[NFC][KeyInstr] Add Atom Group (re)mapping (#133479)

Add:
mapAtomInstance - map the atom group number to a new group.
RemapSourceAtom - apply the mapped atom group number to this instruction.

Modify:
CloneBasicBlock - Call mapAtomInstance on cloned instruction's DebugLocs
if MapAtoms is true (default). Setting to false could
lead to a degraded debugging experience. See code comment.

Optimisations like loop unroll that duplicate instructions need to remap source
atom groups so that each duplicated source construct instance is considered
distinct when determining is_stmt locations.

This commit adds the remapping functionality and a unittest.

RFC:
https://discourse.llvm.org/t/rfc-improving-is-stmt-placement-for-better-interactive-debugging/82668

Author: OCHyams

llvm/include/llvm/IR/ValueMap.h

@@ -87,6 +87,8 @@ class ValueMap {
   using ValueMapCVH = ValueMapCallbackVH<KeyT, ValueT, Config>;
   using MapT = DenseMap<ValueMapCVH, ValueT, DenseMapInfo<ValueMapCVH>>;
   using MDMapT = DenseMap<const Metadata *, TrackingMDRef>;
+  /// Map {(InlinedAt, old atom number) -> new atom number}.
+  using DMAtomT = SmallDenseMap<std::pair<Metadata *, uint64_t>, uint64_t>;
   using ExtraData = typename Config::ExtraData;
 
   MapT Map;
@@ -117,6 +119,8 @@ class ValueMap {
     return *MDMap;
   }
   std::optional<MDMapT> &getMDMap() { return MDMap; }
+  /// Map {(InlinedAt, old atom number) -> new atom number}.
+  DMAtomT AtomMap;
 
   /// Get the mapped metadata, if it's in the map.
   std::optional<Metadata *> getMappedMD(const Metadata *MD) const {
@@ -145,6 +149,7 @@ class ValueMap {
   void clear() {
     Map.clear();
     MDMap.reset();
+    AtomMap.clear();
   }
 
   /// Return 1 if the specified key is in the map, 0 otherwise.

llvm/include/llvm/Transforms/Utils/Cloning.h

@@ -22,6 +22,7 @@
 #include "llvm/Analysis/AssumptionCache.h"
 #include "llvm/Analysis/InlineCost.h"
 #include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/DebugLoc.h"
 #include "llvm/IR/ValueHandle.h"
 #include "llvm/Transforms/Utils/ValueMapper.h"
 #include <functional>
@@ -117,9 +118,22 @@ struct ClonedCodeInfo {
 /// If you would like to collect additional information about the cloned
 /// function, you can specify a ClonedCodeInfo object with the optional fifth
 /// parameter.
+///
+/// \p MapAtoms indicates whether source location atoms should be mapped for
+/// later remapping. Must be true when you duplicate a code path and a source
+/// location is intended to appear twice in the generated instructions. Can be
+/// set to false if you are transplanting code from one place to another.
+/// Setting true (default) is always safe (won't produce incorrect debug info)
+/// but is sometimes unnecessary, causing extra work that could be avoided by
+/// setting the parameter to false.
 BasicBlock *CloneBasicBlock(const BasicBlock *BB, ValueToValueMapTy &VMap,
                             const Twine &NameSuffix = "", Function *F = nullptr,
-                            ClonedCodeInfo *CodeInfo = nullptr);
+                            ClonedCodeInfo *CodeInfo = nullptr,
+                            bool MapAtoms = true);
+
+/// Mark a cloned instruction as a new instance so that its source loc can
+/// be updated when remapped.
+void mapAtomInstance(const DebugLoc &DL, ValueToValueMapTy &VMap);
 
