https://github.com/MaskRay updated 
https://github.com/llvm/llvm-project/pull/198982

>From eae139cf8cf8dbae40c769b2b00b9e3de657b31e Mon Sep 17 00:00:00 2001
From: Fangrui Song <[email protected]>
Date: Mon, 11 May 2026 23:37:03 -0700
Subject: [PATCH 1/4] [llvm] Don't assume non-erased DenseMap entries remain
 valid after erase. NFC

In preparation for switching DenseMap from tombstone deletion to TAOCP
Algorithm 6.4R, improving performance, don't rely on the iterators of
non-erased elements being kept.

Switch to the collect-then-erase idiom.

Aided by Claude Opus 4.7
---
 llvm/include/llvm/ADT/SetOperations.h         | 27 ++++++++--------
 llvm/lib/Analysis/LoopAccessAnalysis.cpp      |  5 ++-
 llvm/lib/Analysis/ScalarEvolution.cpp         | 31 +++++++++----------
 llvm/lib/CodeGen/MachineCopyPropagation.cpp   |  2 +-
 llvm/lib/CodeGen/MachineLateInstrsCleanup.cpp | 15 +++++----
 llvm/lib/CodeGen/PeepholeOptimizer.cpp        | 13 +++++---
 .../CodeGen/RemoveRedundantDebugValues.cpp    |  5 ++-
 llvm/lib/IR/LegacyPassManager.cpp             | 29 +++++++++--------
 llvm/lib/MC/MCObjectStreamer.cpp              | 13 +++++---
 llvm/lib/Transforms/IPO/FunctionImport.cpp    | 12 +++----
 .../Utils/PromoteMemoryToRegister.cpp         | 14 ++++-----
 11 files changed, 90 insertions(+), 76 deletions(-)

diff --git a/llvm/include/llvm/ADT/SetOperations.h 
b/llvm/include/llvm/ADT/SetOperations.h
index 4d4ff4045f813..36943452cd422 100644
--- a/llvm/include/llvm/ADT/SetOperations.h
+++ b/llvm/include/llvm/ADT/SetOperations.h
@@ -16,6 +16,7 @@
 #define LLVM_ADT_SETOPERATIONS_H
 
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SmallVector.h"
 
 namespace llvm {
 
@@ -60,12 +61,12 @@ template <class S1Ty, class S2Ty> void set_intersect(S1Ty 
&S1, const S2Ty &S2) {
   if constexpr (detail::HasMemberRemoveIf<S1Ty, decltype(Pred)>) {
     S1.remove_if(Pred);
   } else {
-    typename S1Ty::iterator Next;
-    for (typename S1Ty::iterator I = S1.begin(); I != S1.end(); I = Next) {
-      Next = std::next(I);
-      if (!S2.count(*I))
-        S1.erase(I); // Erase element if not in S2
-    }
+    SmallVector<typename S1Ty::value_type> ToRemove;
+    for (const auto &E : S1)
+      if (!S2.count(E))
+        ToRemove.push_back(E);
+    for (const auto &E : ToRemove)
+      S1.erase(E);
   }
 }
 
@@ -117,13 +118,13 @@ template <class S1Ty, class S2Ty> void set_subtract(S1Ty 
&S1, const S2Ty &S2) {
       }
     } else if constexpr (detail::HasMemberEraseIter<S1Ty>) {
       if (S1.size() < S2.size()) {
-        typename S1Ty::iterator Next;
-        for (typename S1Ty::iterator SI = S1.begin(), SE = S1.end(); SI != SE;
-             SI = Next) {
-          Next = std::next(SI);
-          if (S2.contains(*SI))
-            S1.erase(SI);
-        }
+        // Collect-then-erase: see the matching comment in set_intersect.
+        SmallVector<typename S1Ty::value_type> ToRemove;
+        for (const auto &E : S1)
+          if (S2.contains(E))
+            ToRemove.push_back(E);
+        for (const auto &E : ToRemove)
+          S1.erase(E);
         return;
       }
     }
diff --git a/llvm/lib/Analysis/LoopAccessAnalysis.cpp 
b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
index 2b9efd22131c6..365cd630278d0 100644
--- a/llvm/lib/Analysis/LoopAccessAnalysis.cpp
+++ b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
@@ -3229,12 +3229,15 @@ void LoopAccessInfoManager::clear() {
   // analyzed loop or SCEVs that may have been modified or invalidated. At the
   // moment, that is loops requiring memory or SCEV runtime checks, as those 
cache
   // SCEVs, e.g. for pointer expressions.
+  SmallVector<Loop *> ToRemove;
   for (const auto &[L, LAI] : LoopAccessInfoMap) {
     if (LAI->getRuntimePointerChecking()->getChecks().empty() &&
         LAI->getPSE().getPredicate().isAlwaysTrue())
       continue;
-    LoopAccessInfoMap.erase(L);
+    ToRemove.push_back(L);
   }
+  for (Loop *L : ToRemove)
+    LoopAccessInfoMap.erase(L);
 }
 
 bool LoopAccessInfoManager::invalidate(
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp 
b/llvm/lib/Analysis/ScalarEvolution.cpp
index 6dbcd82744f8b..c345f449acaf1 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -8749,8 +8749,9 @@ void ScalarEvolution::visitAndClearUsers(
     ValueExprMapType::iterator It =
         ValueExprMap.find_as(static_cast<Value *>(I));
     if (It != ValueExprMap.end()) {
+      SCEVUse Mapped = It->second;
       eraseValueFromMap(It->first);
-      ToForget.push_back(It->second);
+      ToForget.push_back(Mapped);
       if (PHINode *PN = dyn_cast<PHINode>(I))
         ConstantEvolutionLoopExitValue.erase(PN);
     }
@@ -8774,14 +8775,12 @@ void ScalarEvolution::forgetLoop(const Loop *L) {
     forgetBackedgeTakenCounts(CurrL, /* Predicated */ true);
 
     // Drop information about predicated SCEV rewrites for this loop.
-    for (auto I = PredicatedSCEVRewrites.begin();
-         I != PredicatedSCEVRewrites.end();) {
-      std::pair<const SCEV *, const Loop *> Entry = I->first;
-      if (Entry.second == CurrL)
-        PredicatedSCEVRewrites.erase(I++);
-      else
-        ++I;
-    }
+    SmallVector<std::pair<const SCEVUnknown *, const Loop *>> ToRemove;
+    for (const auto &Entry : PredicatedSCEVRewrites)
+      if (Entry.first.second == CurrL)
+        ToRemove.push_back(Entry.first);
+    for (const auto &K : ToRemove)
+      PredicatedSCEVRewrites.erase(K);
 
     auto LoopUsersItr = LoopUsers.find(CurrL);
     if (LoopUsersItr != LoopUsers.end())
@@ -14567,14 +14566,12 @@ void 
ScalarEvolution::forgetMemoizedResults(ArrayRef<SCEVUse> SCEVs) {
   for (const auto *S : ToForget)
     forgetMemoizedResultsImpl(S);
 
