Implement initial support for function attributes, including parser, printer,
resolver support.

Still TODO are verifier support (to make sure you don't use an attribute for a
function in another module) and the TODO in ModuleParser::finalizeModule that I
will handle in the next patch.

PiperOrigin-RevId: 209361648
diff --git a/lib/IR/AsmPrinter.cpp b/lib/IR/AsmPrinter.cpp
index 896b76e..8303f57 100644
--- a/lib/IR/AsmPrinter.cpp
+++ b/lib/IR/AsmPrinter.cpp
@@ -415,6 +415,16 @@
   case Attribute::Kind::Type:
     printType(cast<TypeAttr>(attr)->getValue());
     break;
+  case Attribute::Kind::Function: {
+    auto *function = cast<FunctionAttr>(attr)->getValue();
+    if (!function) {
+      os << "<<FUNCTION ATTR FOR DELETED FUNCTION>>";
+    } else {
+      os << '@' << function->getName() << " : ";
+      printType(function->getType());
+    }
+    break;
+  }
   }
 }
 
@@ -784,6 +794,11 @@
         }
       } else if (auto intOp = op->getAs<ConstantAffineIntOp>()) {
         specialName << 'c' << intOp->getValue();
+      } else if (auto constant = op->getAs<ConstantOp>()) {
+        if (isa<FunctionAttr>(constant->getValue()))
+          specialName << 'f';
+        else
+          specialName << "cst";
       }
     }
 
@@ -909,7 +924,7 @@
   // Filter out any attributes that shouldn't be included.
   SmallVector<NamedAttribute, 8> filteredAttrs;
   for (auto attr : attrs) {
-    auto attrName = attr.first.ref();
+    auto attrName = attr.first.strref();
     // Never print attributes that start with a colon.  These are internal
     // attributes that represent location or other internal metadata.
     if (attrName.startswith(":"))
@@ -946,7 +961,7 @@
 
   // Check to see if this is a known operation.  If so, use the registered
   // custom printer hook.
-  if (auto *opInfo = state.operationSet->lookup(op->getName().ref())) {
+  if (auto *opInfo = state.operationSet->lookup(op->getName())) {
     opInfo->printAssembly(op, this);
     return;
   }
@@ -957,7 +972,7 @@
 
 void FunctionPrinter::printDefaultOp(const Operation *op) {
   os << '"';
-  printEscapedString(op->getName().ref(), os);
+  printEscapedString(op->getName(), os);
   os << "\"(";
 
   interleaveComma(op->getOperands(),
diff --git a/lib/IR/Builders.cpp b/lib/IR/Builders.cpp
index be334ef..5fce94c 100644
--- a/lib/IR/Builders.cpp
+++ b/lib/IR/Builders.cpp
@@ -111,6 +111,10 @@
   return TypeAttr::get(type, context);
 }
 
+FunctionAttr *Builder::getFunctionAttr(Function *value) {
+  return FunctionAttr::get(value, context);
+}
+
 //===----------------------------------------------------------------------===//
 // Affine Expressions, Affine Maps, and Integet Sets.
 //===----------------------------------------------------------------------===//
diff --git a/lib/IR/Function.cpp b/lib/IR/Function.cpp
index df3d7c8..bfcfd6f 100644
--- a/lib/IR/Function.cpp
+++ b/lib/IR/Function.cpp
@@ -15,6 +15,7 @@
 // limitations under the License.
 // =============================================================================
 
+#include "mlir/IR/Attributes.h"
 #include "mlir/IR/CFGFunction.h"
 #include "mlir/IR/MLFunction.h"
 #include "mlir/IR/Module.h"
@@ -27,6 +28,11 @@
 Function::Function(StringRef name, FunctionType *type, Kind kind)
     : kind(kind), name(Identifier::get(name, type->getContext())), type(type) {}
 
+Function::~Function() {
+  // Clean up function attributes referring to this function.
+  FunctionAttr::dropFunctionReference(this);
+}
+
 MLIRContext *Function::getContext() const { return getType()->getContext(); }
 
