[MLIR][LLVM] Implement LLVM dialect support for global aliases

This includes support for module translation and module import and add tests for both.

ClangIR cannot currently lower global aliases to LLVM because of missing
support for this.

Author: bcardosolopes

mlir/include/mlir/Dialect/LLVMIR/LLVMOps.td

@@ -1443,6 +1443,92 @@ def LLVM_GlobalDtorsOp : LLVM_Op<"mlir.global_dtors", [
   let hasVerifier = 1;
 }
 
+def LLVM_AliasOp : LLVM_Op<"mlir.alias",
+    [IsolatedFromAbove, SingleBlockImplicitTerminator<"ReturnOp">, Symbol]> {
+  let arguments = (ins
+    TypeAttr:$alias_type,
+    StrAttr:$sym_name,
+    Linkage:$linkage,
+    UnitAttr:$dso_local,
+    UnitAttr:$thread_local_,
+    UnitAttr:$externally_initialized,
+    DefaultValuedAttr<ConfinedAttr<I32Attr, [IntNonNegative]>, "0">:$addr_space,
+    OptionalAttr<UnnamedAddr>:$unnamed_addr,
+    OptionalAttr<StrAttr>:$section,
+    OptionalAttr<SymbolRefAttr>:$comdat,
+    DefaultValuedAttr<Visibility, "mlir::LLVM::Visibility::Default">:$visibility_
+  );
+  let summary = "LLVM dialect alias.";
+  let description = [{
+    `llvm.mlir.alias` is a top level operation that defines a global alias for
+    global variables and functions. The operation is always initialized by
+    using a initializer region which could be a direct map to another global
+    value or contain some address computation on top of it.
+
+    It uses an @-identifier for its value, which will be uniqued by the module
+    with respect to other @-identifiers in it.
+
+    Similarly to functions and globals, they can also have a linkage attribute.
+    This attribute is placed between `llvm.mlir.alias` and the symbol name. If
+    the attribute is omitted, `external` linkage is assumed by default.
+
+    Examples:
+
+    ```mlir
+    // Global alias use @-identifiers.
+    llvm.mlir.alias external @foo_alias {addr_space = 0 : i32} : !llvm.ptr {
+      %0 = llvm.mlir.addressof @some_function : !llvm.ptr
+      llvm.return %0 : !llvm.ptr
+    }
+
+    // More complex initialization.
+    llvm.mlir.alias linkonce_odr hidden @glob
+    {addr_space = 0 : i32, dso_local} : !llvm.array<32 x i32> {
+      %0 = llvm.mlir.constant(1234 : i64) : i64
+      %1 = llvm.mlir.addressof @glob.private : !llvm.ptr
+      %2 = llvm.ptrtoint %1 : !llvm.ptr to i64
+      %3 = llvm.add %2, %0 : i64
+      %4 = llvm.inttoptr %3 : i64 to !llvm.ptr
+      llvm.return %4 : !llvm.ptr
+    }
+    ```
+  }];
+  let regions = (region AnyRegion:$initializer);
+
+  let builders = [
+    OpBuilder<(ins "Type":$type, "Linkage":$linkage,
+      "StringRef":$name,
+      CArg<"unsigned", "0">:$addrSpace,
+      CArg<"bool", "false">:$dsoLocal,
+      CArg<"bool", "false">:$thread_local_,
+      CArg<"SymbolRefAttr", "{}">:$comdat,
+      CArg<"ArrayRef<NamedAttribute>", "{}">:$attrs,
+      CArg<"ArrayRef<Attribute>", "{}">:$dbgExprs)>
+  ];
+
+  let extraClassDeclaration = [{
+    /// Return the LLVM type of the global alias.
+    Type getType() {
+      return getAliasType();
+    }
+    /// Return the initializer region. This may be empty, but if it is not it
+    /// terminates in an `llvm.return` op with the initializer value.
+    Region &getInitializerRegion() {
+      return getOperation()->getRegion(0);
+    }
+    /// Return the initializer block. The initializer region always exist
+    /// (differently from llvm.global) and it terminates with an `llvm.return`
+    /// op with the aliasee value.
+    Block *getInitializerBlock() {
+      return &getInitializerRegion().front();
+    }
+  }];
+
+  let hasCustomAssemblyFormat = 1;
+  let hasVerifier = 1;
+  let hasRegionVerifier = 1;
+}
+
 def LLVM_ComdatSelectorOp : LLVM_Op<"comdat_selector", [Symbol]> {
   let arguments = (ins
     SymbolNameAttr:$sym_name,