-  for (auto I = PredicatedSCEVRewrites.begin();
-       I != PredicatedSCEVRewrites.end();) {
-    std::pair<const SCEV *, const Loop *> Entry = I->first;
-    if (ToForget.count(Entry.first))
-      PredicatedSCEVRewrites.erase(I++);
-    else
-      ++I;
-  }
+  SmallVector<std::pair<const SCEVUnknown *, const Loop *>> ToRemove;
+  for (const auto &Entry : PredicatedSCEVRewrites)
+    if (ToForget.count(Entry.first.first))
+      ToRemove.push_back(Entry.first);
+  for (const auto &K : ToRemove)
+    PredicatedSCEVRewrites.erase(K);
 }
 
 void ScalarEvolution::forgetMemoizedResultsImpl(const SCEV *S) {
diff --git a/llvm/lib/CodeGen/MachineCopyPropagation.cpp 
b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
index 1d8fdcab64909..6bfd21b215706 100644
--- a/llvm/lib/CodeGen/MachineCopyPropagation.cpp
+++ b/llvm/lib/CodeGen/MachineCopyPropagation.cpp
@@ -255,7 +255,7 @@ class CopyTracker {
         }
       }
       // Now we can erase the copy.
-      Copies.erase(I);
+      Copies.erase(Unit);
     }
   }
 
diff --git a/llvm/lib/CodeGen/MachineLateInstrsCleanup.cpp 
b/llvm/lib/CodeGen/MachineLateInstrsCleanup.cpp
index 4f281fa1361ca..946fa687b067c 100644
--- a/llvm/lib/CodeGen/MachineLateInstrsCleanup.cpp
+++ b/llvm/lib/CodeGen/MachineLateInstrsCleanup.cpp
@@ -243,15 +243,18 @@ bool 
MachineLateInstrsCleanup::processBlock(MachineBasicBlock *MBB) {
     }
 
     // Clear any entries in map that MI clobbers.
-    for (auto DefI : llvm::make_early_inc_range(MBBDefs)) {
-      Register Reg = DefI.first;
-      if (MI.modifiesRegister(Reg, TRI)) {
-        MBBDefs.erase(Reg);
-        MBBKills.erase(Reg);
-      } else if (MI.findRegisterUseOperandIdx(Reg, TRI, true /*isKill*/) != -1)
+    SmallVector<Register> Clobbered;
+    for (auto [Reg, DefMI] : MBBDefs) {
+      if (MI.modifiesRegister(Reg, TRI))
+        Clobbered.push_back(Reg);
+      else if (MI.findRegisterUseOperandIdx(Reg, TRI, true /*isKill*/) != -1)
         // Keep track of all instructions that fully or partially kills Reg.
         MBBKills[Reg].push_back(&MI);
     }
+    for (Register Reg : Clobbered) {
+      MBBDefs.erase(Reg);
+      MBBKills.erase(Reg);
+    }
 
     // Record this MI for potential later reuse.
     if (IsCandidate) {
diff --git a/llvm/lib/CodeGen/PeepholeOptimizer.cpp 
b/llvm/lib/CodeGen/PeepholeOptimizer.cpp
index cfbffb920ef36..8ba7a78c1d014 100644
--- a/llvm/lib/CodeGen/PeepholeOptimizer.cpp
+++ b/llvm/lib/CodeGen/PeepholeOptimizer.cpp
@@ -1813,13 +1813,16 @@ bool PeepholeOptimizer::run(MachineFunction &MF) {
             }
           } else if (MO.isRegMask()) {
             const uint32_t *RegMask = MO.getRegMask();
+            SmallVector<Register> ClobberedDefs;
             for (auto &RegMI : NAPhysToVirtMIs) {
               Register Def = RegMI.first;
-              if (MachineOperand::clobbersPhysReg(RegMask, Def)) {
-                LLVM_DEBUG(dbgs()
-                           << "NAPhysCopy: invalidating because of " << *MI);
-                NAPhysToVirtMIs.erase(Def);
-              }
+              if (MachineOperand::clobbersPhysReg(RegMask, Def))
+                ClobberedDefs.push_back(Def);
+            }
+            for (Register Def : ClobberedDefs) {
+              LLVM_DEBUG(dbgs()
+                         << "NAPhysCopy: invalidating because of " << *MI);
+              NAPhysToVirtMIs.erase(Def);
             }
           }
         }
diff --git a/llvm/lib/CodeGen/RemoveRedundantDebugValues.cpp 
b/llvm/lib/CodeGen/RemoveRedundantDebugValues.cpp
index 11468245f8400..c2fd20084efcd 100644
--- a/llvm/lib/CodeGen/RemoveRedundantDebugValues.cpp
+++ b/llvm/lib/CodeGen/RemoveRedundantDebugValues.cpp
@@ -128,11 +128,14 @@ static bool reduceDbgValsForwardScan(MachineBasicBlock 
&MBB) {
       continue;
 
     // Stop tracking any location that is clobbered by this instruction.
+    SmallVector<DebugVariable> Clobbered;
     for (auto &Var : VariableMap) {
       auto &LocOp = Var.second.first;
       if (MI.modifiesRegister(LocOp->getReg(), TRI))
-        VariableMap.erase(Var.first);
+        Clobbered.push_back(Var.first);
     }
+    for (const DebugVariable &Var : Clobbered)
+      VariableMap.erase(Var);
   }
 
   for (auto &Instr : DbgValsToBeRemoved) {
diff --git a/llvm/lib/IR/LegacyPassManager.cpp 
b/llvm/lib/IR/LegacyPassManager.cpp
index 7b9ad89038dc6..d1d5237e28146 100644
--- a/llvm/lib/IR/LegacyPassManager.cpp
+++ b/llvm/lib/IR/LegacyPassManager.cpp
@@ -903,20 +903,21 @@ void PMDataManager::removeNotPreservedAnalysis(Pass *P) {
     return;
 
   const AnalysisUsage::VectorType &PreservedSet = AnUsage->getPreservedSet();
-  for (auto I = AvailableAnalysis.begin(), E = AvailableAnalysis.end();
-       I != E;) {
-    auto Info = I++;
-    if (Info->second->getAsImmutablePass() == nullptr &&
-        !is_contained(PreservedSet, Info->first)) {
+  SmallVector<AnalysisID, 16> ToRemove;
+  for (auto &Entry : AvailableAnalysis) {
+    if (Entry.second->getAsImmutablePass() == nullptr &&
+        !is_contained(PreservedSet, Entry.first)) {
       // Remove this analysis
       if (PassDebugging >= Details) {
-        Pass *S = Info->second;
+        Pass *S = Entry.second;
         dbgs() << " -- '" <<  P->getPassName() << "' is not preserving '";
         dbgs() << S->getPassName() << "'\n";
       }
-      AvailableAnalysis.erase(Info);
+      ToRemove.push_back(Entry.first);
     }
   }
+  for (AnalysisID ID : ToRemove)
+    AvailableAnalysis.erase(ID);
 
   // Check inherited analysis also. If P is not preserving analysis
   // provided by parent manager then remove it here.
@@ -924,19 +925,21 @@ void PMDataManager::removeNotPreservedAnalysis(Pass *P) {
     if (!IA)
       continue;
 
-    for (auto I = IA->begin(), E = IA->end(); I != E;) {
-      auto Info = I++;
-      if (Info->second->getAsImmutablePass() == nullptr &&
-          !is_contained(PreservedSet, Info->first)) {
+    ToRemove.clear();
+    for (auto &Entry : *IA) {
+      if (Entry.second->getAsImmutablePass() == nullptr &&
+          !is_contained(PreservedSet, Entry.first)) {
         // Remove this analysis
         if (PassDebugging >= Details) {
-          Pass *S = Info->second;
+          Pass *S = Entry.second;
           dbgs() << " -- '" <<  P->getPassName() << "' is not preserving '";
           dbgs() << S->getPassName() << "'\n";
         }
-        IA->erase(Info);
+        ToRemove.push_back(Entry.first);
       }
     }
+    for (AnalysisID ID : ToRemove)
+      IA->erase(ID);
   }
 }
 
diff --git a/llvm/lib/MC/MCObjectStreamer.cpp b/llvm/lib/MC/MCObjectStreamer.cpp
index 88dafb94a4aaa..2bf5f05c1c315 100644
--- a/llvm/lib/MC/MCObjectStreamer.cpp
+++ b/llvm/lib/MC/MCObjectStreamer.cpp
@@ -272,12 +272,15 @@ void MCObjectStreamer::emitLabel(MCSymbol *Symbol, SMLoc 
Loc) {
 
 void MCObjectStreamer::emitPendingAssignments(MCSymbol *Symbol) {
   auto Assignments = pendingAssignments.find(Symbol);
-  if (Assignments != pendingAssignments.end()) {
-    for (const PendingAssignment &A : Assignments->second)
-      emitAssignment(A.Symbol, A.Value);
+  if (Assignments == pendingAssignments.end())
+    return;
 
