IR: Split Metadata from Value

Split `Metadata` away from the `Value` class hierarchy, as part of
PR21532.  Assembly and bitcode changes are in the wings, but this is the
bulk of the change for the IR C++ API.

I have a follow-up patch prepared for `clang`.  If this breaks other
sub-projects, I apologize in advance :(.  Help me compile it on Darwin
I'll try to fix it.  FWIW, the errors should be easy to fix, so it may
be simpler to just fix it yourself.

This breaks the build for all metadata-related code that's out-of-tree.
Rest assured the transition is mechanical and the compiler should catch
almost all of the problems.

Here's a quick guide for updating your code:

  - `Metadata` is the root of a class hierarchy with three main classes:
    `MDNode`, `MDString`, and `ValueAsMetadata`.  It is distinct from
    the `Value` class hierarchy.  It is typeless -- i.e., instances do
    *not* have a `Type`.

  - `MDNode`'s operands are all `Metadata *` (instead of `Value *`).

  - `TrackingVH<MDNode>` and `WeakVH` referring to metadata can be
    replaced with `TrackingMDNodeRef` and `TrackingMDRef`, respectively.

    If you're referring solely to resolved `MDNode`s -- post graph
    construction -- just use `MDNode*`.

  - `MDNode` (and the rest of `Metadata`) have only limited support for
    `replaceAllUsesWith()`.

    As long as an `MDNode` is pointing at a forward declaration -- the
    result of `MDNode::getTemporary()` -- it maintains a side map of its
    uses and can RAUW itself.  Once the forward declarations are fully
    resolved RAUW support is dropped on the ground.  This means that
    uniquing collisions on changing operands cause nodes to become
    "distinct".  (This already happened fairly commonly, whenever an
    operand went to null.)

    If you're constructing complex (non self-reference) `MDNode` cycles,
    you need to call `MDNode::resolveCycles()` on each node (or on a
    top-level node that somehow references all of the nodes).  Also,
    don't do that.  Metadata cycles (and the RAUW machinery needed to
    construct them) are expensive.

  - An `MDNode` can only refer to a `Constant` through a bridge called
    `ConstantAsMetadata` (one of the subclasses of `ValueAsMetadata`).

    As a side effect, accessing an operand of an `MDNode` that is known
    to be, e.g., `ConstantInt`, takes three steps: first, cast from
    `Metadata` to `ConstantAsMetadata`; second, extract the `Constant`;
    third, cast down to `ConstantInt`.

    The eventual goal is to introduce `MDInt`/`MDFloat`/etc. and have
    metadata schema owners transition away from using `Constant`s when
    the type isn't important (and they don't care about referring to
    `GlobalValue`s).

    In the meantime, I've added transitional API to the `mdconst`
    namespace that matches semantics with the old code, in order to
    avoid adding the error-prone three-step equivalent to every call
    site.  If your old code was:

        MDNode *N = foo();
        bar(isa             <ConstantInt>(N->getOperand(0)));
        baz(cast            <ConstantInt>(N->getOperand(1)));
        bak(cast_or_null    <ConstantInt>(N->getOperand(2)));
        bat(dyn_cast        <ConstantInt>(N->getOperand(3)));
        bay(dyn_cast_or_null<ConstantInt>(N->getOperand(4)));

    you can trivially match its semantics with:

        MDNode *N = foo();
        bar(mdconst::hasa               <ConstantInt>(N->getOperand(0)));
        baz(mdconst::extract            <ConstantInt>(N->getOperand(1)));
        bak(mdconst::extract_or_null    <ConstantInt>(N->getOperand(2)));
        bat(mdconst::dyn_extract        <ConstantInt>(N->getOperand(3)));
        bay(mdconst::dyn_extract_or_null<ConstantInt>(N->getOperand(4)));

    and when you transition your metadata schema to `MDInt`:

        MDNode *N = foo();
        bar(isa             <MDInt>(N->getOperand(0)));
        baz(cast            <MDInt>(N->getOperand(1)));
        bak(cast_or_null    <MDInt>(N->getOperand(2)));
        bat(dyn_cast        <MDInt>(N->getOperand(3)));
        bay(dyn_cast_or_null<MDInt>(N->getOperand(4)));

  - A `CallInst` -- specifically, intrinsic instructions -- can refer to
    metadata through a bridge called `MetadataAsValue`.  This is a
    subclass of `Value` where `getType()->isMetadataTy()`.

