IR: Remove uselist for constantdata

This is a resurrected version of the patch attached to this RFC:

https://discourse.llvm.org/t/rfc-constantdata-should-not-have-use-lists/42606

In this adaptation, there are a few differences. In the original patch, the Use's
use list was replaced with an unsigned* to the reference count in the value. This
version leaves them as null and leaves the ref counting only in Value.

Remove use-lists from instances of ConstantData (which are shared
across modules and have no operands).

To continue supporting most of the use-list API, store a ref-count in
place of the use-list; this is for API like Value::use_empty and
Value::hasNUses.  Operations that actually need the use-list -- like
Value::use_begin -- will assert.

This change has three benefits:

 1. The compiler output cannot in any way depend on the use-list order
    of instances of ConstantData.

 2. There's no use-list traffic when adding and removing simple
    constants from operand lists (although there is ref-count traffic;
    YMMV).

 3. It's cheaper to serialize use-lists (since we're no longer
    serializing the use-list order of things like i32 0).

The downside is that you can't look at all the users of ConstantData,
but traversals of users of i32 0 are already ill-advised.

Possible follow-ups:
  - Track if an instance of a ConstantVector/ConstantArray/etc. is known
    to have all ConstantData arguments, and drop the use-lists to
    ref-counts in those cases.  Callers need to check Value::hasUseList
    before iterating through the use-list.
  - Remove even the ref-counts.  I'm not sure they have any benefit
    besides minimizing the scope of this commit, and maintaining the
    counts is not free.

Fixes #58629

Author: arsenm

llvm/docs/ReleaseNotes.md

@@ -56,6 +56,8 @@ Makes programs 10x faster by doing Special New Thing.
 Changes to the LLVM IR
 ----------------------
 
+* It is no longer permitted to inspect the uses of ConstantData
+
 * The `nocapture` attribute has been replaced by `captures(none)`.
 * The constant expression variants of the following instructions have been
   removed:

llvm/include/llvm/IR/Constants.h

@@ -50,6 +50,8 @@ template <class ConstantClass> struct ConstantAggrKeyType;
 /// These constants have no operands; they represent their data directly.
 /// Since they can be in use by unrelated modules (and are never based on
 /// GlobalValues), it never makes sense to RAUW them.
+///
+/// These do not have use lists. It is illegal to inspect the uses.
 class ConstantData : public Constant {
   constexpr static IntrusiveOperandsAllocMarker AllocMarker{0};
 

llvm/include/llvm/IR/Use.h

@@ -23,6 +23,7 @@
 namespace llvm {
 
 template <typename> struct simplify_type;
+class ConstantData;
 class User;
 class Value;
 
@@ -42,10 +43,7 @@ class Use {
 
 private:
   /// Destructor - Only for zap()
-  ~Use() {
-    if (Val)
-      removeFromList();
-  }
+  ~Use();
 
   /// Constructor
   Use(User *Parent) : Parent(Parent) {}
@@ -87,19 +85,10 @@ class Use {
   Use **Prev = nullptr;
   User *Parent = nullptr;
 
-  void addToList(Use **List) {
-    Next = *List;
-    if (Next)
-      Next->Prev = &Next;
-    Prev = List;
-    *Prev = this;
-  }
-
-  void removeFromList() {
-    *Prev = Next;
-    if (Next)
-      Next->Prev = Prev;
-  }
+  inline void addToList(unsigned &Count);
+  inline void addToList(Use *&List);
+  inline void removeFromList(unsigned &Count);
+  inline void removeFromList(Use *&List);
 };
 
 /// Allow clients to treat uses just like values when using

llvm/include/llvm/IR/Value.h

@@ -116,7 +116,10 @@ class Value {
 
 private:
   Type *VTy;
-  Use *UseList;
+  union {
+    Use *List = nullptr;
+    unsigned Count;
+  } Uses;
 
   friend class ValueAsMetadata; // Allow access to IsUsedByMD.
   friend class ValueHandleBase; // Allow access to HasValueHandle.
@@ -339,21 +342,28 @@ class Value {
 #endif
   }
 