 /// Delete this object.
diff --git a/lib/IR/MLIRContext.cpp b/lib/IR/MLIRContext.cpp
index 644a74a..a960223 100644
--- a/lib/IR/MLIRContext.cpp
+++ b/lib/IR/MLIRContext.cpp
@@ -20,6 +20,7 @@
 #include "mlir/IR/AffineExpr.h"
 #include "mlir/IR/AffineMap.h"
 #include "mlir/IR/Attributes.h"
+#include "mlir/IR/Function.h"
 #include "mlir/IR/Identifier.h"
 #include "mlir/IR/IntegerSet.h"
 #include "mlir/IR/OperationSet.h"
@@ -254,6 +255,7 @@
   using AttributeListSet =
       DenseSet<AttributeListStorage *, AttributeListKeyInfo>;
   AttributeListSet attributeLists;
+  DenseMap<Function *, FunctionAttr *> functionAttrs;
 
 public:
   MLIRContextImpl() : identifiers(allocator) {
@@ -633,6 +635,34 @@
   return result;
 }
 
+FunctionAttr *FunctionAttr::get(Function *value, MLIRContext *context) {
+  auto *&result = context->getImpl().functionAttrs[value];
+  if (result)
+    return result;
+
+  result = context->getImpl().allocator.Allocate<FunctionAttr>();
+  new (result) FunctionAttr(value);
+  return result;
+}
+
+/// This function is used by the internals of the Function class to null out
+/// attributes refering to functions that are about to be deleted.
+void FunctionAttr::dropFunctionReference(Function *value) {
+  // Check to see if there was an attribute referring to this function.
+  auto &functionAttrs = value->getContext()->getImpl().functionAttrs;
+
+  // If not, then we're done.
+  auto it = functionAttrs.find(value);
+  if (it == functionAttrs.end())
+    return;
+
+  // If so, null out the function reference in the attribute (to avoid dangling
+  // pointers) and remove the entry from the map so the map doesn't contain
+  // dangling keys.
+  it->second->value = nullptr;
+  functionAttrs.erase(it);
+}
+
 /// Perform a three-way comparison between the names of the specified
 /// NamedAttributes.
 static int compareNamedAttributes(const NamedAttribute *lhs,
diff --git a/lib/IR/Module.cpp b/lib/IR/Module.cpp
index 5a33d26..36b892a 100644
--- a/lib/IR/Module.cpp
+++ b/lib/IR/Module.cpp
@@ -21,13 +21,13 @@
 Module::Module(MLIRContext *context) : context(context) {}
 
 /// Look up a function with the specified name, returning null if no such
-/// name exists.
+/// name exists.  Function names never include the @ on them.
 Function *Module::getNamedFunction(StringRef name) {
   return getNamedFunction(Identifier::get(name, context));
 }
 
 /// Look up a function with the specified name, returning null if no such
-/// name exists.
+/// name exists.  Function names never include the @ on them.
 Function *Module::getNamedFunction(Identifier name) {
   auto it = symbolTable.find(name);
   return it != symbolTable.end() ? it->second : nullptr;
diff --git a/lib/IR/StandardOps.cpp b/lib/IR/StandardOps.cpp
index 723dcc2..6d8c366 100644
--- a/lib/IR/StandardOps.cpp
+++ b/lib/IR/StandardOps.cpp
@@ -204,16 +204,25 @@
 void ConstantOp::print(OpAsmPrinter *p) const {
   *p << "constant " << *getValue();
   p->printOptionalAttrDict(getAttrs(), /*elidedAttrs=*/"value");
-  *p << " : " << *getType();
+
+  if (!isa<FunctionAttr>(getValue()))
+    *p << " : " << *getType();
 }
 
 bool ConstantOp::parse(OpAsmParser *parser, OperationState *result) {
   Attribute *valueAttr;
   Type *type;
 
-  return parser->parseAttribute(valueAttr, "value", result->attributes) ||
-         parser->parseOptionalAttributeDict(result->attributes) ||
-         parser->parseColonType(type) ||
+  if (parser->parseAttribute(valueAttr, "value", result->attributes) ||
+      parser->parseOptionalAttributeDict(result->attributes))
+    return true;
+
+  // 'constant' taking a function reference doesn't get a redundant type
+  // specifier.  The attribute itself carries it.
+  if (auto *fnAttr = dyn_cast<FunctionAttr>(valueAttr))
+    return parser->addTypeToList(fnAttr->getValue()->getType(), result->types);
+
+  return parser->parseColonType(type) ||
          parser->addTypeToList(type, result->types);
 }
 
@@ -244,7 +253,9 @@
   }
 
   if (isa<FunctionType>(type)) {
-    // TODO: Verify a function attr.
+    if (!isa<FunctionAttr>(value))
+      return "requires 'value' to be a function reference";
+    return nullptr;
   }
 
   return "requires a result type that aligns with the 'value' attribute";