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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Valery Pykhtin (vpykhtin)

<details>
<summary>Changes</summary>

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×.

---
Full diff: https://github.com/llvm/llvm-project/pull/130568.diff


1 Files Affected:

- (modified) llvm/lib/Transforms/Scalar/StructurizeCFG.cpp (+21-15) 


``````````diff
diff --git a/llvm/lib/Transforms/Scalar/StructurizeCFG.cpp b/llvm/lib/Transforms/Scalar/StructurizeCFG.cpp
index 89a2a7ac9be3f..db6321cc2f71e 100644
--- a/llvm/lib/Transforms/Scalar/StructurizeCFG.cpp
+++ b/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 +996,10 @@ 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);
+  auto *Term = Dominator->getTerminator();
+  if (const DebugLoc &DL =
+          Term ? Term->getDebugLoc() : TermDL.lookup(Dominator))
+    TermDL[Flow] = DL;
 
   DT->addNewBlock(Flow, Dominator);
   ParentRegion->getRegionInfo()->setRegionFor(Flow, ParentRegion);
@@ -1088,7 +1094,7 @@ 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 +1135,7 @@ 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);

``````````

</details>


https://github.com/llvm/llvm-project/pull/130568