 /// Return a copy of the specified function and add it to that
 /// function's module.  Also, any references specified in the VMap are changed

llvm/include/llvm/Transforms/Utils/ValueMapper.h

@@ -105,6 +105,13 @@ enum RemapFlags {
   /// Any global values not in value map are mapped to null instead of mapping
   /// to self.  Illegal if RF_IgnoreMissingLocals is also set.
   RF_NullMapMissingGlobalValues = 8,
+
+  /// Do not remap source location atoms. Only safe if to do this if the cloned
+  /// instructions being remapped are inserted into a new function, or an
+  /// existing function where the inlined-at fields are updated. If in doubt,
+  /// don't use this flag. It's used when remapping is known to be un-necessary
+  /// to save some compile-time.
+  RF_DoNotRemapAtoms = 16,
 };
 
 inline RemapFlags operator|(RemapFlags LHS, RemapFlags RHS) {
@@ -284,6 +291,12 @@ inline void RemapInstruction(Instruction *I, ValueToValueMapTy &VM,
       .remapInstruction(*I);
 }
 
+/// Remap source location atom. Called by RemapInstruction. This updates the
+/// instruction's atom group number if it has been mapped (e.g. with
+/// llvm::mapAtomInstance), which is necessary to distinguish source code
+/// atoms on duplicated code paths.
+void RemapSourceAtom(Instruction *I, ValueToValueMapTy &VM);
+
 /// Remap the Values used in the DbgRecord \a DR using the value map \a
 /// VM.
 inline void RemapDbgRecord(Module *M, DbgRecord *DR, ValueToValueMapTy &VM,

llvm/lib/Transforms/Utils/CloneFunction.cpp

@@ -13,6 +13,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Analysis/DomTreeUpdater.h"
 #include "llvm/Analysis/InstructionSimplify.h"
@@ -40,6 +41,27 @@ using namespace llvm;
 
 #define DEBUG_TYPE "clone-function"
 
+STATISTIC(RemappedAtomMax, "Highest global NextAtomGroup (after mapping)");
+
+void llvm::mapAtomInstance(const DebugLoc &DL, ValueToValueMapTy &VMap) {
+  auto CurGroup = DL.get()->getAtomGroup();
+  if (!CurGroup)
+    return;
+
+  // Try inserting a new entry. If there's already a mapping for this atom
+  // then there's nothing to do.
+  auto [It, Inserted] = VMap.AtomMap.insert({{DL.getInlinedAt(), CurGroup}, 0});
+  if (!Inserted)
+    return;
+
+  // Map entry to a new atom group.
+  uint64_t NewGroup = DL->getContext().incNextDILocationAtomGroup();
+  assert(NewGroup > CurGroup && "Next should always be greater than current");
+  It->second = NewGroup;
+
+  RemappedAtomMax = std::max<uint64_t>(NewGroup, RemappedAtomMax);
+}
+
 namespace {
 void collectDebugInfoFromInstructions(const Function &F,
                                       DebugInfoFinder &DIFinder) {
@@ -79,7 +101,7 @@ MetadataPredicate createIdentityMDPredicate(const Function &F,
 /// See comments in Cloning.h.
 BasicBlock *llvm::CloneBasicBlock(const BasicBlock *BB, ValueToValueMapTy &VMap,
                                   const Twine &NameSuffix, Function *F,
-                                  ClonedCodeInfo *CodeInfo) {
+                                  ClonedCodeInfo *CodeInfo, bool MapAtoms) {
   BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), "", F);
   NewBB->IsNewDbgInfoFormat = BB->IsNewDbgInfoFormat;
   if (BB->hasName())
@@ -98,6 +120,11 @@ BasicBlock *llvm::CloneBasicBlock(const BasicBlock *BB, ValueToValueMapTy &VMap,
 
     VMap[&I] = NewInst; // Add instruction map to value.
 
