[AMDGPU] Improve StructurizeCFG pass performance: avoid redundant DebugLoc map initialization. NFC. (#130568)

Previously, the TermDL (BB terminator → DebugLoc) map was initialized at
the start of processing each function's region, creating entries for the
entire function. This could be inefficient for large functions.

This patch improves performance by creating map entries only when
needed—when a terminator is being killed or when a flow block is
created. Additionally, entries are removed immediately after use,
preventing unnecessary map growth and ensuring DebugLocs are not
"retracked."

A mapless variant was also explored, but due to limited familiarity with
the structurizer, it was not pursued further.

In my cases, this change improves performance by 2-3×.

Author: vpykhtin

llvm/lib/Transforms/Scalar/StructurizeCFG.cpp

@@ -361,6 +361,8 @@ class StructurizeCFG {
 
   void rebuildSSA();
 
+  void setDebugLoc(BranchInst *Br, BasicBlock *BB);
+
 public:
   void init(Region *R);
   bool run(Region *R, DominatorTree *DT);
@@ -595,14 +597,6 @@ void StructurizeCFG::collectInfos() {
     // Find the last back edges
     analyzeLoops(RN);
   }
-
-  // Reset the collected term debug locations
-  TermDL.clear();
-
-  for (BasicBlock &BB : *Func) {
-    if (const DebugLoc &DL = BB.getTerminator()->getDebugLoc())
-      TermDL[&BB] = DL;
-  }
 }
 
 /// Insert the missing branch conditions
@@ -924,12 +918,24 @@ void StructurizeCFG::simplifyAffectedPhis() {
   } while (Changed);
 }
 
+void StructurizeCFG::setDebugLoc(BranchInst *Br, BasicBlock *BB) {
+  auto I = TermDL.find(BB);
+  if (I == TermDL.end())
+    return;
+
+  Br->setDebugLoc(I->second);
+  TermDL.erase(I);
+}
+
 /// Remove phi values from all successors and then remove the terminator.
 void StructurizeCFG::killTerminator(BasicBlock *BB) {
   Instruction *Term = BB->getTerminator();
   if (!Term)
     return;
 
+  if (const DebugLoc &DL = Term->getDebugLoc())
+    TermDL[BB] = DL;
+
   for (BasicBlock *Succ : successors(BB))
     delPhiValues(BB, Succ);
 
@@ -974,7 +980,7 @@ void StructurizeCFG::changeExit(RegionNode *Node, BasicBlock *NewExit,
     BasicBlock *BB = Node->getNodeAs<BasicBlock>();
     killTerminator(BB);
     BranchInst *Br = BranchInst::Create(NewExit, BB);
-    Br->setDebugLoc(TermDL[BB]);
+    setDebugLoc(Br, BB);
     addPhiValues(BB, NewExit);
     if (IncludeDominator)
       DT->changeImmediateDominator(NewExit, BB);
@@ -990,10 +996,14 @@ BasicBlock *StructurizeCFG::getNextFlow(BasicBlock *Dominator) {
                                         Func, Insert);
   FlowSet.insert(Flow);
 
-  // use a temporary variable to avoid a use-after-free if the map's storage is
-  // reallocated
-  DebugLoc DL = TermDL[Dominator];
-  TermDL[Flow] = std::move(DL);
+  if (auto *Term = Dominator->getTerminator()) {
+    if (const DebugLoc &DL = Term->getDebugLoc())
+      TermDL[Flow] = DL;
+  } else if (DebugLoc DLTemp = TermDL.lookup(Dominator)) {
+    // Use a temporary copy to avoid a use-after-free if the map's storage
+    // is reallocated.
+    TermDL[Flow] = DLTemp;
+  }
 
   DT->addNewBlock(Flow, Dominator);
   ParentRegion->getRegionInfo()->setRegionFor(Flow, ParentRegion);
@@ -1088,7 +1098,7 @@ void StructurizeCFG::wireFlow(bool ExitUseAllowed,
 
     // let it point to entry and next block
     BranchInst *Br = BranchInst::Create(Entry, Next, BoolPoison, Flow);
-    Br->setDebugLoc(TermDL[Flow]);
+    setDebugLoc(Br, Flow);
     Conditions.push_back(Br);
     addPhiValues(Flow, Entry);
     DT->changeImmediateDominator(Entry, Flow);
@@ -1129,7 +1139,7 @@ void StructurizeCFG::handleLoops(bool ExitUseAllowed,
   LoopEnd = needPrefix(false);
   BasicBlock *Next = needPostfix(LoopEnd, ExitUseAllowed);
   BranchInst *Br = BranchInst::Create(Next, LoopStart, BoolPoison, LoopEnd);
-  Br->setDebugLoc(TermDL[LoopEnd]);
+  setDebugLoc(Br, LoopEnd);
   LoopConds.push_back(Br);
   addPhiValues(LoopEnd, LoopStart);
   setPrevNode(Next);