-    pendingAssignments.erase(Assignments);
-  }
+  // emitAssignment can recursively re-enter emitPendingAssignments for
+  // other symbols, so move the list out and erase before iterating.
+  SmallVector<PendingAssignment, 1> Pending = std::move(Assignments->second);
+  pendingAssignments.erase(Assignments);
+  for (const PendingAssignment &A : Pending)
+    emitAssignment(A.Symbol, A.Value);
 }
 
 // Emit a label at a previously emitted fragment/offset position. This must be
diff --git a/llvm/lib/Transforms/IPO/FunctionImport.cpp 
b/llvm/lib/Transforms/IPO/FunctionImport.cpp
index 456a9b116cc30..6f96d464f0a60 100644
--- a/llvm/lib/Transforms/IPO/FunctionImport.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionImport.cpp
@@ -1284,12 +1284,12 @@ void llvm::ComputeCrossModuleImport(
     // exporting module. We do this after the above insertion since we may hit
     // the same ref/call target multiple times in above loop, and it is more
     // efficient to avoid a set lookup each time.
-    for (auto EI = NewExports.begin(); EI != NewExports.end();) {
-      if (!DefinedGVSummaries.count(EI->getGUID()))
-        NewExports.erase(EI++);
-      else
-        ++EI;
-    }
+    SmallVector<ValueInfo, 8> ToRemove;
+    for (const auto &VI : NewExports)
+      if (!DefinedGVSummaries.count(VI.getGUID()))
+        ToRemove.push_back(VI);
+    for (const auto &VI : ToRemove)
+      NewExports.erase(VI);
     ELI.second.insert_range(NewExports);
   }
 
diff --git a/llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp 
b/llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
index b635d805bf13d..631190a61cc6d 100644
--- a/llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
+++ b/llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
@@ -929,22 +929,20 @@ void PromoteMem2Reg::run() {
     // simplify and RAUW them as we go.  If it was not, we could add uses to
     // the values we replace with in a non-deterministic order, thus creating
     // non-deterministic def->use chains.
-    for (DenseMap<std::pair<unsigned, unsigned>, PHINode *>::iterator
-             I = NewPhiNodes.begin(),
-             E = NewPhiNodes.end();
-         I != E;) {
-      PHINode *PN = I->second;
+    SmallVector<std::pair<unsigned, unsigned>> ToRemove;
+    for (auto &Entry : NewPhiNodes) {
+      PHINode *PN = Entry.second;
 
       // If this PHI node merges one value and/or undefs, get the value.
       if (Value *V = simplifyInstruction(PN, SQ)) {
         PN->replaceAllUsesWith(V);
         PN->eraseFromParent();
-        NewPhiNodes.erase(I++);
+        ToRemove.push_back(Entry.first);
         EliminatedAPHI = true;
-        continue;
       }
-      ++I;
     }
+    for (auto &K : ToRemove)
+      NewPhiNodes.erase(K);
   }
 
   // At this point, the renamer has added entries to PHI nodes for all 
reachable

>From 51ba1a081c525679dd0ca449f78724ddd7b362f0 Mon Sep 17 00:00:00 2001
From: Fangrui Song <[email protected]>
Date: Thu, 21 May 2026 10:26:42 -0700
Subject: [PATCH 2/4] simplify

---
 llvm/lib/IR/LegacyPassManager.cpp          | 2 +-
 llvm/lib/MC/MCObjectStreamer.cpp           | 2 +-
 llvm/lib/Transforms/IPO/FunctionImport.cpp | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/llvm/lib/IR/LegacyPassManager.cpp 
b/llvm/lib/IR/LegacyPassManager.cpp
index d1d5237e28146..457f290feebbe 100644
--- a/llvm/lib/IR/LegacyPassManager.cpp
+++ b/llvm/lib/IR/LegacyPassManager.cpp
@@ -903,7 +903,7 @@ void PMDataManager::removeNotPreservedAnalysis(Pass *P) {
     return;
 
   const AnalysisUsage::VectorType &PreservedSet = AnUsage->getPreservedSet();
-  SmallVector<AnalysisID, 16> ToRemove;
+  SmallVector<AnalysisID> ToRemove;
   for (auto &Entry : AvailableAnalysis) {
     if (Entry.second->getAsImmutablePass() == nullptr &&
         !is_contained(PreservedSet, Entry.first)) {
diff --git a/llvm/lib/MC/MCObjectStreamer.cpp b/llvm/lib/MC/MCObjectStreamer.cpp
index 2bf5f05c1c315..e6b622331036a 100644
--- a/llvm/lib/MC/MCObjectStreamer.cpp
+++ b/llvm/lib/MC/MCObjectStreamer.cpp
@@ -277,7 +277,7 @@ void MCObjectStreamer::emitPendingAssignments(MCSymbol 
*Symbol) {
 
   // emitAssignment can recursively re-enter emitPendingAssignments for
   // other symbols, so move the list out and erase before iterating.
-  SmallVector<PendingAssignment, 1> Pending = std::move(Assignments->second);
+  SmallVector<PendingAssignment> Pending = std::move(Assignments->second);
   pendingAssignments.erase(Assignments);
   for (const PendingAssignment &A : Pending)
     emitAssignment(A.Symbol, A.Value);
diff --git a/llvm/lib/Transforms/IPO/FunctionImport.cpp 
b/llvm/lib/Transforms/IPO/FunctionImport.cpp
index 6f96d464f0a60..c55674495646d 100644
--- a/llvm/lib/Transforms/IPO/FunctionImport.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionImport.cpp
@@ -1284,7 +1284,7 @@ void llvm::ComputeCrossModuleImport(
     // exporting module. We do this after the above insertion since we may hit
     // the same ref/call target multiple times in above loop, and it is more
     // efficient to avoid a set lookup each time.
-    SmallVector<ValueInfo, 8> ToRemove;
+    SmallVector<ValueInfo> ToRemove;
     for (const auto &VI : NewExports)
       if (!DefinedGVSummaries.count(VI.getGUID()))
         ToRemove.push_back(VI);

>From 90be26bb48afafe8bf7571b06e939bf9ad322056 Mon Sep 17 00:00:00 2001
From: Fangrui Song <[email protected]>
Date: Fri, 22 May 2026 00:02:50 -0700
Subject: [PATCH 3/4] add remove_if and simplify code with it

