[llvm] [compiler-rt] [libcxx] [clang] [mlir] [clang-tools-extra] [RegAllocFast] Lazily initialize InstrPosIndexes for each MBB (PR #76275)

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https://github.com/HaohaiWen updated https://github.com/llvm/llvm-project/pull/76275

>From 23882772e4dac82b45cac2b3ea4fba12415764f6 Mon Sep 17 00:00:00 2001
From: Haohai Wen <haohai.wen at intel.com>
Date: Sat, 23 Dec 2023 09:55:37 +0800
Subject: [PATCH] [RegAllocFast] Lazily initialize InstrPosIndexes for each MBB

Most basic block do not need to query dominates. Defer initialization of
InstrPosIndexes to first query for each MBB.
---
 llvm/lib/CodeGen/RegAllocFast.cpp | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/CodeGen/RegAllocFast.cpp b/llvm/lib/CodeGen/RegAllocFast.cpp
index d7edaa1d7ea47d..e81d4793013682 100644
--- a/llvm/lib/CodeGen/RegAllocFast.cpp
+++ b/llvm/lib/CodeGen/RegAllocFast.cpp
@@ -66,6 +66,8 @@ namespace {
 /// can be used to determine dominance between instructions in same MBB.
 class InstrPosIndexes {
 public:
+  void unsetInitialized() { IsInitialized = false; }
+
   void init(const MachineBasicBlock &MBB) {
     CurMBB = &MBB;
     Instr2PosIndex.clear();
@@ -80,6 +82,13 @@ class InstrPosIndexes {
   /// index without affecting existing instruction's index. Return true if all
   /// instructions index has been reassigned.
   bool getIndex(const MachineInstr &MI, uint64_t &Index) {
+    if (!IsInitialized) {
+      init(*MI.getParent());
+      IsInitialized = true;
+      Index = Instr2PosIndex.at(&MI);
+      return true;
+    }
+
     assert(MI.getParent() == CurMBB && "MI is not in CurMBB");
     auto It = Instr2PosIndex.find(&MI);
     if (It != Instr2PosIndex.end()) {
@@ -159,6 +168,7 @@ class InstrPosIndexes {
   }
 
 private:
+  bool IsInitialized = false;
   enum { InstrDist = 1024 };
   const MachineBasicBlock *CurMBB = nullptr;
   DenseMap<const MachineInstr *, uint64_t> Instr2PosIndex;
@@ -1665,7 +1675,7 @@ void RegAllocFast::allocateBasicBlock(MachineBasicBlock &MBB) {
   this->MBB = &MBB;
   LLVM_DEBUG(dbgs() << "\nAllocating " << MBB);
 
-  PosIndexes.init(MBB);
+  PosIndexes.unsetInitialized();
   RegUnitStates.assign(TRI->getNumRegUnits(), regFree);
   assert(LiveVirtRegs.empty() && "Mapping not cleared from last block?");