[SlotIndexes] Implement support for poison checks

In debug builds, mark slot indexes for deleted instructions as poisoned
and add an isPoisoned method to allow writing assertions elsewhere in
the compiler. This restores some of the functionality that was removed
by 4cf8da9.

Author: jayfoad

llvm/include/llvm/CodeGen/SlotIndexes.h

@@ -43,21 +43,40 @@ class raw_ostream;
   /// SlotIndex & SlotIndexes classes for the public interface to this
   /// information.
   class IndexListEntry : public ilist_node<IndexListEntry> {
-    MachineInstr *mi;
+#if NDEBUG
+    // Disable poison checks such that setPoison will do nothing and isPoisoned
+    // will return false.
+    static constexpr unsigned PoisonBits = 0;
+    static constexpr unsigned PoisonVal = 0;
+#else
+    static constexpr unsigned PoisonBits = 1;
+    static constexpr unsigned PoisonVal = 1;
+#endif
+
+    PointerIntPair<MachineInstr *, PoisonBits, unsigned> mi;
     unsigned index;
 
   public:
-    IndexListEntry(MachineInstr *mi, unsigned index) : mi(mi), index(index) {}
+    IndexListEntry(MachineInstr *mi, unsigned index) : mi(mi, 0), index(index) {
+    }
 
-    MachineInstr* getInstr() const { return mi; }
+    MachineInstr* getInstr() const { return mi.getPointer(); }
     void setInstr(MachineInstr *mi) {
-      this->mi = mi;
+      this->mi.setPointer(mi);
     }
 
     unsigned getIndex() const { return index; }
     void setIndex(unsigned index) {
       this->index = index;
     }
+
+    void setPoison() {
+      mi.setInt(PoisonVal);
+    }
+
+    bool isPoisoned() const {
+      return mi.getInt();
+    }
   };
 
   template <>
@@ -285,6 +304,10 @@ class raw_ostream;
     SlotIndex getPrevIndex() const {
       return SlotIndex(&*--listEntry()->getIterator(), getSlot());
     }
+
+    bool isPoisoned() const {
+      return listEntry()->isPoisoned();
+    }
   };
 
   inline raw_ostream& operator<<(raw_ostream &os, SlotIndex li) {

llvm/lib/CodeGen/SlotIndexes.cpp

@@ -126,6 +126,7 @@ void SlotIndexes::removeMachineInstrFromMaps(MachineInstr &MI,
   mi2iMap.erase(mi2iItr);
   // FIXME: Eventually we want to actually delete these indexes.
   MIEntry.setInstr(nullptr);
+  MIEntry.setPoison();
 }
 
 void SlotIndexes::removeSingleMachineInstrFromMaps(MachineInstr &MI) {
@@ -152,6 +153,7 @@ void SlotIndexes::removeSingleMachineInstrFromMaps(MachineInstr &MI) {
   } else {
     // FIXME: Eventually we want to actually delete these indexes.
     MIEntry.setInstr(nullptr);
+    MIEntry.setPoison();
   }
 }