diff --git a/llvm/include/llvm/IR/LLVMContext.h b/llvm/include/llvm/IR/LLVMContext.h
index bbd125fd38cf1..6dde7a968b570 100644
--- a/llvm/include/llvm/IR/LLVMContext.h
+++ b/llvm/include/llvm/IR/LLVMContext.h
@@ -335,6 +335,14 @@ class LLVMContext {
   StringRef getDefaultTargetFeatures();
   void setDefaultTargetFeatures(StringRef Features);
 
+  /// Key Instructions: update the highest number atom group emitted for any
+  /// function.
+  void updateDILocationAtomGroupWaterline(uint64_t G);
+
+  /// Key Instructions: get the next free atom group number and increment
+  /// the global tracker.
+  uint64_t incNextDILocationAtomGroup();
+
 private:
   // Module needs access to the add/removeModule methods.
   friend class Module;
diff --git a/llvm/lib/IR/DebugInfoMetadata.cpp b/llvm/lib/IR/DebugInfoMetadata.cpp
index e8a58f12e3e57..0973fcf94ee90 100644
--- a/llvm/lib/IR/DebugInfoMetadata.cpp
+++ b/llvm/lib/IR/DebugInfoMetadata.cpp
@@ -74,6 +74,9 @@ DILocation::DILocation(LLVMContext &C, StorageType Storage, unsigned Line,
 #ifdef EXPERIMENTAL_KEY_INSTRUCTIONS
   assert(AtomRank <= 7 && "AtomRank number should fit in 3 bits");
 #endif
+  if (AtomGroup)
+    C.updateDILocationAtomGroupWaterline(AtomGroup + 1);
+
   assert((MDs.size() == 1 || MDs.size() == 2) &&
          "Expected a scope and optional inlined-at");
   // Set line and column.
diff --git a/llvm/lib/IR/LLVMContext.cpp b/llvm/lib/IR/LLVMContext.cpp
index 447e5d92e0b99..57532cd491dd6 100644
--- a/llvm/lib/IR/LLVMContext.cpp
+++ b/llvm/lib/IR/LLVMContext.cpp
@@ -377,3 +377,11 @@ StringRef LLVMContext::getDefaultTargetFeatures() {
 void LLVMContext::setDefaultTargetFeatures(StringRef Features) {
   pImpl->DefaultTargetFeatures = Features;
 }
+
+void LLVMContext::updateDILocationAtomGroupWaterline(uint64_t V) {
+  pImpl->NextAtomGroup = std::max(pImpl->NextAtomGroup, V);
+}
+
+uint64_t LLVMContext::incNextDILocationAtomGroup() {
+  return pImpl->NextAtomGroup++;
+}
diff --git a/llvm/lib/IR/LLVMContextImpl.h b/llvm/lib/IR/LLVMContextImpl.h
index 5c2b5cd3a19cc..21f5c06ea24f3 100644
--- a/llvm/lib/IR/LLVMContextImpl.h
+++ b/llvm/lib/IR/LLVMContextImpl.h
@@ -1880,6 +1880,16 @@ class LLVMContextImpl {
 
   std::string DefaultTargetCPU;
   std::string DefaultTargetFeatures;
+
+  /// The next available source atom group number. The front end is responsible
+  /// for assigning source atom numbers, but certain optimisations need to
+  /// assign new group numbers to a set of instructions. Most often code
+  /// duplication optimisations like loop unroll. Tracking a global maximum
+  /// value means we can know (cheaply) we're never using a group number that's
+  /// already used within this function.
+  ///
+  /// Start a 1 because 0 means the source location isn't part of an atom group.
+  uint64_t NextAtomGroup = 1;
 };
 
 } // end namespace llvm
diff --git a/llvm/unittests/IR/MetadataTest.cpp b/llvm/unittests/IR/MetadataTest.cpp
index 45860d60d8f4c..9b25d0b91abdd 100644
--- a/llvm/unittests/IR/MetadataTest.cpp
+++ b/llvm/unittests/IR/MetadataTest.cpp
@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/IR/Metadata.h"
+#include "../lib/IR/LLVMContextImpl.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/IR/Constants.h"
@@ -1591,6 +1592,44 @@ TEST_F(DILocationTest, discriminatorSpecialCases) {
   EXPECT_EQ(std::nullopt, L4->cloneByMultiplyingDuplicationFactor(0x1000));
 }
 
+TEST_F(DILocationTest, KeyInstructions) {
+  Context.pImpl->NextAtomGroup = 1;
+
+  EXPECT_EQ(Context.pImpl->NextAtomGroup, 1u);
+  DILocation *A1 =
+      DILocation::get(Context, 1, 0, getSubprogram(), nullptr, false, 1, 2);
+  // The group is only applied to the DILocation if we've built LLVM with
+  // EXPERIMENTAL_KEY_INSTRUCTIONS.
+#ifdef EXPERIMENTAL_KEY_INSTRUCTIONS
+  EXPECT_EQ(A1->getAtomGroup(), 1u);
+  EXPECT_EQ(A1->getAtomRank(), 2u);
+#else
+  EXPECT_EQ(A1->getAtomGroup(), 0u);
+  EXPECT_EQ(A1->getAtomRank(), 0u);
+#endif
+
+  // Group number 1 has been "used" so next available is 2.
+  EXPECT_EQ(Context.pImpl->NextAtomGroup, 2u);
+
+  // Set a group number higher than current + 1, then check the waterline.
+  DILocation::get(Context, 2, 0, getSubprogram(), nullptr, false, 5, 1);
+  EXPECT_EQ(Context.pImpl->NextAtomGroup, 6u);
+
+  // The waterline should be unchanged (group <= next).
+  DILocation::get(Context, 3, 0, getSubprogram(), nullptr, false, 4, 1);
+  EXPECT_EQ(Context.pImpl->NextAtomGroup, 6u);
+  DILocation::get(Context, 3, 0, getSubprogram(), nullptr, false, 5, 1);
+  EXPECT_EQ(Context.pImpl->NextAtomGroup, 6u);
+
+  // Check the waterline gets incremented by 1.
+  EXPECT_EQ(Context.incNextDILocationAtomGroup(), 6u);
+  EXPECT_EQ(Context.pImpl->NextAtomGroup, 7u);
+
+  Context.updateDILocationAtomGroupWaterline(8);
+  EXPECT_EQ(Context.pImpl->NextAtomGroup, 8u);
+  Context.updateDILocationAtomGroupWaterline(7);
+  EXPECT_EQ(Context.pImpl->NextAtomGroup, 8u);
+}
 
 typedef MetadataTest GenericDINodeTest;