diff --git a/llvm/include/llvm/AsmParser/LLParser.h b/llvm/include/llvm/AsmParser/LLParser.h
index b0d02eac8e672..cf358c384f520 100644
--- a/llvm/include/llvm/AsmParser/LLParser.h
+++ b/llvm/include/llvm/AsmParser/LLParser.h
@@ -331,6 +331,7 @@ namespace llvm {
 
     // Top-Level Entities
     bool parseTopLevelEntities();
+    void dropUnknownMetadataReferences();
     bool validateEndOfModule(bool UpgradeDebugInfo);
     bool validateEndOfIndex();
     bool parseTargetDefinitions(DataLayoutCallbackTy DataLayoutCallback);
diff --git a/llvm/include/llvm/IR/GlobalObject.h b/llvm/include/llvm/IR/GlobalObject.h
index ae8e616824448..b6a974d8bb9f0 100644
--- a/llvm/include/llvm/IR/GlobalObject.h
+++ b/llvm/include/llvm/IR/GlobalObject.h
@@ -133,6 +133,7 @@ class GlobalObject : public GlobalValue {
   using Value::addMetadata;
   using Value::clearMetadata;
   using Value::eraseMetadata;
+  using Value::eraseMetadataIf;
   using Value::getAllMetadata;
   using Value::getMetadata;
   using Value::hasMetadata;
diff --git a/llvm/include/llvm/IR/Instruction.h b/llvm/include/llvm/IR/Instruction.h
index 0211b5076131c..fcd2ba838e7fd 100644
--- a/llvm/include/llvm/IR/Instruction.h
+++ b/llvm/include/llvm/IR/Instruction.h
@@ -384,6 +384,9 @@ class Instruction : public User,
   void copyMetadata(const Instruction &SrcInst,
                     ArrayRef<unsigned> WL = ArrayRef<unsigned>());
 
+  /// Erase all metadata that matches the predicate.
+  void eraseMetadataIf(function_ref<bool(unsigned, MDNode *)> Pred);
+
   /// If the instruction has "branch_weights" MD_prof metadata and the MDNode
   /// has three operands (including name string), swap the order of the
   /// metadata.
diff --git a/llvm/include/llvm/IR/Metadata.h b/llvm/include/llvm/IR/Metadata.h
index befb6975ca18d..db1f44fea3b45 100644
--- a/llvm/include/llvm/IR/Metadata.h
+++ b/llvm/include/llvm/IR/Metadata.h
@@ -417,6 +417,8 @@ class ReplaceableMetadataImpl {
   /// is resolved.
   void resolveAllUses(bool ResolveUsers = true);
 
+  unsigned getNumUses() const { return UseMap.size(); }
+
 private:
   void addRef(void *Ref, OwnerTy Owner);
   void dropRef(void *Ref);
@@ -1243,6 +1245,11 @@ class MDNode : public Metadata {
   bool isReplaceable() const { return isTemporary() || isAlwaysReplaceable(); }
   bool isAlwaysReplaceable() const { return getMetadataID() == DIAssignIDKind; }
 
+  unsigned getNumTemporaryUses() const {
+    assert(isTemporary() && "Only for temporaries");
+    return Context.getReplaceableUses()->getNumUses();
+  }
+
   /// RAUW a temporary.
   ///
   /// \pre \a isTemporary() must be \c true.
diff --git a/llvm/include/llvm/IR/Value.h b/llvm/include/llvm/IR/Value.h
index 61f9d34eef356..945081b77e953 100644
--- a/llvm/include/llvm/IR/Value.h
+++ b/llvm/include/llvm/IR/Value.h
@@ -618,6 +618,9 @@ class Value {
   /// \returns true if any metadata was removed.
   bool eraseMetadata(unsigned KindID);
 
+  /// Erase all metadata attachments matching the given predicate.
+  void eraseMetadataIf(function_ref<bool(unsigned, MDNode *)> Pred);
+
   /// Erase all metadata attached to this Value.
   void clearMetadata();
 
diff --git a/llvm/lib/AsmParser/LLParser.cpp b/llvm/lib/AsmParser/LLParser.cpp
index ea7f9a00d719f..d6c5993797de1 100644
--- a/llvm/lib/AsmParser/LLParser.cpp
+++ b/llvm/lib/AsmParser/LLParser.cpp
@@ -13,8 +13,8 @@
 #include "llvm/AsmParser/LLParser.h"
 #include "llvm/ADT/APSInt.h"
 #include "llvm/ADT/DenseMap.h"
-#include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/AsmParser/LLToken.h"
 #include "llvm/AsmParser/SlotMapping.h"
@@ -32,7 +32,9 @@
 #include "llvm/IR/GlobalIFunc.h"
 #include "llvm/IR/GlobalObject.h"
 #include "llvm/IR/InlineAsm.h"
+#include "llvm/IR/InstIterator.h"
 #include "llvm/IR/Instructions.h"
+#include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Intrinsics.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/Metadata.h"
@@ -54,6 +56,12 @@
 
 using namespace llvm;
 
