[llvm] [RegAllocFast] Refactor dominates algorithm for large basic block (PR #72250)

[Phoebe Wang via llvm-commits]

================
@@ -62,6 +62,71 @@ static RegisterRegAlloc fastRegAlloc("fast", "fast register allocator",
 
 namespace {
 
+/// Assign ascending index for instructions in machine basic block. The index
+/// can be used to determine dominance between instructions in same MBB.
+class InstrPosIndexes {
+public:
+  void init(const MachineBasicBlock &MBB) {
+    CurMBB = &MBB;
+    Instr2PosIndex.clear();
+    uint64_t LastIndex = 0;
+    for (const MachineInstr &MI : MBB) {
+      LastIndex += InstrDist;
+      Instr2PosIndex[&MI] = LastIndex;
+    }
+  }
+
+  /// Set \p Index to index of \p MI. If \p MI is new inserted, it try to assign
+  /// index without affecting existing instruction's index. Return true if all
+  /// instructions index has been reassigned.
+  bool getIndex(const MachineInstr &MI, uint64_t &Index) {
+    assert(MI.getParent() == CurMBB && "MI is not in CurMBB");
+    if (Instr2PosIndex.count(&MI)) {
+      Index = Instr2PosIndex[&MI];
+      return false;
+    }
+
+    unsigned Distance = 1;
+    MachineBasicBlock::const_iterator Start = MI.getIterator(),
+                                      End = std::next(Start);
+    while (Start != CurMBB->begin() &&
+           !Instr2PosIndex.count(&*std::prev(Start))) {
+      --Start;
+      ++Distance;
+    }
+    while (End != CurMBB->end() && !Instr2PosIndex.count(&*(End))) {
+      ++End;
+      ++Distance;
+    }
+
+    uint64_t LastIndex =
+        Start == CurMBB->begin() ? 0 : Instr2PosIndex.at(&*std::prev(Start));
+    uint64_t Step = End == CurMBB->end()
+                        ? static_cast<uint64_t>(InstrDist)
+                        : (Instr2PosIndex.at(&*End) - LastIndex - 1) / Distance;
+
+    // Reassign index for all instructions if number of new inserted
+    // instructions exceed slot or all instructions are new.
+    if (LLVM_UNLIKELY(!Step || (!LastIndex && Step == InstrDist))) {
+      init(*CurMBB);
+      Index = Instr2PosIndex.at(&MI);
+      return true;
+    }
+
+    for (auto I = Start; I != End; ++I) {
+      LastIndex += Step;
+      Instr2PosIndex[&*I] = LastIndex;
+    }
----------------
phoebewang wrote:

Do we know when new MI being inserted? I think we can simply assgin it a value if we do, e.g.,
```
MachineInstr *New = BuildMI(MBB, II, ...
uint64_t NewPos = Instr2PosIndex[&*II];
if (LLVM_UNLIKELY(NewPos & (InstrDist - 1) == 0)) ...
Instr2PosIndex[New] = NewPos;
```

https://github.com/llvm/llvm-project/pull/72250