---
 llvm/include/llvm/ADT/DenseMap.h              | 27 +++++++++++
 llvm/include/llvm/ADT/DenseSet.h              | 10 +++++
 llvm/include/llvm/ADT/SetOperations.h         | 27 ++++++-----
 llvm/lib/Analysis/LoopAccessAnalysis.cpp      | 14 +++---
 llvm/lib/Analysis/ScalarEvolution.cpp         | 16 ++-----
 llvm/lib/CodeGen/MachineLateInstrsCleanup.cpp | 19 ++++----
 llvm/lib/CodeGen/PeepholeOptimizer.cpp        | 14 +++---
 .../CodeGen/RemoveRedundantDebugValues.cpp    | 11 ++---
 llvm/lib/IR/LegacyPassManager.cpp             | 43 ++++++------------
 llvm/lib/MC/MCObjectStreamer.cpp              |  2 +-
 llvm/lib/Transforms/IPO/FunctionImport.cpp    |  8 +---
 .../Utils/PromoteMemoryToRegister.cpp         | 12 ++---
 llvm/unittests/ADT/DenseMapTest.cpp           | 45 +++++++++++++++++++
 llvm/unittests/ADT/DenseSetTest.cpp           | 16 +++++++
 14 files changed, 157 insertions(+), 107 deletions(-)

diff --git a/llvm/include/llvm/ADT/DenseMap.h b/llvm/include/llvm/ADT/DenseMap.h
index b8b548a31acbc..e13b64b3e6bf4 100644
--- a/llvm/include/llvm/ADT/DenseMap.h
+++ b/llvm/include/llvm/ADT/DenseMap.h
@@ -344,6 +344,33 @@ class DenseMapBase : public DebugEpochBase {
     incrementNumTombstones();
   }
 
+  /// Remove entries that match the given predicate. \p Pred is invoked
+  /// with a reference to each live bucket and must not access the map being
+  /// modified. This is the safe replacement for erase-while-iterating.
+  ///
+  /// Returns whether anything was removed. If so, all iterators and references
+  /// into the map are invalidated.
+  template <typename Predicate> bool remove_if(Predicate Pred) {
+    const KeyT EmptyKey = KeyInfoT::getEmptyKey();
+    const KeyT TombstoneKey = KeyInfoT::getTombstoneKey();
+    bool Removed = false;
+    for (BucketT &B : buckets()) {
+      if (KeyInfoT::isEqual(B.getFirst(), EmptyKey) ||
+          KeyInfoT::isEqual(B.getFirst(), TombstoneKey))
+        continue;
+      if (Pred(B)) {
+        B.getSecond().~ValueT();
+        B.getFirst() = TombstoneKey;
+        decrementNumEntries();
+        incrementNumTombstones();
+        Removed = true;
+      }
+    }
+    if (Removed)
+      incrementEpoch();
+    return Removed;
+  }
+
   ValueT &operator[](const KeyT &Key) {
     return lookupOrInsertIntoBucket(Key).first->second;
   }
diff --git a/llvm/include/llvm/ADT/DenseSet.h b/llvm/include/llvm/ADT/DenseSet.h
index eec800d07b6df..645d6d1568f35 100644
--- a/llvm/include/llvm/ADT/DenseSet.h
+++ b/llvm/include/llvm/ADT/DenseSet.h
@@ -99,6 +99,16 @@ class DenseSetImpl {
 
   bool erase(const ValueT &V) { return TheMap.erase(V); }
 
+  /// Remove all elements for which \p Pred returns true.  This is the safe
+  /// replacement for erase-while-iterating; see DenseMap::remove_if.  The
+  /// predicate must not access the set being modified.  Returns whether
+  /// anything was removed; if so, all iterators are invalidated.
+  template <typename Predicate> bool remove_if(Predicate Pred) {
+    return TheMap.remove_if([&](const typename MapTy::value_type &KV) {
+      return Pred(KV.getFirst());
+    });
+  }
+
   void swap(DenseSetImpl &RHS) { TheMap.swap(RHS.TheMap); }
 
 private:
diff --git a/llvm/include/llvm/ADT/SetOperations.h 
b/llvm/include/llvm/ADT/SetOperations.h
index 36943452cd422..4d4ff4045f813 100644
--- a/llvm/include/llvm/ADT/SetOperations.h
+++ b/llvm/include/llvm/ADT/SetOperations.h
@@ -16,7 +16,6 @@
 #define LLVM_ADT_SETOPERATIONS_H
 
 #include "llvm/ADT/STLExtras.h"
-#include "llvm/ADT/SmallVector.h"
 
 namespace llvm {
 
@@ -61,12 +60,12 @@ template <class S1Ty, class S2Ty> void set_intersect(S1Ty 
&S1, const S2Ty &S2) {
   if constexpr (detail::HasMemberRemoveIf<S1Ty, decltype(Pred)>) {
     S1.remove_if(Pred);
   } else {
-    SmallVector<typename S1Ty::value_type> ToRemove;
-    for (const auto &E : S1)
-      if (!S2.count(E))
-        ToRemove.push_back(E);
-    for (const auto &E : ToRemove)
-      S1.erase(E);
+    typename S1Ty::iterator Next;
+    for (typename S1Ty::iterator I = S1.begin(); I != S1.end(); I = Next) {
+      Next = std::next(I);
+      if (!S2.count(*I))
+        S1.erase(I); // Erase element if not in S2
+    }
   }
 }
 
@@ -118,13 +117,13 @@ template <class S1Ty, class S2Ty> void set_subtract(S1Ty 
&S1, const S2Ty &S2) {
       }
     } else if constexpr (detail::HasMemberEraseIter<S1Ty>) {
       if (S1.size() < S2.size()) {
-        // Collect-then-erase: see the matching comment in set_intersect.
-        SmallVector<typename S1Ty::value_type> ToRemove;
-        for (const auto &E : S1)
-          if (S2.contains(E))
-            ToRemove.push_back(E);
-        for (const auto &E : ToRemove)
-          S1.erase(E);
+        typename S1Ty::iterator Next;
+        for (typename S1Ty::iterator SI = S1.begin(), SE = S1.end(); SI != SE;
+             SI = Next) {
+          Next = std::next(SI);
+          if (S2.contains(*SI))
+            S1.erase(SI);
+        }
         return;
       }
     }
diff --git a/llvm/lib/Analysis/LoopAccessAnalysis.cpp 
b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
index 365cd630278d0..60e15b2a5bd82 100644
--- a/llvm/lib/Analysis/LoopAccessAnalysis.cpp
+++ b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
@@ -3229,15 +3229,11 @@ void LoopAccessInfoManager::clear() {
   // analyzed loop or SCEVs that may have been modified or invalidated. At the
   // moment, that is loops requiring memory or SCEV runtime checks, as those 
cache
   // SCEVs, e.g. for pointer expressions.
-  SmallVector<Loop *> ToRemove;
-  for (const auto &[L, LAI] : LoopAccessInfoMap) {
-    if (LAI->getRuntimePointerChecking()->getChecks().empty() &&
-        LAI->getPSE().getPredicate().isAlwaysTrue())
-      continue;
-    ToRemove.push_back(L);
-  }
-  for (Loop *L : ToRemove)
-    LoopAccessInfoMap.erase(L);
+  LoopAccessInfoMap.remove_if([](const auto &Entry) {
+    const auto &LAI = Entry.second;
+    return !(LAI->getRuntimePointerChecking()->getChecks().empty() &&
+             LAI->getPSE().getPredicate().isAlwaysTrue());
+  });
 }
 
 bool LoopAccessInfoManager::invalidate(
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp 
b/llvm/lib/Analysis/ScalarEvolution.cpp
index c345f449acaf1..ec221e5b6ade5 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -8775,12 +8775,8 @@ void ScalarEvolution::forgetLoop(const Loop *L) {
     forgetBackedgeTakenCounts(CurrL, /* Predicated */ true);
 
     // Drop information about predicated SCEV rewrites for this loop.
-    SmallVector<std::pair<const SCEVUnknown *, const Loop *>> ToRemove;
-    for (const auto &Entry : PredicatedSCEVRewrites)
-      if (Entry.first.second == CurrL)
-        ToRemove.push_back(Entry.first);
-    for (const auto &K : ToRemove)
-      PredicatedSCEVRewrites.erase(K);
+    PredicatedSCEVRewrites.remove_if(
+        [&](const auto &Entry) { return Entry.first.second == CurrL; });
 
     auto LoopUsersItr = LoopUsers.find(CurrL);
     if (LoopUsersItr != LoopUsers.end())
@@ -14566,12 +14562,8 @@ void 
ScalarEvolution::forgetMemoizedResults(ArrayRef<SCEVUse> SCEVs) {
   for (const auto *S : ToForget)
     forgetMemoizedResultsImpl(S);
 