+static cl::opt<bool> AllowIncompleteIR(
+    "allow-incomplete-ir", cl::init(false), cl::Hidden,
+    cl::desc(
+        "Allow incomplete IR on a best effort basis (references to unknown "
+        "metadata will be dropped)"));
+
 static std::string getTypeString(Type *T) {
   std::string Result;
   raw_string_ostream Tmp(Result);
@@ -123,6 +131,55 @@ void LLParser::restoreParsingState(const SlotMapping *Slots) {
         std::make_pair(I.first, std::make_pair(I.second, LocTy())));
 }
 
+static void dropIntrinsicWithUnknownMetadataArgument(IntrinsicInst *II) {
+  // White-list intrinsics that are safe to drop.
+  if (!isa<DbgInfoIntrinsic>(II) &&
+      II->getIntrinsicID() != Intrinsic::experimental_noalias_scope_decl)
+    return;
+
+  SmallVector<MetadataAsValue *> MVs;
+  for (Value *V : II->args())
+    if (auto *MV = dyn_cast<MetadataAsValue>(V))
+      if (auto *MD = dyn_cast<MDNode>(MV->getMetadata()))
+        if (MD->isTemporary())
+          MVs.push_back(MV);
+
+  if (!MVs.empty()) {
+    assert(II->use_empty() && "Cannot have uses");
+    II->eraseFromParent();
+
+    // Also remove no longer used MetadataAsValue wrappers.
+    for (MetadataAsValue *MV : MVs)
+      if (MV->use_empty())
+        delete MV;
+  }
+}
+
+void LLParser::dropUnknownMetadataReferences() {
+  auto Pred = [](unsigned MDKind, MDNode *Node) { return Node->isTemporary(); };
+  for (Function &F : *M) {
+    F.eraseMetadataIf(Pred);
+    for (Instruction &I : make_early_inc_range(instructions(F))) {
+      I.eraseMetadataIf(Pred);
+
+      if (auto *II = dyn_cast<IntrinsicInst>(&I))
+        dropIntrinsicWithUnknownMetadataArgument(II);
+    }
+  }
+
+  for (GlobalVariable &GV : M->globals())
+    GV.eraseMetadataIf(Pred);
+
+  for (const auto &[ID, Info] : make_early_inc_range(ForwardRefMDNodes)) {
+    // Check whether there is only a single use left, which would be in our
+    // own NumberedMetadata.
+    if (Info.first->getNumTemporaryUses() == 1) {
+      NumberedMetadata.erase(ID);
+      ForwardRefMDNodes.erase(ID);
+    }
+  }
+}
+
 /// validateEndOfModule - Do final validity and basic correctness checks at the
 /// end of the module.
 bool LLParser::validateEndOfModule(bool UpgradeDebugInfo) {
@@ -284,6 +341,9 @@ bool LLParser::validateEndOfModule(bool UpgradeDebugInfo) {
                  "use of undefined value '@" +
                      Twine(ForwardRefValIDs.begin()->first) + "'");
 
+  if (AllowIncompleteIR && !ForwardRefMDNodes.empty())
+    dropUnknownMetadataReferences();
+
   if (!ForwardRefMDNodes.empty())
     return error(ForwardRefMDNodes.begin()->second.second,
                  "use of undefined metadata '!" +
@@ -297,10 +357,14 @@ bool LLParser::validateEndOfModule(bool UpgradeDebugInfo) {
 
   for (auto *Inst : InstsWithTBAATag) {
     MDNode *MD = Inst->getMetadata(LLVMContext::MD_tbaa);
-    assert(MD && "UpgradeInstWithTBAATag should have a TBAA tag");
-    auto *UpgradedMD = UpgradeTBAANode(*MD);
-    if (MD != UpgradedMD)
-      Inst->setMetadata(LLVMContext::MD_tbaa, UpgradedMD);
+    // With incomplete IR, the tbaa metadata may have been dropped.
+    if (!AllowIncompleteIR)
+      assert(MD && "UpgradeInstWithTBAATag should have a TBAA tag");
+    if (MD) {
+      auto *UpgradedMD = UpgradeTBAANode(*MD);
+      if (MD != UpgradedMD)
+        Inst->setMetadata(LLVMContext::MD_tbaa, UpgradedMD);
+    }
   }
 
   // Look for intrinsic functions and CallInst that need to be upgraded.  We use
diff --git a/llvm/lib/IR/Metadata.cpp b/llvm/lib/IR/Metadata.cpp
index eff4fd371a73f..37017a222d485 100644
--- a/llvm/lib/IR/Metadata.cpp
+++ b/llvm/lib/IR/Metadata.cpp
@@ -1533,6 +1533,21 @@ bool Value::eraseMetadata(unsigned KindID) {
   return Changed;
 }
 