    `MetadataAsValue` is the *only* class that can legally refer to a
    `LocalAsMetadata`, which is a bridged form of non-`Constant` values
    like `Argument` and `Instruction`.  It can also refer to any other
    `Metadata` subclass.

(I'll break all your testcases in a follow-up commit, when I propagate
this change to assembly.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223802 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/Verifier.cpp

@@ -102,6 +102,13 @@ struct VerifierSupport {
     }
   }
 
+  void WriteMetadata(const Metadata *MD) {
+    if (!MD)
+      return;
+    MD->printAsOperand(OS, true, M);
+    OS << '\n';
+  }
+
   void WriteType(Type *T) {
     if (!T)
       return;
@@ -128,6 +135,24 @@ struct VerifierSupport {
     Broken = true;
   }
 
+  void CheckFailed(const Twine &Message, const Metadata *V1, const Metadata *V2,
+                   const Metadata *V3 = nullptr, const Metadata *V4 = nullptr) {
+    OS << Message.str() << "\n";
+    WriteMetadata(V1);
+    WriteMetadata(V2);
+    WriteMetadata(V3);
+    WriteMetadata(V4);
+    Broken = true;
+  }
+
+  void CheckFailed(const Twine &Message, const Metadata *V1,
+                   const Value *V2 = nullptr) {
+    OS << Message.str() << "\n";
+    WriteMetadata(V1);
+    WriteValue(V2);
+    Broken = true;
+  }
+
   void CheckFailed(const Twine &Message, const Value *V1, Type *T2,
                    const Value *V3 = nullptr) {
     OS << Message.str() << "\n";
@@ -167,7 +192,7 @@ class Verifier : public InstVisitor<Verifier>, VerifierSupport {
   SmallPtrSet<Instruction *, 16> InstsInThisBlock;
 
   /// \brief Keep track of the metadata nodes that have been checked already.
-  SmallPtrSet<MDNode *, 32> MDNodes;
+  SmallPtrSet<Metadata *, 32> MDNodes;
 
   /// \brief The personality function referenced by the LandingPadInsts.
   /// All LandingPadInsts within the same function must use the same
@@ -261,7 +286,9 @@ private:
   void visitAliaseeSubExpr(SmallPtrSetImpl<const GlobalAlias *> &Visited,
                            const GlobalAlias &A, const Constant &C);
   void visitNamedMDNode(const NamedMDNode &NMD);
-  void visitMDNode(MDNode &MD, Function *F);
+  void visitMDNode(MDNode &MD);
+  void visitMetadataAsValue(MetadataAsValue &MD, Function *F);
+  void visitValueAsMetadata(ValueAsMetadata &MD, Function *F);
   void visitComdat(const Comdat &C);
   void visitModuleIdents(const Module &M);
   void visitModuleFlags(const Module &M);
@@ -560,46 +587,77 @@ void Verifier::visitNamedMDNode(const NamedMDNode &NMD) {
     if (!MD)
       continue;
 
-    Assert1(!MD->isFunctionLocal(),
-            "Named metadata operand cannot be function local!", MD);
-    visitMDNode(*MD, nullptr);
+    visitMDNode(*MD);
   }
 }
 
-void Verifier::visitMDNode(MDNode &MD, Function *F) {
+void Verifier::visitMDNode(MDNode &MD) {
   // Only visit each node once.  Metadata can be mutually recursive, so this
   // avoids infinite recursion here, as well as being an optimization.
   if (!MDNodes.insert(&MD).second)
     return;
 