-  SmallVector<std::pair<const SCEVUnknown *, const Loop *>> ToRemove;
-  for (const auto &Entry : PredicatedSCEVRewrites)
-    if (ToForget.count(Entry.first.first))
-      ToRemove.push_back(Entry.first);
-  for (const auto &K : ToRemove)
-    PredicatedSCEVRewrites.erase(K);
+  PredicatedSCEVRewrites.remove_if(
+      [&](const auto &Entry) { return ToForget.count(Entry.first.first); });
 }
 
 void ScalarEvolution::forgetMemoizedResultsImpl(const SCEV *S) {
diff --git a/llvm/lib/CodeGen/MachineLateInstrsCleanup.cpp 
b/llvm/lib/CodeGen/MachineLateInstrsCleanup.cpp
index 946fa687b067c..811cc4fe65f3f 100644
--- a/llvm/lib/CodeGen/MachineLateInstrsCleanup.cpp
+++ b/llvm/lib/CodeGen/MachineLateInstrsCleanup.cpp
@@ -243,18 +243,17 @@ bool 
MachineLateInstrsCleanup::processBlock(MachineBasicBlock *MBB) {
     }
 
     // Clear any entries in map that MI clobbers.
-    SmallVector<Register> Clobbered;
-    for (auto [Reg, DefMI] : MBBDefs) {
-      if (MI.modifiesRegister(Reg, TRI))
-        Clobbered.push_back(Reg);
-      else if (MI.findRegisterUseOperandIdx(Reg, TRI, true /*isKill*/) != -1)
+    MBBDefs.remove_if([&](const auto &Entry) {
+      Register Reg = Entry.first;
+      if (MI.modifiesRegister(Reg, TRI)) {
+        MBBKills.erase(Reg);
+        return true;
+      }
+      if (MI.findRegisterUseOperandIdx(Reg, TRI, true /*isKill*/) != -1)
         // Keep track of all instructions that fully or partially kills Reg.
         MBBKills[Reg].push_back(&MI);
-    }
-    for (Register Reg : Clobbered) {
-      MBBDefs.erase(Reg);
-      MBBKills.erase(Reg);
-    }
+      return false;
+    });
 
     // Record this MI for potential later reuse.
     if (IsCandidate) {
diff --git a/llvm/lib/CodeGen/PeepholeOptimizer.cpp 
b/llvm/lib/CodeGen/PeepholeOptimizer.cpp
index 8ba7a78c1d014..19a81e3363086 100644
--- a/llvm/lib/CodeGen/PeepholeOptimizer.cpp
+++ b/llvm/lib/CodeGen/PeepholeOptimizer.cpp
@@ -1813,17 +1813,13 @@ bool PeepholeOptimizer::run(MachineFunction &MF) {
             }
           } else if (MO.isRegMask()) {
             const uint32_t *RegMask = MO.getRegMask();
-            SmallVector<Register> ClobberedDefs;
-            for (auto &RegMI : NAPhysToVirtMIs) {
-              Register Def = RegMI.first;
-              if (MachineOperand::clobbersPhysReg(RegMask, Def))
-                ClobberedDefs.push_back(Def);
-            }
-            for (Register Def : ClobberedDefs) {
+            NAPhysToVirtMIs.remove_if([&](const auto &RegMI) {
+              if (!MachineOperand::clobbersPhysReg(RegMask, RegMI.first))
+                return false;
               LLVM_DEBUG(dbgs()
                          << "NAPhysCopy: invalidating because of " << *MI);
-              NAPhysToVirtMIs.erase(Def);
-            }
+              return true;
+            });
           }
         }
       }
diff --git a/llvm/lib/CodeGen/RemoveRedundantDebugValues.cpp 
b/llvm/lib/CodeGen/RemoveRedundantDebugValues.cpp
index c2fd20084efcd..30057317a6383 100644
--- a/llvm/lib/CodeGen/RemoveRedundantDebugValues.cpp
+++ b/llvm/lib/CodeGen/RemoveRedundantDebugValues.cpp
@@ -128,14 +128,9 @@ static bool reduceDbgValsForwardScan(MachineBasicBlock 
&MBB) {
       continue;
 
     // Stop tracking any location that is clobbered by this instruction.
-    SmallVector<DebugVariable> Clobbered;
-    for (auto &Var : VariableMap) {
-      auto &LocOp = Var.second.first;
-      if (MI.modifiesRegister(LocOp->getReg(), TRI))
-        Clobbered.push_back(Var.first);
-    }
-    for (const DebugVariable &Var : Clobbered)
-      VariableMap.erase(Var);
+    VariableMap.remove_if([&](const auto &Var) {
+      return MI.modifiesRegister(Var.second.first->getReg(), TRI);
+    });
   }
 
   for (auto &Instr : DbgValsToBeRemoved) {
diff --git a/llvm/lib/IR/LegacyPassManager.cpp 
b/llvm/lib/IR/LegacyPassManager.cpp
index 457f290feebbe..b8efa7a399734 100644
--- a/llvm/lib/IR/LegacyPassManager.cpp
+++ b/llvm/lib/IR/LegacyPassManager.cpp
@@ -903,43 +903,26 @@ void PMDataManager::removeNotPreservedAnalysis(Pass *P) {
     return;
 
   const AnalysisUsage::VectorType &PreservedSet = AnUsage->getPreservedSet();
-  SmallVector<AnalysisID> ToRemove;
-  for (auto &Entry : AvailableAnalysis) {
-    if (Entry.second->getAsImmutablePass() == nullptr &&
-        !is_contained(PreservedSet, Entry.first)) {
-      // Remove this analysis
-      if (PassDebugging >= Details) {
-        Pass *S = Entry.second;
-        dbgs() << " -- '" <<  P->getPassName() << "' is not preserving '";
-        dbgs() << S->getPassName() << "'\n";
-      }
-      ToRemove.push_back(Entry.first);
+  auto IsNotPreserved = [&](const auto &Entry) {
+    if (Entry.second->getAsImmutablePass() != nullptr ||
+        is_contained(PreservedSet, Entry.first))
+      return false;
+    // Remove this analysis
+    if (PassDebugging >= Details) {
+      Pass *S = Entry.second;
+      dbgs() << " -- '" << P->getPassName() << "' is not preserving '";
+      dbgs() << S->getPassName() << "'\n";
     }
-  }
-  for (AnalysisID ID : ToRemove)
-    AvailableAnalysis.erase(ID);
+    return true;
+  };
+  AvailableAnalysis.remove_if(IsNotPreserved);
 
   // Check inherited analysis also. If P is not preserving analysis
   // provided by parent manager then remove it here.
   for (DenseMap<AnalysisID, Pass *> *IA : InheritedAnalysis) {
     if (!IA)
       continue;
-
-    ToRemove.clear();
-    for (auto &Entry : *IA) {
-      if (Entry.second->getAsImmutablePass() == nullptr &&
-          !is_contained(PreservedSet, Entry.first)) {
-        // Remove this analysis
-        if (PassDebugging >= Details) {
-          Pass *S = Entry.second;
-          dbgs() << " -- '" <<  P->getPassName() << "' is not preserving '";
-          dbgs() << S->getPassName() << "'\n";
-        }
-        ToRemove.push_back(Entry.first);
-      }
-    }
-    for (AnalysisID ID : ToRemove)
-      IA->erase(ID);
+    IA->remove_if(IsNotPreserved);
   }
 }
 
diff --git a/llvm/lib/MC/MCObjectStreamer.cpp b/llvm/lib/MC/MCObjectStreamer.cpp
index e6b622331036a..2bf5f05c1c315 100644
--- a/llvm/lib/MC/MCObjectStreamer.cpp
+++ b/llvm/lib/MC/MCObjectStreamer.cpp
@@ -277,7 +277,7 @@ void MCObjectStreamer::emitPendingAssignments(MCSymbol 
*Symbol) {
 