mlir/include/mlir/Target/LLVMIR/ModuleImport.h

@@ -67,6 +67,9 @@ class ModuleImport {
   /// Converts all global variables of the LLVM module to MLIR global variables.
   LogicalResult convertGlobals();
 
+  /// Converts all aliases of the LLVM module to MLIR variables.
+  LogicalResult convertAliases();
+
   /// Converts the data layout of the LLVM module to an MLIR data layout
   /// specification.
   LogicalResult convertDataLayout();
@@ -288,6 +291,9 @@ class ModuleImport {
   LogicalResult convertGlobal(llvm::GlobalVariable *globalVar);
   /// Imports the magic globals "global_ctors" and "global_dtors".
   LogicalResult convertGlobalCtorsAndDtors(llvm::GlobalVariable *globalVar);
+  /// Converts an LLVM global alias variable into an MLIR LLVM dialect alias
+  /// operation if a conversion exists. Otherwise, returns failure.
+  LogicalResult convertAlias(llvm::GlobalAlias *alias);
   /// Returns personality of `func` as a FlatSymbolRefAttr.
   FlatSymbolRefAttr getPersonalityAsAttr(llvm::Function *func);
   /// Imports `bb` into `block`, which must be initially empty.
@@ -406,6 +412,8 @@ class ModuleImport {
   Operation *constantInsertionOp = nullptr;
   /// Operation to insert the next global after.
   Operation *globalInsertionOp = nullptr;
+  /// Operation to insert the next alias after.
+  Operation *aliasInsertionOp = nullptr;
   /// Operation to insert comdat selector operations into.
   ComdatOp globalComdatOp = nullptr;
   /// The current context.

mlir/include/mlir/Target/LLVMIR/ModuleTranslation.h

@@ -188,6 +188,12 @@ class ModuleTranslation {
     return globalsMapping.lookup(op);
   }
 
+  /// Finds an LLVM IR global value that corresponds to the given MLIR operation
+  /// defining a global alias value.
+  llvm::GlobalValue *lookupAlias(Operation *op) {
+    return aliasesMapping.lookup(op);
+  }
+
   /// Returns the OpenMP IR builder associated with the LLVM IR module being
   /// constructed.
   llvm::OpenMPIRBuilder *getOpenMPBuilder();
@@ -322,6 +328,7 @@ class ModuleTranslation {
   LogicalResult convertFunctions();
   LogicalResult convertComdats();
   LogicalResult convertGlobals();
+  LogicalResult convertAliases();
   LogicalResult convertOneFunction(LLVMFuncOp func);
   LogicalResult convertBlockImpl(Block &bb, bool ignoreArguments,
                                  llvm::IRBuilderBase &builder,
@@ -366,6 +373,10 @@ class ModuleTranslation {
   /// Mappings between llvm.mlir.global definitions and corresponding globals.
   DenseMap<Operation *, llvm::GlobalValue *> globalsMapping;
 
+  /// Mappings between llvm.mlir.alias definitions and corresponding global
+  /// aliases.
+  DenseMap<Operation *, llvm::GlobalValue *> aliasesMapping;
+
   /// A stateful object used to translate types.
   TypeToLLVMIRTranslator typeTranslator;
 

mlir/lib/Dialect/LLVMIR/IR/LLVMDialect.cpp

@@ -2422,6 +2422,178 @@ LogicalResult GlobalDtorsOp::verify() {
   return success();
 }
 
