[KeyInstr] Add Atom Group waterline to LLVMContext

The waterline is managed through DILocation creation,
LLVMContext::incNextAtomGroup, and LLVMContext::updateAtomGroupWaterline.

Add unittest.

llvm/include/llvm/IR/LLVMContext.h

@@ -335,6 +335,14 @@ public:
   StringRef getDefaultTargetFeatures();
   void setDefaultTargetFeatures(StringRef Features);
 
+  /// Key Instructions: update the highest number atom group emitted for any
+  /// function.
+  void updateAtomGroupWaterline(uint64_t G);
+
+  /// Key Instructions: get the next free atom group number and increment
+  /// the global tracker.
+  uint64_t incNextAtomGroup();
+
 private:
   // Module needs access to the add/removeModule methods.
   friend class Module;

llvm/lib/IR/DebugInfoMetadata.cpp

@@ -67,6 +67,9 @@ DILocation::DILocation(LLVMContext &C, StorageType Storage, unsigned Line,
 #ifdef EXPERIMENTAL_KEY_INSTRUCTIONS
   assert(AtomRank <= 7 && "AtomRank number should fit in 3 bits");
 #endif
+  if (AtomRank)
+    C.updateAtomGroupWaterline(AtomGroup + 1);
+
   assert((MDs.size() == 1 || MDs.size() == 2) &&
          "Expected a scope and optional inlined-at");
   // Set line and column.

llvm/lib/IR/LLVMContext.cpp

@@ -377,3 +377,11 @@ StringRef LLVMContext::getDefaultTargetFeatures() {
 void LLVMContext::setDefaultTargetFeatures(StringRef Features) {
   pImpl->DefaultTargetFeatures = Features;
 }
+
+void LLVMContext::updateAtomGroupWaterline(uint64_t V) {
+  pImpl->NextAtomGroup = std::max(pImpl->NextAtomGroup, V);
+}
+
+uint64_t LLVMContext::incNextAtomGroup() {
+  return pImpl->NextAtomGroup++;
+}

llvm/lib/IR/LLVMContextImpl.h

@@ -1832,6 +1832,16 @@ public:
 
   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

llvm/unittests/IR/MetadataTest.cpp

@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "../lib/IR/LLVMContextImpl.h"
 #include "llvm/IR/Metadata.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/STLExtras.h"
@@ -1366,6 +1367,43 @@ 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 the build has opted into
+  // the additional DILocation fields needed for the feature.
+#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.incNextAtomGroup(), 6u);
+  EXPECT_EQ(Context.pImpl->NextAtomGroup, 7u);
+
+  Context.updateAtomGroupWaterline(8);
+  EXPECT_EQ(Context.pImpl->NextAtomGroup, 8u);
+  Context.updateAtomGroupWaterline(7);
+  EXPECT_EQ(Context.pImpl->NextAtomGroup, 8u);
+}
 
 typedef MetadataTest GenericDINodeTest;