[llvm] [AMDGPU][CodeGen] Incrementally update reserved regs for SIPreAllocateWWMRegs pass in RegisterClassInfo (PR #212201)

[Carl Ritson via llvm-commits]

================
@@ -120,6 +120,71 @@ void RegisterClassInfo::runOnMachineFunction(const MachineFunction &mf,
   }
 }
 
+void RegisterClassInfo::updateReservedRegs(const BitVector &ReservedInput) {
+  assert(MF && TRI && RegClass &&
+         "RegisterClassInfo must be initialized before updating reserved regs");
+  assert(ReservedInput.size() == Reserved.size() &&
+         "Reserved register bit vectors must have the same size");
+  if (ReservedInput == Reserved)
+    return;
+
+  // Cached orders cannot regain unreserved registers; recompute them lazily.
+  bool OnlyNewReservations = Reserved.subsetOf(ReservedInput);
+
+  // Subtract the old set to find newly reserved registers.
+  BitVector NewReservations = ReservedInput;
+  NewReservations.reset(Reserved);
+
+  Reserved = ReservedInput;
+
+  // Pressure limits depend on the number of allocatable registers.
+  std::fill_n(PSetLimits.get(), TRI->getNumRegPressureSets(), 0);
+
+  // NumRegs may hide entries beyond the stress limit, so those orders cannot
+  // safely be compacted using only their visible prefix.
+  if (!OnlyNewReservations || StressRA) {
+    ++Tag;
+    return;
+  }
+
+  for (const TargetRegisterClass &RC : TRI->regclasses()) {
+    RCInfo &Info = RegClass[RC.getID()];
+
+    // Skip stale class information.
+    if (Info.Tag != Tag)
+      continue;
+
+    unsigned NewNumRegs = 0;
+    uint8_t MinCost = uint8_t(~0u);
+    uint8_t LastCost = uint8_t(~0u);
+    unsigned LastCostChange = 0;
+
+    for (unsigned I = 0; I != Info.NumRegs; ++I) {
+      MCPhysReg PhysReg = Info.Order[I];
+      if (NewReservations.test(PhysReg))
----------------
perlfu wrote:

Shouldn't this just be testing `Reserved`?
By testing only `NewReservations` the cost of existing reservations will be included in the cost calculation?

Should costs values be covered by the unit tests?

https://github.com/llvm/llvm-project/pull/212201