   for (unsigned i = 0, e = MD.getNumOperands(); i != e; ++i) {
-    Value *Op = MD.getOperand(i);
+    Metadata *Op = MD.getOperand(i);
     if (!Op)
       continue;
-    if (isa<Constant>(Op) || isa<MDString>(Op))
-      continue;
-    if (MDNode *N = dyn_cast<MDNode>(Op)) {
-      Assert2(MD.isFunctionLocal() || !N->isFunctionLocal(),
-              "Global metadata operand cannot be function local!", &MD, N);
-      visitMDNode(*N, F);
+    Assert2(!isa<LocalAsMetadata>(Op), "Invalid operand for global metadata!",
+            &MD, Op);
+    if (auto *N = dyn_cast<MDNode>(Op)) {
+      visitMDNode(*N);
+      continue;
+    }
+    if (auto *V = dyn_cast<ValueAsMetadata>(Op)) {
+      visitValueAsMetadata(*V, nullptr);
       continue;
     }
-    Assert2(MD.isFunctionLocal(), "Invalid operand for global metadata!", &MD, Op);
-
-    // If this was an instruction, bb, or argument, verify that it is in the
-    // function that we expect.
-    Function *ActualF = nullptr;
-    if (Instruction *I = dyn_cast<Instruction>(Op))
-      ActualF = I->getParent()->getParent();
-    else if (BasicBlock *BB = dyn_cast<BasicBlock>(Op))
-      ActualF = BB->getParent();
-    else if (Argument *A = dyn_cast<Argument>(Op))
-      ActualF = A->getParent();
-    assert(ActualF && "Unimplemented function local metadata case!");
-
-    Assert2(ActualF == F, "function-local metadata used in wrong function",
-            &MD, Op);
   }
+
+  // Check these last, so we diagnose problems in operands first.
+  Assert1(!isa<MDNodeFwdDecl>(MD), "Expected no forward declarations!", &MD);
+  Assert1(MD.isResolved(), "All nodes should be resolved!", &MD);
+}
+
+void Verifier::visitValueAsMetadata(ValueAsMetadata &MD, Function *F) {
+  Assert1(MD.getValue(), "Expected valid value", &MD);
+  Assert2(!MD.getValue()->getType()->isMetadataTy(),
+          "Unexpected metadata round-trip through values", &MD, MD.getValue());
+
+  auto *L = dyn_cast<LocalAsMetadata>(&MD);
+  if (!L)
+    return;
+
+  Assert1(F, "function-local metadata used outside a function", L);
+
+  // If this was an instruction, bb, or argument, verify that it is in the
+  // function that we expect.
+  Function *ActualF = nullptr;
+  if (Instruction *I = dyn_cast<Instruction>(L->getValue())) {
+    Assert2(I->getParent(), "function-local metadata not in basic block", L, I);
+    ActualF = I->getParent()->getParent();
+  } else if (BasicBlock *BB = dyn_cast<BasicBlock>(L->getValue()))
+    ActualF = BB->getParent();
+  else if (Argument *A = dyn_cast<Argument>(L->getValue()))
+    ActualF = A->getParent();
+  assert(ActualF && "Unimplemented function local metadata case!");
+
+  Assert1(ActualF == F, "function-local metadata used in wrong function", L);
+}
+
+void Verifier::visitMetadataAsValue(MetadataAsValue &MDV, Function *F) {
+  Metadata *MD = MDV.getMetadata();
+  if (auto *N = dyn_cast<MDNode>(MD)) {
+    visitMDNode(*N);
+    return;
+  }
+
+  // Only visit each node once.  Metadata can be mutually recursive, so this
+  // avoids infinite recursion here, as well as being an optimization.
+  if (!MDNodes.insert(MD).second)
+    return;
+
+  if (auto *V = dyn_cast<ValueAsMetadata>(MD))
+    visitValueAsMetadata(*V, F);
 }
 
 void Verifier::visitComdat(const Comdat &C) {
@@ -650,7 +708,7 @@ void Verifier::visitModuleFlags(const Module &M) {
   for (unsigned I = 0, E = Requirements.size(); I != E; ++I) {
     const MDNode *Requirement = Requirements[I];
     const MDString *Flag = cast<MDString>(Requirement->getOperand(0));
-    const Value *ReqValue = Requirement->getOperand(1);
+    const Metadata *ReqValue = Requirement->getOperand(1);
 
     const MDNode *Op = SeenIDs.lookup(Flag);
     if (!Op) {
@@ -679,7 +737,7 @@ Verifier::visitModuleFlag(const MDNode *Op,
   Module::ModFlagBehavior MFB;
   if (!Module::isValidModFlagBehavior(Op->getOperand(0), MFB)) {
     Assert1(
-        dyn_cast<ConstantInt>(Op->getOperand(0)),
+        mdconst::dyn_extract<ConstantInt>(Op->getOperand(0)),
         "invalid behavior operand in module flag (expected constant integer)",
         Op->getOperand(0));
     Assert1(false,
@@ -1907,9 +1965,11 @@ void Verifier::visitRangeMetadata(Instruction& I,
   