+  /// Check if this Value has a use-list.
+  bool hasUseList() const { return !isa<ConstantData>(this); }
+
   bool use_empty() const {
     assertModuleIsMaterialized();
-    return UseList == nullptr;
+    return hasUseList() ? Uses.List == nullptr : Uses.Count == 0;
   }
 
   bool materialized_use_empty() const {
-    return UseList == nullptr;
+    return hasUseList() ? Uses.List == nullptr : !Uses.Count;
   }
 
   using use_iterator = use_iterator_impl<Use>;
   using const_use_iterator = use_iterator_impl<const Use>;
 
-  use_iterator materialized_use_begin() { return use_iterator(UseList); }
+  use_iterator materialized_use_begin() {
+    assert(hasUseList());
+    return use_iterator(Uses.List);
+  }
   const_use_iterator materialized_use_begin() const {
-    return const_use_iterator(UseList);
+    assert(hasUseList());
+    return const_use_iterator(Uses.List);
   }
   use_iterator use_begin() {
     assertModuleIsMaterialized();
@@ -380,17 +390,18 @@ class Value {
     return materialized_uses();
   }
 
-  bool user_empty() const {
-    assertModuleIsMaterialized();
-    return UseList == nullptr;
-  }
+  bool user_empty() const { return use_empty(); }
 
   using user_iterator = user_iterator_impl<User>;
   using const_user_iterator = user_iterator_impl<const User>;
 
-  user_iterator materialized_user_begin() { return user_iterator(UseList); }
+  user_iterator materialized_user_begin() {
+    assert(hasUseList());
+    return user_iterator(Uses.List);
+  }
   const_user_iterator materialized_user_begin() const {
-    return const_user_iterator(UseList);
+    assert(hasUseList());
+    return const_user_iterator(Uses.List);
   }
   user_iterator user_begin() {
     assertModuleIsMaterialized();
@@ -429,7 +440,11 @@ class Value {
   ///
   /// This is specialized because it is a common request and does not require
   /// traversing the whole use list.
-  bool hasOneUse() const { return hasSingleElement(uses()); }
+  bool hasOneUse() const {
+    if (!hasUseList())
+      return Uses.Count == 1;
+    return hasSingleElement(uses());
+  }
 
   /// Return true if this Value has exactly N uses.
   bool hasNUses(unsigned N) const;
@@ -491,6 +506,8 @@ class Value {
   static void dropDroppableUse(Use &U);
 
   /// Check if this value is used in the specified basic block.
+  ///
+  /// Not supported for ConstantData.
   bool isUsedInBasicBlock(const BasicBlock *BB) const;
 
   /// This method computes the number of uses of this Value.
@@ -500,7 +517,19 @@ class Value {
   unsigned getNumUses() const;
 
   /// This method should only be used by the Use class.
-  void addUse(Use &U) { U.addToList(&UseList); }
+  void addUse(Use &U) {
+    if (hasUseList())
+      U.addToList(Uses.List);
+    else
+      U.addToList(Uses.Count);
+  }
+
+  void removeUse(Use &U) {
+    if (hasUseList())
+      U.removeFromList(Uses.List);
+    else
+      U.removeFromList(Uses.Count);
+  }
 
   /// Concrete subclass of this.
   ///
@@ -841,7 +870,8 @@ class Value {
   ///
   /// \return the first element in the list.
   ///
-  /// \note Completely ignores \a Use::Prev (doesn't read, doesn't update).
+  /// \note Completely ignores \a Use::PrevOrCount (doesn't read, doesn't
+  /// update).
   template <class Compare>
   static Use *mergeUseLists(Use *L, Use *R, Compare Cmp) {
     Use *Merged;
@@ -887,10 +917,50 @@ inline raw_ostream &operator<<(raw_ostream &OS, const Value &V) {
   return OS;
 }
 