+void Value::eraseMetadataIf(function_ref<bool(unsigned, MDNode *)> Pred) {
+  if (!HasMetadata)
+    return;
+
+  auto &MetadataStore = getContext().pImpl->ValueMetadata;
+  MDAttachments &Info = MetadataStore.find(this)->second;
+  assert(!Info.empty() && "bit out of sync with hash table");
+  Info.remove_if([Pred](const MDAttachments::Attachment &I) {
+    return Pred(I.MDKind, I.Node);
+  });
+
+  if (Info.empty())
+    clearMetadata();
+}
+
 void Value::clearMetadata() {
   if (!HasMetadata)
     return;
@@ -1556,6 +1571,13 @@ MDNode *Instruction::getMetadataImpl(StringRef Kind) const {
   return Value::getMetadata(KindID);
 }
 
+void Instruction::eraseMetadataIf(function_ref<bool(unsigned, MDNode *)> Pred) {
+  if (DbgLoc && Pred(LLVMContext::MD_dbg, DbgLoc.getAsMDNode()))
+    DbgLoc = {};
+
+  Value::eraseMetadataIf(Pred);
+}
+
 void Instruction::dropUnknownNonDebugMetadata(ArrayRef<unsigned> KnownIDs) {
   if (!Value::hasMetadata())
     return; // Nothing to remove!
@@ -1566,17 +1588,9 @@ void Instruction::dropUnknownNonDebugMetadata(ArrayRef<unsigned> KnownIDs) {
   // A DIAssignID attachment is debug metadata, don't drop it.
   KnownSet.insert(LLVMContext::MD_DIAssignID);
 
-  auto &MetadataStore = getContext().pImpl->ValueMetadata;
-  MDAttachments &Info = MetadataStore.find(this)->second;
-  assert(!Info.empty() && "bit out of sync with hash table");
-  Info.remove_if([&KnownSet](const MDAttachments::Attachment &I) {
-    return !KnownSet.count(I.MDKind);
+  Value::eraseMetadataIf([&KnownSet](unsigned MDKind, MDNode *Node) {
+    return !KnownSet.count(MDKind);
   });
-
-  if (Info.empty()) {
-    // Drop our entry at the store.
-    clearMetadata();
-  }
 }
 
 void Instruction::updateDIAssignIDMapping(DIAssignID *ID) {
diff --git a/llvm/test/Assembler/incomplete-ir-metadata-unsupported.ll b/llvm/test/Assembler/incomplete-ir-metadata-unsupported.ll
new file mode 100644
index 0000000000000..e320472921ed9
--- /dev/null
+++ b/llvm/test/Assembler/incomplete-ir-metadata-unsupported.ll
@@ -0,0 +1,9 @@
+; RUN: not llvm-as -allow-incomplete-ir < %s 2>&1 | FileCheck %s
+
+; CHECK: error: use of undefined metadata '!1'
+define void @test(ptr %p) {
+  %v = load i8, ptr %p, !noalias !0
+  ret void
+}
+
+!0 = !{!1}
diff --git a/llvm/test/Assembler/incomplete-ir-metadata.ll b/llvm/test/Assembler/incomplete-ir-metadata.ll
new file mode 100644
index 0000000000000..fdf7b4ee0ebd2
--- /dev/null
+++ b/llvm/test/Assembler/incomplete-ir-metadata.ll
@@ -0,0 +1,34 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt -S -allow-incomplete-ir < %s | FileCheck %s
+
+@g = global i8 0, !exclude !4
+
+define void @test(ptr %p) !dbg !3 {
+; CHECK-LABEL: define void @test(
+; CHECK-SAME: ptr [[P:%.*]]) {
+; CHECK-NEXT:    [[V1:%.*]] = load i8, ptr [[P]], align 1
+; CHECK-NEXT:    [[V2:%.*]] = load i8, ptr [[P]], align 1
+; CHECK-NEXT:    [[V3:%.*]] = load i8, ptr [[P]], align 1, !noalias [[META0:![0-9]+]]
+; CHECK-NEXT:    call void @llvm.experimental.noalias.scope.decl(metadata [[META0]])
+; CHECK-NEXT:    ret void
+;
+  %v1 = load i8, ptr %p, !noalias !0
+  %v2 = load i8, ptr %p, !tbaa !1
+  %v3 = load i8, ptr %p, !dbg !2, !noalias !100
+  call void @llvm.experimental.noalias.scope.decl(metadata !5)
+  call void @llvm.dbg.value(metadata i32 0, metadata !7, metadata !8)
+  call void @llvm.experimental.noalias.scope.decl(metadata !100)
+  ret void
+}
+
+declare void @llvm.experimental.noalias.scope.decl(metadata)
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!100 = !{!101}
+!101 = !{!101, !102}
+!102 = !{!102}
+;.
+; CHECK: [[META0]] = !{[[META1:![0-9]+]]}
+; CHECK: [[META1]] = distinct !{[[META1]], [[META2:![0-9]+]]}
+; CHECK: [[META2]] = distinct !{[[META2]]}
+;.