   ConstantRange LastRange(1); // Dummy initial value
   for (unsigned i = 0; i < NumRanges; ++i) {
-    ConstantInt *Low = dyn_cast<ConstantInt>(Range->getOperand(2*i));
+    ConstantInt *Low =
+        mdconst::dyn_extract<ConstantInt>(Range->getOperand(2 * i));
     Assert1(Low, "The lower limit must be an integer!", Low);
-    ConstantInt *High = dyn_cast<ConstantInt>(Range->getOperand(2*i + 1));
+    ConstantInt *High =
+        mdconst::dyn_extract<ConstantInt>(Range->getOperand(2 * i + 1));
     Assert1(High, "The upper limit must be an integer!", High);
     Assert1(High->getType() == Low->getType() &&
             High->getType() == Ty, "Range types must match instruction type!",
@@ -1932,9 +1992,9 @@ void Verifier::visitRangeMetadata(Instruction& I,
   }
   if (NumRanges > 2) {
     APInt FirstLow =
-      dyn_cast<ConstantInt>(Range->getOperand(0))->getValue();
+        mdconst::dyn_extract<ConstantInt>(Range->getOperand(0))->getValue();
     APInt FirstHigh =
-      dyn_cast<ConstantInt>(Range->getOperand(1))->getValue();
+        mdconst::dyn_extract<ConstantInt>(Range->getOperand(1))->getValue();
     ConstantRange FirstRange(FirstLow, FirstHigh);
     Assert1(FirstRange.intersectWith(LastRange).isEmptySet(),
             "Intervals are overlapping", Range);
@@ -2278,8 +2338,8 @@ void Verifier::visitInstruction(Instruction &I) {
     Assert1(I.getType()->isFPOrFPVectorTy(),
             "fpmath requires a floating point result!", &I);
     Assert1(MD->getNumOperands() == 1, "fpmath takes one operand!", &I);
-    Value *Op0 = MD->getOperand(0);
-    if (ConstantFP *CFP0 = dyn_cast_or_null<ConstantFP>(Op0)) {
+    if (ConstantFP *CFP0 =
+            mdconst::dyn_extract_or_null<ConstantFP>(MD->getOperand(0))) {
       APFloat Accuracy = CFP0->getValueAPF();
       Assert1(Accuracy.isFiniteNonZero() && !Accuracy.isNegative(),
               "fpmath accuracy not a positive number!", &I);
@@ -2496,8 +2556,8 @@ void Verifier::visitIntrinsicFunctionCall(Intrinsic::ID ID, CallInst &CI) {
   // If the intrinsic takes MDNode arguments, verify that they are either global
   // or are local to *this* function.
   for (unsigned i = 0, e = CI.getNumArgOperands(); i != e; ++i)
-    if (MDNode *MD = dyn_cast<MDNode>(CI.getArgOperand(i)))
-      visitMDNode(*MD, CI.getParent()->getParent());
+    if (auto *MD = dyn_cast<MetadataAsValue>(CI.getArgOperand(i)))
+      visitMetadataAsValue(*MD, CI.getParent()->getParent());
 
   switch (ID) {
   default:
@@ -2509,11 +2569,8 @@ void Verifier::visitIntrinsicFunctionCall(Intrinsic::ID ID, CallInst &CI) {
             "constant int", &CI);
     break;
   case Intrinsic::dbg_declare: {  // llvm.dbg.declare
-    Assert1(CI.getArgOperand(0) && isa<MDNode>(CI.getArgOperand(0)),
-                "invalid llvm.dbg.declare intrinsic call 1", &CI);
-    MDNode *MD = cast<MDNode>(CI.getArgOperand(0));
-    Assert1(MD->getNumOperands() == 1,
-                "invalid llvm.dbg.declare intrinsic call 2", &CI);
+    Assert1(CI.getArgOperand(0) && isa<MetadataAsValue>(CI.getArgOperand(0)),
+            "invalid llvm.dbg.declare intrinsic call 1", &CI);
   } break;
   case Intrinsic::memcpy:
   case Intrinsic::memmove: