Use getMBBEndIdx(MBB).getPrevSlot() as getMBBLastIdx result.

Author: vpykhtin

llvm/include/llvm/CodeGen/SlotIndexes.h

@@ -481,13 +481,11 @@ class raw_ostream;
     /// This index corresponds to the dead slot of the last non-debug
     /// instruction and can be used to find live-out ranges of the block. Note
     /// that getMBBEndIdx returns the start index of the next block, which is
-    /// also used as the start index for segments with phi-def values. Returns
-    /// an invalid SlotIndex if the block has no non-debug instructions.
+    /// also used as the start index for segments with phi-def values. If the
+    /// basic block doesn't contain any non-debug instructions, this returns
+    /// the same as getMBBStartIdx.getDeadSlot().
     SlotIndex getMBBLastIdx(const MachineBasicBlock *MBB) const {
-      auto LastInstrI = MBB->getLastNonDebugInstr();
-      return LastInstrI == MBB->end()
-                 ? SlotIndex()
-                 : getInstructionIndex(*LastInstrI).getDeadSlot();
+      return getMBBEndIdx(MBB).getPrevSlot();
     }
 
     /// Iterator over the idx2MBBMap (sorted pairs of slot index of basic block

llvm/lib/Target/AMDGPU/GCNRegPressure.cpp

@@ -907,13 +907,13 @@ bool GCNRegPressurePrinter::runOnMachineFunction(MachineFunction &MF) {
     OS << ":\n";
 
     SlotIndex MBBStartSlot = LIS.getSlotIndexes()->getMBBStartIdx(&MBB);
-    SlotIndex MBBEndSlot = LIS.getSlotIndexes()->getMBBEndIdx(&MBB);
     SlotIndex MBBLastSlot = LIS.getSlotIndexes()->getMBBLastIdx(&MBB);
+    bool MBBHasNonDebugInstrs = MBBStartSlot != MBBLastSlot.getBaseIndex();
 
     GCNRPTracker::LiveRegSet LiveIn, LiveOut;
     GCNRegPressure RPAtMBBEnd;
 
-    if (!MBBLastSlot.isValid()) { // MBB doesn't have any non-debug instrs.
+    if (!MBBHasNonDebugInstrs) {
       LiveIn = LiveOut = getLiveRegs(MBBStartSlot, LIS, MRI);
       RPAtMBBEnd = getRegPressure(MRI, LiveIn);
     } else if (UseDownwardTracker) {
@@ -966,13 +966,14 @@ bool GCNRegPressurePrinter::runOnMachineFunction(MachineFunction &MF) {
     OS << printRP(RPAtMBBEnd) << '\n';
 
     OS << PFX "  Live-out:" << llvm::print(LiveOut, MRI);
-    if (UseDownwardTracker && MBBLastSlot.isValid())
+    if (UseDownwardTracker)
       ReportLISMismatchIfAny(LiveOut, getLiveRegs(MBBLastSlot, LIS, MRI));
 
     GCNRPTracker::LiveRegSet LiveThrough;
     for (auto [Reg, Mask] : LiveIn) {
       LaneBitmask MaskIntersection = Mask & LiveOut.lookup(Reg);
       if (MaskIntersection.any()) {
+        SlotIndex MBBEndSlot = LIS.getSlotIndexes()->getMBBEndIdx(&MBB);
         LaneBitmask LTMask = getRegLiveThroughMask(
             MRI, LIS, Reg, MBBStartSlot, MBBEndSlot, MaskIntersection);
         if (LTMask.any())