+    if (MapAtoms) {
+      if (const DebugLoc &DL = NewInst->getDebugLoc())
+        mapAtomInstance(DL.get(), VMap);
+    }
+
     if (isa<CallInst>(I) && !I.isDebugOrPseudoInst()) {
       hasCalls = true;
       hasMemProfMetadata |= I.hasMetadata(LLVMContext::MD_memprof);

llvm/lib/Transforms/Utils/ValueMapper.cpp

@@ -37,6 +37,7 @@
 #include "llvm/IR/Type.h"
 #include "llvm/IR/Value.h"
 #include "llvm/Support/Casting.h"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
 #include <cassert>
 #include <limits>
@@ -1010,6 +1011,10 @@ void Mapper::remapInstruction(Instruction *I) {
       I->setMetadata(MI.first, New);
   }
 
+  // Remap source location atom instance.
+  if (!(Flags & RF_DoNotRemapAtoms))
+    RemapSourceAtom(I, getVM());
+
   if (!TypeMapper)
     return;
 
@@ -1292,3 +1297,24 @@ void ValueMapper::scheduleMapGlobalIFunc(GlobalIFunc &GI, Constant &Resolver,
 void ValueMapper::scheduleRemapFunction(Function &F, unsigned MCID) {
   getAsMapper(pImpl)->scheduleRemapFunction(F, MCID);
 }
+
+void llvm::RemapSourceAtom(Instruction *I, ValueToValueMapTy &VM) {
+  const DebugLoc &DL = I->getDebugLoc();
+  if (!DL)
+    return;
+
+  auto AtomGroup = DL->getAtomGroup();
+  if (!AtomGroup)
+    return;
+
+  auto R = VM.AtomMap.find({DL->getInlinedAt(), AtomGroup});
+  if (R == VM.AtomMap.end())
+    return;
+  AtomGroup = R->second;
+
+  // Remap the atom group and copy all other fields.
+  DILocation *New = DILocation::get(
+      I->getContext(), DL.getLine(), DL.getCol(), DL.getScope(),
+      DL.getInlinedAt(), DL.isImplicitCode(), AtomGroup, DL->getAtomRank());
+  I->setDebugLoc(New);
+}

llvm/unittests/Transforms/Utils/CloningTest.cpp

@@ -14,18 +14,21 @@
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/AsmParser/Parser.h"
 #include "llvm/IR/Argument.h"
+#include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/Constant.h"
 #include "llvm/IR/DIBuilder.h"
 #include "llvm/IR/DebugInfo.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/InstIterator.h"
+#include "llvm/IR/Instruction.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/Verifier.h"
 #include "llvm/Support/SourceMgr.h"
+#include "llvm/Transforms/Utils/ValueMapper.h"
 #include "gtest/gtest.h"
 
 using namespace llvm;
@@ -1162,4 +1165,105 @@ declare i64 @foo(i32 noundef) local_unnamed_addr
   auto NewM = llvm::CloneModule(*M);
   EXPECT_FALSE(verifyModule(*NewM, &errs()));
 }
+
+TEST_F(CloneInstruction, cloneKeyInstructions) {
+  LLVMContext Context;
+
+  std::unique_ptr<Module> M = parseIR(Context, R"(
+    define void @test(ptr align 4 %dst) !dbg !3 {
+      store i64 1, ptr %dst, align 4, !dbg !6
+      store i64 2, ptr %dst, align 4, !dbg !7
+      store i64 3, ptr %dst, align 4, !dbg !8
+      ret void, !dbg !9
+    }
+
+    !llvm.dbg.cu = !{!0}
+    !llvm.module.flags = !{!2}
+    !0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1)
+    !1 = !DIFile(filename: "test.cpp",  directory: "")
+    !2 = !{i32 1, !"Debug Info Version", i32 3}
+    !3 = distinct !DISubprogram(name: "test", scope: !0, unit: !0)
+    !4 = distinct !DISubprogram(name: "inlined", scope: !0, unit: !0, retainedNodes: !{!5})
+    !5 = !DILocalVariable(name: "awaitables", scope: !4)
+    !6 = !DILocation(line: 1, scope: !4, inlinedAt: !8, atomGroup: 1, atomRank: 1)
+    !7 = !DILocation(line: 2, scope: !3, atomGroup: 1, atomRank: 1)
+    !8 = !DILocation(line: 3, scope: !3, atomGroup: 1, atomRank: 1)
+    !9 = !DILocation(line: 4, scope: !3, atomGroup: 2, atomRank: 1)
+  )");
+
+  ASSERT_FALSE(verifyModule(*M, &errs()));
+
+#ifdef EXPERIMENTAL_KEY_INSTRUCTIONS
+#define EXPECT_ATOM(Inst, G)                                                   \
+  EXPECT_TRUE(Inst->getDebugLoc());                                            \
+  EXPECT_EQ(Inst->getDebugLoc()->getAtomGroup(), uint64_t(G));
+#else
+#define EXPECT_ATOM(Inst, G) (void)Inst;
+#endif
+
+  Function *F = M->getFunction("test");
+  BasicBlock *BB = &*F->begin();
+  ASSERT_TRUE(F);
+  Instruction *Store1 = &*BB->begin();
+  Instruction *Store2 = Store1->getNextNode();
+  Instruction *Store3 = Store2->getNextNode();
+  Instruction *Ret = Store3->getNextNode();
+
+  // Test the remapping works as expected outside of cloning.
+  ValueToValueMapTy VM;
+  // Store1 and Store2 have the same atomGroup number, but have different
+  // inlining scopes, so only Store1 should change group.
+  mapAtomInstance(Store1->getDebugLoc(), VM);
+  for (Instruction &I : *BB)
+    RemapSourceAtom(&I, VM);
+  EXPECT_ATOM(Store1, 3);
+  EXPECT_ATOM(Store2, 1);
+  EXPECT_ATOM(Store3, 1);
+  EXPECT_ATOM(Ret, 2);
+  VM.clear();
+
+  // Store2 and Store3 have the same group number; both should get remapped.
+  mapAtomInstance(Store2->getDebugLoc(), VM);
+  for (Instruction &I : *BB)
+    RemapSourceAtom(&I, VM);
+  EXPECT_ATOM(Store1, 3);
+  EXPECT_ATOM(Store2, 4);
+  EXPECT_ATOM(Store3, 4);
+  EXPECT_ATOM(Ret, 2);
+  VM.clear();
+
+  // Cloning BB with MapAtoms=false should clone the atom numbers.
+  BasicBlock *BB2 =
+      CloneBasicBlock(BB, VM, "", nullptr, nullptr, /*MapAtoms*/ false);
+  for (Instruction &I : *BB2)
+    RemapSourceAtom(&I, VM);
+  Store1 = &*BB2->begin();
+  Store2 = Store1->getNextNode();
+  Store3 = Store2->getNextNode();
+  Ret = Store3->getNextNode();
+  EXPECT_ATOM(Store1, 3);
+  EXPECT_ATOM(Store2, 4);
+  EXPECT_ATOM(Store3, 4);
+  EXPECT_ATOM(Ret, 2);
+  VM.clear();
+  delete BB2;
+
+  // Cloning BB with MapAtoms=true should map the atom numbers.
+  BasicBlock *BB3 =
+      CloneBasicBlock(BB, VM, "", nullptr, nullptr, /*MapAtoms*/ true);
+  for (Instruction &I : *BB3)
+    RemapSourceAtom(&I, VM);
+  Store1 = &*BB3->begin();
+  Store2 = Store1->getNextNode();
+  Store3 = Store2->getNextNode();
+  Ret = Store3->getNextNode();
+  EXPECT_ATOM(Store1, 5);
+  EXPECT_ATOM(Store2, 6);
+  EXPECT_ATOM(Store3, 6);
+  EXPECT_ATOM(Ret, 7);
+  VM.clear();
+  delete BB3;
+#undef EXPECT_ATOM
+}
+
 } // namespace