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.)

llvm-svn: 223802

1 files changed, 18 insertions, 13 deletions

diff --git a/llvm/lib/IR/Module.cpp b/llvm/lib/IR/Module.cpp
index 065c66e..0781ea4 100644
--- a/llvm/lib/IR/Module.cpp
+++ b/llvm/lib/IR/Module.cpp
@@ -260,8 +260,8 @@ void Module::eraseNamedMetadata(NamedMDNode *NMD) {
   NamedMDList.erase(NMD);
 }
 
-bool Module::isValidModFlagBehavior(Value *V, ModFlagBehavior &MFB) {
-  if (ConstantInt *Behavior = dyn_cast<ConstantInt>(V)) {
+bool Module::isValidModFlagBehavior(Metadata *MD, ModFlagBehavior &MFB) {
+  if (ConstantInt *Behavior = mdconst::dyn_extract<ConstantInt>(MD)) {
     uint64_t Val = Behavior->getLimitedValue();
     if (Val >= ModFlagBehaviorFirstVal && Val <= ModFlagBehaviorLastVal) {
       MFB = static_cast<ModFlagBehavior>(Val);
@@ -285,7 +285,7 @@ getModuleFlagsMetadata(SmallVectorImpl<ModuleFlagEntry> &Flags) const {
       // Check the operands of the MDNode before accessing the operands.
       // The verifier will actually catch these failures.
       MDString *Key = cast<MDString>(Flag->getOperand(1));
-      Value *Val = Flag->getOperand(2);
+      Metadata *Val = Flag->getOperand(2);
       Flags.push_back(ModuleFlagEntry(MFB, Key, Val));
     }
   }
@@ -293,7 +293,7 @@ getModuleFlagsMetadata(SmallVectorImpl<ModuleFlagEntry> &Flags) const {
 
 /// Return the corresponding value if Key appears in module flags, otherwise
 /// return null.
-Value *Module::getModuleFlag(StringRef Key) const {
+Metadata *Module::getModuleFlag(StringRef Key) const {
   SmallVector<Module::ModuleFlagEntry, 8> ModuleFlags;
   getModuleFlagsMetadata(ModuleFlags);
   for (const ModuleFlagEntry &MFE : ModuleFlags) {
@@ -321,14 +321,18 @@ NamedMDNode *Module::getOrInsertModuleFlagsMetadata() {
 /// metadata. It will create the module-level flags named metadata if it doesn't
 /// already exist.
 void Module::addModuleFlag(ModFlagBehavior Behavior, StringRef Key,
-                           Value *Val) {
+                           Metadata *Val) {
   Type *Int32Ty = Type::getInt32Ty(Context);
-  Value *Ops[3] = {
-    ConstantInt::get(Int32Ty, Behavior), MDString::get(Context, Key), Val
-  };
+  Metadata *Ops[3] = {
+      ConstantAsMetadata::get(ConstantInt::get(Int32Ty, Behavior)),
+      MDString::get(Context, Key), Val};
   getOrInsertModuleFlagsMetadata()->addOperand(MDNode::get(Context, Ops));
 }
 void Module::addModuleFlag(ModFlagBehavior Behavior, StringRef Key,
+                           Constant *Val) {
+  addModuleFlag(Behavior, Key, ConstantAsMetadata::get(Val));
+}
+void Module::addModuleFlag(ModFlagBehavior Behavior, StringRef Key,
                            uint32_t Val) {
   Type *Int32Ty = Type::getInt32Ty(Context);
   addModuleFlag(Behavior, Key, ConstantInt::get(Int32Ty, Val));
@@ -336,7 +340,7 @@ void Module::addModuleFlag(ModFlagBehavior Behavior, StringRef Key,
 void Module::addModuleFlag(MDNode *Node) {
   assert(Node->getNumOperands() == 3 &&
          "Invalid number of operands for module flag!");
-  assert(isa<ConstantInt>(Node->getOperand(0)) &&
+  assert(mdconst::hasa<ConstantInt>(Node->getOperand(0)) &&
          isa<MDString>(Node->getOperand(1)) &&
          "Invalid operand types for module flag!");
   getOrInsertModuleFlagsMetadata()->addOperand(Node);
@@ -459,10 +463,10 @@ void Module::dropAllReferences() {
 }
 
 unsigned Module::getDwarfVersion() const {
-  Value *Val = getModuleFlag("Dwarf Version");
+  auto *Val = cast_or_null<ConstantAsMetadata>(getModuleFlag("Dwarf Version"));
   if (!Val)
     return dwarf::DWARF_VERSION;
-  return cast<ConstantInt>(Val)->getZExtValue();
+  return cast<ConstantInt>(Val->getValue())->getZExtValue();
 }
 
 Comdat *Module::getOrInsertComdat(StringRef Name) {
@@ -472,12 +476,13 @@ Comdat *Module::getOrInsertComdat(StringRef Name) {
 }
 
 PICLevel::Level Module::getPICLevel() const {
-  Value *Val = getModuleFlag("PIC Level");
+  auto *Val = cast_or_null<ConstantAsMetadata>(getModuleFlag("PIC Level"));
 
   if (Val == NULL)
     return PICLevel::Default;
 
-  return static_cast<PICLevel::Level>(cast<ConstantInt>(Val)->getZExtValue());
+  return static_cast<PICLevel::Level>(
+      cast<ConstantInt>(Val->getValue())->getZExtValue());
 }
 
 void Module::setPICLevel(PICLevel::Level PL) {