+inline Use::~Use() {
+  if (Val)
+    Val->removeUse(*this);
+}
+
+void Use::addToList(unsigned &Count) {
+  assert(isa<ConstantData>(Val) && "Only ConstantData is ref-counted");
+  ++Count;
+
+  // We don't have a uselist - clear the remnant if we are replacing a
+  // non-constant value.
+  Prev = nullptr;
+  Next = nullptr;
+}
+
+void Use::addToList(Use *&List) {
+  assert(!isa<ConstantData>(Val) && "ConstantData has no use-list");
+
+  Next = List;
+  if (Next)
+    Next->Prev = &Next;
+  Prev = &List;
+  List = this;
+}
+
+void Use::removeFromList(unsigned &Count) {
+  assert(isa<ConstantData>(Val));
+  assert(Count > 0 && "reference count underflow");
+  assert(!Prev && !Next && "should not have uselist remnant");
+  --Count;
+}
+
+void Use::removeFromList(Use *&List) {
+  *Prev = Next;
+  if (Next)
+    Next->Prev = Prev;
+}
+
 void Use::set(Value *V) {
-  if (Val) removeFromList();
+  if (Val)
+    Val->removeUse(*this);
   Val = V;
-  if (V) V->addUse(*this);
+  if (V)
+    V->addUse(*this);
 }
 
 Value *Use::operator=(Value *RHS) {
@@ -904,7 +974,7 @@ const Use &Use::operator=(const Use &RHS) {
 }
 
 template <class Compare> void Value::sortUseList(Compare Cmp) {
-  if (!UseList || !UseList->Next)
+  if (!hasUseList() || !Uses.List || !Uses.List->Next)
     // No need to sort 0 or 1 uses.
     return;
 
@@ -917,10 +987,10 @@ template <class Compare> void Value::sortUseList(Compare Cmp) {
   Use *Slots[MaxSlots];
 
   // Collect the first use, turning it into a single-item list.
-  Use *Next = UseList->Next;
-  UseList->Next = nullptr;
+  Use *Next = Uses.List->Next;
+  Uses.List->Next = nullptr;
   unsigned NumSlots = 1;
-  Slots[0] = UseList;
+  Slots[0] = Uses.List;
 
   // Collect all but the last use.
   while (Next->Next) {
@@ -956,15 +1026,15 @@ template <class Compare> void Value::sortUseList(Compare Cmp) {
   // Merge all the lists together.
   assert(Next && "Expected one more Use");
   assert(!Next->Next && "Expected only one Use");
-  UseList = Next;
+  Uses.List = Next;
   for (unsigned I = 0; I < NumSlots; ++I)
     if (Slots[I])
-      // Since the uses in Slots[I] originally preceded those in UseList, send
+      // Since the uses in Slots[I] originally preceded those in Uses.List, send
       // Slots[I] in as the left parameter to maintain a stable sort.
-      UseList = mergeUseLists(Slots[I], UseList, Cmp);
+      Uses.List = mergeUseLists(Slots[I], Uses.List, Cmp);
 
   // Fix the Prev pointers.
-  for (Use *I = UseList, **Prev = &UseList; I; I = I->Next) {
+  for (Use *I = Uses.List, **Prev = &Uses.List; I; I = I->Next) {
     I->Prev = Prev;
     Prev = &I->Next;
   }

llvm/lib/Analysis/TypeMetadataUtils.cpp

@@ -54,6 +54,9 @@ findCallsAtConstantOffset(SmallVectorImpl<DevirtCallSite> &DevirtCalls,
 static void findLoadCallsAtConstantOffset(
     const Module *M, SmallVectorImpl<DevirtCallSite> &DevirtCalls, Value *VPtr,
     int64_t Offset, const CallInst *CI, DominatorTree &DT) {
+  if (!VPtr->hasUseList())
+    return;
+
   for (const Use &U : VPtr->uses()) {
     Value *User = U.getUser();
     if (isa<BitCastInst>(User)) {

llvm/lib/AsmParser/LLParser.cpp

@@ -8857,6 +8857,8 @@ bool LLParser::parseMDNodeVector(SmallVectorImpl<Metadata *> &Elts) {
 //===----------------------------------------------------------------------===//
 bool LLParser::sortUseListOrder(Value *V, ArrayRef<unsigned> Indexes,
                                 SMLoc Loc) {
+  if (!V->hasUseList())
+    return false;
   if (V->use_empty())
     return error(Loc, "value has no uses");
 

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -3856,6 +3856,10 @@ Error BitcodeReader::parseUseLists() {
         V = FunctionBBs[ID];
       } else
         V = ValueList[ID];
+
+      if (isa<ConstantData>(V))
+        break;
+
       unsigned NumUses = 0;
       SmallDenseMap<const Use *, unsigned, 16> Order;
       for (const Use &U : V->materialized_uses()) {

llvm/lib/Bitcode/Writer/ValueEnumerator.cpp

@@ -230,6 +230,9 @@ static void predictValueUseListOrderImpl(const Value *V, const Function *F,
 
 static void predictValueUseListOrder(const Value *V, const Function *F,
                                      OrderMap &OM, UseListOrderStack &Stack) {
+  if (isa<ConstantData>(V))
+    return;
+
   auto &IDPair = OM[V];
   assert(IDPair.first && "Unmapped value");
   if (IDPair.second)

llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp

@@ -4002,7 +4002,7 @@ static void emitGlobalConstantImpl(const DataLayout &DL, const Constant *CV,
   // Globals with sub-elements such as combinations of arrays and structs
   // are handled recursively by emitGlobalConstantImpl. Keep track of the
   // constant symbol base and the current position with BaseCV and Offset.
-  if (!BaseCV && CV->hasOneUse())
+  if (!isa<ConstantData>(CV) && !BaseCV && CV->hasOneUse())
     BaseCV = dyn_cast<Constant>(CV->user_back());
 
   if (isa<ConstantAggregateZero>(CV)) {

llvm/lib/CodeGen/CodeGenPrepare.cpp

@@ -8591,6 +8591,9 @@ static bool optimizeBranch(BranchInst *Branch, const TargetLowering &TLI,
     return false;
 
   Value *X = Cmp->getOperand(0);
+  if (!X->hasUseList())
+    return false;
+
   APInt CmpC = cast<ConstantInt>(Cmp->getOperand(1))->getValue();
 
   for (auto *U : X->users()) {

llvm/lib/CodeGen/ComplexDeinterleavingPass.cpp

@@ -1034,6 +1034,9 @@ ComplexDeinterleavingGraph::identifyPartialReduction(Value *R, Value *I) {
   if (!isa<VectorType>(R->getType()) || !isa<VectorType>(I->getType()))
     return nullptr;
 
+  if (!R->hasUseList() || !I->hasUseList())
+    return nullptr;
+
   auto CommonUser =
       findCommonBetweenCollections<Value *>(R->users(), I->users());
   if (!CommonUser)

llvm/lib/IR/AsmWriter.cpp

@@ -125,11 +125,15 @@ static void orderValue(const Value *V, OrderMap &OM) {
   if (OM.lookup(V))
     return;
 
-  if (const Constant *C = dyn_cast<Constant>(V))
+  if (const Constant *C = dyn_cast<Constant>(V)) {
+    if (isa<ConstantData>(C))
+      return;
+
     if (C->getNumOperands() && !isa<GlobalValue>(C))
       for (const Value *Op : C->operands())
         if (!isa<BasicBlock>(Op) && !isa<GlobalValue>(Op))
           orderValue(Op, OM);
+  }
 
   // Note: we cannot cache this lookup above, since inserting into the map
   // changes the map's size, and thus affects the other IDs.
@@ -275,7 +279,8 @@ static UseListOrderMap predictUseListOrder(const Module *M) {
   UseListOrderMap ULOM;
   for (const auto &Pair : OM) {
     const Value *V = Pair.first;
-    if (V->use_empty() || std::next(V->use_begin()) == V->use_end())
+    if (!V->hasUseList() || V->use_empty() ||
+        std::next(V->use_begin()) == V->use_end())
       continue;
 
     std::vector<unsigned> Shuffle =

llvm/lib/IR/Instruction.cpp

@@ -373,7 +373,9 @@ std::optional<BasicBlock::iterator> Instruction::getInsertionPointAfterDef() {
 }
 
 bool Instruction::isOnlyUserOfAnyOperand() {
-  return any_of(operands(), [](Value *V) { return V->hasOneUser(); });
+  return any_of(operands(), [](const Value *V) {
+    return V->hasUseList() && V->hasOneUser();
+  });
 }
 
 void Instruction::setHasNoUnsignedWrap(bool b) {