
+      continue;
+    if (MO.isDead())
+      continue;
+    auto &LiveMask = LiveRegs[Reg];
+    auto PrevMask = LiveMask;
+    LiveMask |= getDefRegMask(MO, *MRI);
+    CurPressure.inc(Reg, PrevMask, LiveMask, *MRI);
+  }
+
+  SlotIndex SI = LIS->getInstructionIndex(MI).getBoundaryIndex();
+  assert(SI.isValid());
+
+  // Remove dead registers or mask bits.
+  SmallSet<Register, 8> SeenRegs;
+  for (auto &MO : MI.operands()) {
+    if (!MO.isReg() || !MO.getReg().isVirtual())
+      continue;
+    if (!MO.isUse())
+      continue;
+    if (!MO.readsReg())
+      continue;
----------------
arsenm wrote:

Checking !isUse before !readsReg is almost certainly wrong. You almost always want readsReg over isUse 

https://github.com/llvm/llvm-project/pull/88797