   // emitAssignment can recursively re-enter emitPendingAssignments for
   // other symbols, so move the list out and erase before iterating.
-  SmallVector<PendingAssignment> Pending = std::move(Assignments->second);
+  SmallVector<PendingAssignment, 1> Pending = std::move(Assignments->second);
   pendingAssignments.erase(Assignments);
   for (const PendingAssignment &A : Pending)
     emitAssignment(A.Symbol, A.Value);
diff --git a/llvm/lib/Transforms/IPO/FunctionImport.cpp 
b/llvm/lib/Transforms/IPO/FunctionImport.cpp
index c55674495646d..d305eadc12f35 100644
--- a/llvm/lib/Transforms/IPO/FunctionImport.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionImport.cpp
@@ -1284,12 +1284,8 @@ void llvm::ComputeCrossModuleImport(
     // exporting module. We do this after the above insertion since we may hit
     // the same ref/call target multiple times in above loop, and it is more
     // efficient to avoid a set lookup each time.
-    SmallVector<ValueInfo> ToRemove;
-    for (const auto &VI : NewExports)
-      if (!DefinedGVSummaries.count(VI.getGUID()))
-        ToRemove.push_back(VI);
-    for (const auto &VI : ToRemove)
-      NewExports.erase(VI);
+    NewExports.remove_if(
+        [&](ValueInfo VI) { return !DefinedGVSummaries.count(VI.getGUID()); });
     ELI.second.insert_range(NewExports);
   }
 
diff --git a/llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp 
b/llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
index 631190a61cc6d..ed0e864fd6905 100644
--- a/llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
+++ b/llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
@@ -929,20 +929,16 @@ void PromoteMem2Reg::run() {
     // simplify and RAUW them as we go.  If it was not, we could add uses to
     // the values we replace with in a non-deterministic order, thus creating
     // non-deterministic def->use chains.
-    SmallVector<std::pair<unsigned, unsigned>> ToRemove;
-    for (auto &Entry : NewPhiNodes) {
+    EliminatedAPHI = NewPhiNodes.remove_if([&](const auto &Entry) {
       PHINode *PN = Entry.second;
-
       // If this PHI node merges one value and/or undefs, get the value.
       if (Value *V = simplifyInstruction(PN, SQ)) {
         PN->replaceAllUsesWith(V);
         PN->eraseFromParent();
-        ToRemove.push_back(Entry.first);
-        EliminatedAPHI = true;
+        return true;
       }
-    }
-    for (auto &K : ToRemove)
-      NewPhiNodes.erase(K);
+      return false;
+    });
   }
 
   // At this point, the renamer has added entries to PHI nodes for all 
reachable
diff --git a/llvm/unittests/ADT/DenseMapTest.cpp 
b/llvm/unittests/ADT/DenseMapTest.cpp
index 553d159d33b1a..c38c0709f615a 100644
--- a/llvm/unittests/ADT/DenseMapTest.cpp
+++ b/llvm/unittests/ADT/DenseMapTest.cpp
@@ -1108,4 +1108,49 @@ TEST(DenseMapCustomTest, ValueDtor) {
   EXPECT_EQ(0u, CtorTester::getNumConstructed());
 }
 
+TEST(DenseMapCustomTest, RemoveIf) {
+  // Use enough entries to exercise the large representation and force the
+  // same-size rehash inside remove_if to restore the probe invariant.
+  DenseMap<int, int> Map;
+  for (int I = 0; I < 100; ++I)
+    Map[I] = I * 10;
+
+  // Remove all even keys.
+  EXPECT_TRUE(Map.remove_if([](const auto &E) { return E.first % 2 == 0; }));
+  EXPECT_EQ(Map.size(), 50u);
+  for (int I = 0; I < 100; ++I) {
+    auto It = Map.find(I);
+    if (I % 2 == 0) {
+      EXPECT_EQ(It, Map.end());
+    } else {
+      ASSERT_NE(It, Map.end());
+      EXPECT_EQ(It->second, I * 10);
+    }
+  }
+
+  // A predicate that matches nothing returns false and leaves the map alone.
+  EXPECT_FALSE(Map.remove_if([](const auto &) { return false; }));
+  EXPECT_EQ(Map.size(), 50u);
+
+  // Remove everything.
+  EXPECT_TRUE(Map.remove_if([](const auto &) { return true; }));
+  EXPECT_TRUE(Map.empty());
+}
+
+TEST(DenseMapCustomTest, RemoveIfValueDtor) {
+  // remove_if must destroy the values of removed entries exactly once, and the
+  // rehash must not leak or double-destroy surviving values.
+  EXPECT_EQ(0u, CtorTester::getNumConstructed());
+  {
+    DenseMap<int, CtorTester> Map;
+    for (int I = 0; I < 16; ++I)
+      Map.try_emplace(I, CtorTester(I));
+    EXPECT_EQ(16u, CtorTester::getNumConstructed());
+    EXPECT_TRUE(Map.remove_if([](const auto &E) { return E.first < 10; }));
+    EXPECT_EQ(6u, CtorTester::getNumConstructed());
+    EXPECT_EQ(Map.size(), 6u);
+  }
+  EXPECT_EQ(0u, CtorTester::getNumConstructed());
+}
+
 } // namespace
diff --git a/llvm/unittests/ADT/DenseSetTest.cpp 
b/llvm/unittests/ADT/DenseSetTest.cpp
index a2a062b151b67..9d214b8649f66 100644
--- a/llvm/unittests/ADT/DenseSetTest.cpp
+++ b/llvm/unittests/ADT/DenseSetTest.cpp
@@ -65,6 +65,22 @@ TEST(SmallDenseSetTest, InsertRange) {
   EXPECT_THAT(set, ::testing::UnorderedElementsAre(7, 8, 9));
 }
 
+TEST(DenseSetTest, RemoveIf) {
+  llvm::DenseSet<unsigned> set;
+  for (unsigned I = 0; I < 100; ++I)
+    set.insert(I);
+
+  EXPECT_TRUE(set.remove_if([](unsigned V) { return V % 2 == 0; }));
+  EXPECT_EQ(set.size(), 50u);
+  for (unsigned I = 0; I < 100; ++I)
+    EXPECT_EQ(set.contains(I), I % 2 == 1);
+
+  EXPECT_FALSE(set.remove_if([](unsigned) { return false; }));
+  EXPECT_EQ(set.size(), 50u);
+  EXPECT_TRUE(set.remove_if([](unsigned) { return true; }));
+  EXPECT_TRUE(set.empty());
+}
+
 struct TestDenseSetInfo {
   static inline unsigned getEmptyKey() { return ~0; }
   static inline unsigned getTombstoneKey() { return ~0U - 1; }

>From 2ba42dfdabf8f4d815da1962e0e665de0996454e Mon Sep 17 00:00:00 2001
From: Fangrui Song <[email protected]>
Date: Sat, 23 May 2026 11:24:48 -0700
Subject: [PATCH 4/4] fix clang as well

---
 clang/lib/AST/ASTImporter.cpp                 | 13 ++++++++----
 clang/lib/CodeGen/CoverageMappingGen.cpp      | 10 +++------
 clang/lib/Interpreter/IncrementalParser.cpp   |  7 ++++++-
 clang/lib/Sema/HLSLExternalSemaSource.cpp     |  6 +++++-
 llvm/lib/Analysis/IRSimilarityIdentifier.cpp  |  7 ++++++-
 llvm/lib/Analysis/ScalarEvolution.cpp         |  3 +--
 .../CodeGen/AssignmentTrackingAnalysis.cpp    | 19 +++++++----------
 llvm/lib/ExecutionEngine/Orc/Core.cpp         | 21 +++++++------------
 llvm/lib/Support/CommandLine.cpp              | 13 +++++++++---
 .../Scalar/LowerMatrixIntrinsics.cpp          |  3 ++-
 llvm/unittests/Support/JSONTest.cpp           |  2 +-
 11 files changed, 59 insertions(+), 45 deletions(-)

diff --git a/clang/lib/AST/ASTImporter.cpp b/clang/lib/AST/ASTImporter.cpp
index 7bab2d7dcddfa..f8527af2bfe6f 100644
--- a/clang/lib/AST/ASTImporter.cpp
+++ b/clang/lib/AST/ASTImporter.cpp
@@ -9864,10 +9864,15 @@ Expected<Decl *> ASTImporter::Import(Decl *FromD) {
     // Failed to import.
 
     auto Pos = ImportedDecls.find(FromD);
-    if (Pos != ImportedDecls.end()) {
+    bool ToDWasCreated = Pos != ImportedDecls.end();
+    // Capture the mapped decl before erasing: the iterator is invalidated by
+    // the erase below under backward-shift deletion, but it is still needed
+    // further down to record the import error.
+    Decl *CreatedToD = ToDWasCreated ? Pos->second : nullptr;
+    if (ToDWasCreated) {
       // Import failed after the object was created.
       // Remove all references to it.
-      auto *ToD = Pos->second;
+      auto *ToD = CreatedToD;
       ImportedDecls.erase(Pos);
 
       // ImportedDecls and ImportedFromDecls are not symmetric.  It may happen
@@ -9903,8 +9908,8 @@ Expected<Decl *> ASTImporter::Import(Decl *FromD) {
                     [&ErrOut](const ASTImportError &E) { ErrOut = E; });
     setImportDeclError(FromD, ErrOut);
     // Set the error for the mapped to Decl, which is in the "to" context.
-    if (Pos != ImportedDecls.end())
-      SharedState->setImportDeclError(Pos->second, ErrOut);
+    if (ToDWasCreated)
+      SharedState->setImportDeclError(CreatedToD, ErrOut);
 
     // Set the error for all nodes which have been created before we
     // recognized the error.
diff --git a/clang/lib/CodeGen/CoverageMappingGen.cpp 
b/clang/lib/CodeGen/CoverageMappingGen.cpp
index eadb6e3bb25a8..c90afacbde293 100644
--- a/clang/lib/CodeGen/CoverageMappingGen.cpp
+++ b/clang/lib/CodeGen/CoverageMappingGen.cpp
@@ -2260,13 +2260,9 @@ struct CounterCoverageMappingBuilder
     (void)FoundCount;
 
     // Tell CodeGenPGO not to instrument.
-    for (auto I = MCDCState.BranchByStmt.begin(),
-              E = MCDCState.BranchByStmt.end();
-         I != E;) {
-      auto II = I++;
-      if (II->second.DecisionStmt == Decision)
-        MCDCState.BranchByStmt.erase(II);
-    }
+    MCDCState.BranchByStmt.remove_if([&](const auto &Entry) {
+      return Entry.second.DecisionStmt == Decision;
+    });
     MCDCState.DecisionByStmt.erase(Decision);
   }
 
diff --git a/clang/lib/Interpreter/IncrementalParser.cpp 
b/clang/lib/Interpreter/IncrementalParser.cpp
index 16a954f3c15e7..f6d2779d64b2b 100644
--- a/clang/lib/Interpreter/IncrementalParser.cpp
+++ b/clang/lib/Interpreter/IncrementalParser.cpp
@@ -174,6 +174,9 @@ IncrementalParser::Parse(llvm::StringRef input) {
 
 void IncrementalParser::CleanUpPTU(TranslationUnitDecl *MostRecentTU) {
   if (StoredDeclsMap *Map = MostRecentTU->getPrimaryContext()->getLookupPtr()) 
{
+    // Collect the keys to erase: erasing during iteration invalidates the map
+    // iterator under backward-shift deletion.
+    llvm::SmallVector<DeclarationName, 16> KeysToErase;
     for (auto &&[Key, List] : *Map) {
       DeclContextLookupResult R = List.getLookupResult();
       std::vector<NamedDecl *> NamedDeclsToRemove;
@@ -185,12 +188,14 @@ void IncrementalParser::CleanUpPTU(TranslationUnitDecl 
*MostRecentTU) {
           RemoveAll = false;
       }
       if (LLVM_LIKELY(RemoveAll)) {
-        Map->erase(Key);
+        KeysToErase.push_back(Key);
       } else {
         for (NamedDecl *D : NamedDeclsToRemove)
           List.remove(D);
       }
     }
+    for (DeclarationName Key : KeysToErase)
+      Map->erase(Key);
   }
 
   ExternCContextDecl *ECCD = S.getASTContext().getExternCContextDecl();
diff --git a/clang/lib/Sema/HLSLExternalSemaSource.cpp 
b/clang/lib/Sema/HLSLExternalSemaSource.cpp
index 449b32a215631..ae61b590a1f71 100644
--- a/clang/lib/Sema/HLSLExternalSemaSource.cpp
+++ b/clang/lib/Sema/HLSLExternalSemaSource.cpp
@@ -726,6 +726,10 @@ void HLSLExternalSemaSource::CompleteType(TagDecl *Tag) {
   auto It = Completions.find(Record);
   if (It == Completions.end())
     return;
-  It->second(Record);
+  // Move out the callback and erase before invoking it: the callback can
+  // re-enter CompleteType and mutate Completions, which invalidates It under
+  // backward-shift deletion.
+  CompletionFunction Fn = std::move(It->second);
   Completions.erase(It);
+  Fn(Record);
 }
diff --git a/llvm/lib/Analysis/IRSimilarityIdentifier.cpp 
b/llvm/lib/Analysis/IRSimilarityIdentifier.cpp
index e5ebd1f908d55..11a824973de60 100644
--- a/llvm/lib/Analysis/IRSimilarityIdentifier.cpp
+++ b/llvm/lib/Analysis/IRSimilarityIdentifier.cpp
@@ -720,7 +720,12 @@ bool IRSimilarityCandidate::compareAssignmentMapping(
   if (!WasInserted && !ValueMappingIt->second.contains(InstValB))
     return false;
   else if (ValueMappingIt->second.size() != 1) {
-    for (unsigned OtherVal : ValueMappingIt->second) {
+    // Snapshot the set before iterating: when InstValA maps to itself the
+    // erase below removes InstValA from the very set being iterated, which
+    // invalidates the range iterator under backward-shift deletion.
+    SmallVector<unsigned> OtherVals(ValueMappingIt->second.begin(),
+                                    ValueMappingIt->second.end());
+    for (unsigned OtherVal : OtherVals) {
       if (OtherVal == InstValB)
         continue;
       auto OtherValIt = ValueNumberMappingA.find(OtherVal);
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp 
b/llvm/lib/Analysis/ScalarEvolution.cpp
index 59232495752b4..855ab3bb5d621 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -8763,9 +8763,8 @@ void ScalarEvolution::visitAndClearUsers(
     ValueExprMapType::iterator It =
         ValueExprMap.find_as(static_cast<Value *>(I));
     if (It != ValueExprMap.end()) {
-      SCEVUse Mapped = It->second;
+      ToForget.push_back(It->second);
       eraseValueFromMap(It->first);
-      ToForget.push_back(Mapped);
       if (PHINode *PN = dyn_cast<PHINode>(I))
         ConstantEvolutionLoopExitValue.erase(PN);
     }
diff --git a/llvm/lib/CodeGen/AssignmentTrackingAnalysis.cpp 
b/llvm/lib/CodeGen/AssignmentTrackingAnalysis.cpp
index 2bd278614f8ac..d4da80a49bc71 100644
--- a/llvm/lib/CodeGen/AssignmentTrackingAnalysis.cpp
+++ b/llvm/lib/CodeGen/AssignmentTrackingAnalysis.cpp
@@ -543,18 +543,15 @@ class MemLocFragmentFill {
     // Meet A and B.
     //
     // Result = meet(a, b) for a in A, b in B where Var(a) == Var(b)
-    for (auto It = A.begin(), End = A.end(); It != End; ++It) {
-      unsigned AVar = It->first;
-      FragsInMemMap &AFrags = It->second;
-      auto BIt = B.find(AVar);
-      if (BIt == B.end()) {
-        A.erase(It);
-        continue; // Var has no bits defined in B.
-      }
+    A.remove_if([&](auto &Entry) {
+      auto BIt = B.find(Entry.first);
+      if (BIt == B.end())
+        return true; // Var has no bits defined in B.
       LLVM_DEBUG(dbgs() << "meet fragment maps for "
-                        << Aggregates[AVar].first->getName() << "\n");
-      AFrags = meetFragments(AFrags, BIt->second);
-    }
+                        << Aggregates[Entry.first].first->getName() << "\n");
+      Entry.second = meetFragments(Entry.second, BIt->second);
+      return false;
+    });
   }
 
   bool meet(const BasicBlock &BB,
diff --git a/llvm/lib/ExecutionEngine/Orc/Core.cpp 
b/llvm/lib/ExecutionEngine/Orc/Core.cpp
index ac780dc82ae5a..1838397f7b270 100644
--- a/llvm/lib/ExecutionEngine/Orc/Core.cpp
+++ b/llvm/lib/ExecutionEngine/Orc/Core.cpp
@@ -1065,9 +1065,7 @@ void JITDylib::removeFromLinkOrder(JITDylib &JD) {
 Error JITDylib::remove(const SymbolNameSet &Names) {
   return ES.runSessionLocked([&]() -> Error {
     assert(State == Open && "JD is defunct");
-    using SymbolMaterializerItrPair =
-        std::pair<SymbolTable::iterator, UnmaterializedInfosMap::iterator>;
-    std::vector<SymbolMaterializerItrPair> SymbolsToRemove;
+    SmallVector<SymbolStringPtr, 0> SymbolsToRemove;
     SymbolNameSet Missing;
     SymbolNameSet Materializing;
 
@@ -1087,10 +1085,7 @@ Error JITDylib::remove(const SymbolNameSet &Names) {
         continue;
       }
 
-      auto UMII = I->second.hasMaterializerAttached()
-                      ? UnmaterializedInfos.find(Name)
-                      : UnmaterializedInfos.end();
-      SymbolsToRemove.push_back(std::make_pair(I, UMII));
+      SymbolsToRemove.push_back(Name);
     }
 
     // If any of the symbols are not defined, return an error.
@@ -1103,18 +1098,18 @@ Error JITDylib::remove(const SymbolNameSet &Names) {
       return make_error<SymbolsCouldNotBeRemoved>(ES.getSymbolStringPool(),
                                                   std::move(Materializing));
 
-    // Remove the symbols.
-    for (auto &SymbolMaterializerItrPair : SymbolsToRemove) {
-      auto UMII = SymbolMaterializerItrPair.second;
-
+    // Remove the symbols. Erase by key rather than holding iterators across 
the
+    // loop: a prior erase invalidates other stored iterators under
+    // backward-shift deletion.
+    for (const SymbolStringPtr &Name : SymbolsToRemove) {
       // If there is a materializer attached, call discard.
+      auto UMII = UnmaterializedInfos.find(Name);
       if (UMII != UnmaterializedInfos.end()) {
         UMII->second->MU->doDiscard(*this, UMII->first);
         UnmaterializedInfos.erase(UMII);
       }
 
-      auto SymI = SymbolMaterializerItrPair.first;
-      Symbols.erase(SymI);
+      Symbols.erase(Name);
     }
 
     shrinkMaterializationInfoMemory();
diff --git a/llvm/lib/Support/CommandLine.cpp b/llvm/lib/Support/CommandLine.cpp
index 0c244b31724f1..5f679d50f8073 100644
--- a/llvm/lib/Support/CommandLine.cpp
+++ b/llvm/lib/Support/CommandLine.cpp
@@ -277,10 +277,11 @@ class CommandLineParser {
       OptionNames.push_back(O->ArgStr);
 
     SubCommand &Sub = *SC;
-    auto End = Sub.OptionsMap.end();
     for (auto Name : OptionNames) {
       auto I = Sub.OptionsMap.find(Name);
-      if (I != End && I->second == O)
+      // Re-query end() each iteration: a prior erase invalidates iterators
+      // (including a cached end()) under backward-shift deletion.
+      if (I != Sub.OptionsMap.end() && I->second == O)
         Sub.OptionsMap.erase(I);
     }
 
@@ -1494,8 +1495,14 @@ void CommandLineParser::ResetAllOptionOccurrences() {
   // Options might be reset twice (they can be reference in both OptionsMap
   // and one of the other members), but that does not harm.
   for (auto *SC : RegisteredSubCommands) {
+    // reset() removes default options from OptionsMap (via removeArgument), so
+    // collect the options first to avoid invalidating the map iterator.
+    SmallVector<Option *, 0> Opts;
+    Opts.reserve(SC->OptionsMap.size());
     for (auto &O : SC->OptionsMap)
-      O.second->reset();
+      Opts.push_back(O.second);
+    for (Option *O : Opts)
+      O->reset();
     for (Option *O : SC->PositionalOpts)
       O->reset();
     for (Option *O : SC->SinkOpts)
diff --git a/llvm/lib/Transforms/Scalar/LowerMatrixIntrinsics.cpp 
b/llvm/lib/Transforms/Scalar/LowerMatrixIntrinsics.cpp
index 04fdd24ad76da..c031574260c3c 100644
--- a/llvm/lib/Transforms/Scalar/LowerMatrixIntrinsics.cpp
+++ b/llvm/lib/Transforms/Scalar/LowerMatrixIntrinsics.cpp
@@ -902,9 +902,10 @@ class LowerMatrixIntrinsics {
     // it conditionally instead.
     auto S = ShapeMap.find(&Old);
     if (S != ShapeMap.end()) {
+      ShapeInfo Shape = S->second;
       ShapeMap.erase(S);
       if (supportsShapeInfo(New))
-        ShapeMap.insert({New, S->second});
+        ShapeMap.insert({New, Shape});
     }
     Old.replaceAllUsesWith(New);
   }
diff --git a/llvm/unittests/Support/JSONTest.cpp 
b/llvm/unittests/Support/JSONTest.cpp
index a58799b4f0455..348f5cb04f9db 100644
--- a/llvm/unittests/Support/JSONTest.cpp
+++ b/llvm/unittests/Support/JSONTest.cpp
@@ -155,8 +155,8 @@ TEST(JSONTest, Object) {
   auto E = O.find("e");
   EXPECT_EQ(E, O.end());
 
-  O.erase("b");
   O.erase(D);
+  O.erase("b");
   EXPECT_EQ(O.size(), 2u);
   EXPECT_EQ(R"({"a":1,"c":3})", s(std::move(O)));
 }

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