+//===----------------------------------------------------------------------===//
+// Builder, printer and verifier for LLVM::GlobalOp.
+//===----------------------------------------------------------------------===//
+
+void AliasOp::build(OpBuilder &builder, OperationState &result, Type type,
+                    Linkage linkage, StringRef name, unsigned addrSpace,
+                    bool dsoLocal, bool threadLocal, SymbolRefAttr comdat,
+                    ArrayRef<NamedAttribute> attrs,
+                    ArrayRef<Attribute> dbgExprs) {
+  result.addAttribute(getSymNameAttrName(result.name),
+                      builder.getStringAttr(name));
+  result.addAttribute(getAliasTypeAttrName(result.name), TypeAttr::get(type));
+  if (dsoLocal)
+    result.addAttribute(getDsoLocalAttrName(result.name),
+                        builder.getUnitAttr());
+  if (threadLocal)
+    result.addAttribute(getThreadLocal_AttrName(result.name),
+                        builder.getUnitAttr());
+  if (comdat)
+    result.addAttribute(getComdatAttrName(result.name), comdat);
+
+  result.addAttribute(getLinkageAttrName(result.name),
+                      LinkageAttr::get(builder.getContext(), linkage));
+  if (addrSpace != 0)
+    result.addAttribute(getAddrSpaceAttrName(result.name),
+                        builder.getI32IntegerAttr(addrSpace));
+  result.attributes.append(attrs.begin(), attrs.end());
+
+  result.addRegion();
+}
+
+void AliasOp::print(OpAsmPrinter &p) {
+  p << ' ' << stringifyLinkage(getLinkage()) << ' ';
+  StringRef visibility = stringifyVisibility(getVisibility_());
+  if (!visibility.empty())
+    p << visibility << ' ';
+  if (getThreadLocal_())
+    p << "thread_local ";
+  if (auto unnamedAddr = getUnnamedAddr()) {
+    StringRef str = stringifyUnnamedAddr(*unnamedAddr);
+    if (!str.empty())
+      p << str << ' ';
+  }
+  p.printSymbolName(getSymName());
+  if (auto comdat = getComdat())
+    p << " comdat(" << *comdat << ')';
+
+  // Note that the alignment attribute is printed using the
+  // default syntax here, even though it is an inherent attribute
+  // (as defined in https://mlir.llvm.org/docs/LangRef/#attributes)
+  p.printOptionalAttrDict((*this)->getAttrs(),
+                          {SymbolTable::getSymbolAttrName(),
+                           getAliasTypeAttrName(), getLinkageAttrName(),
+                           getUnnamedAddrAttrName(), getThreadLocal_AttrName(),
+                           getVisibility_AttrName(), getComdatAttrName(),
+                           getUnnamedAddrAttrName()});
+
+  // Print the trailing type
+  p << " : " << getType();
+
+  Region &initializer = getInitializerRegion();
+  if (!initializer.empty()) {
+    p << ' ';
+    p.printRegion(initializer, /*printEntryBlockArgs=*/false);
+  }
+}
+
+// operation ::= `llvm.mlir.alias` linkage? visibility?
+//               (`unnamed_addr` | `local_unnamed_addr`)?
+//               `thread_local`? `@` identifier
+//               `(` attribute? `)` (`comdat(` symbol-ref-id `)`)?
+//               attribute-list? (`:` type)? region
+//
+// The type can be omitted for string attributes, in which case it will be
+// inferred from the value of the string as [strlen(value) x i8].
+ParseResult AliasOp::parse(OpAsmParser &parser, OperationState &result) {
+  MLIRContext *ctx = parser.getContext();
+  // Parse optional linkage, default to External.
+  result.addAttribute(getLinkageAttrName(result.name),
+                      LLVM::LinkageAttr::get(
+                          ctx, parseOptionalLLVMKeyword<Linkage>(
+                                   parser, result, LLVM::Linkage::External)));
+
+  // Parse optional visibility, default to Default.
+  result.addAttribute(getVisibility_AttrName(result.name),
+                      parser.getBuilder().getI64IntegerAttr(
+                          parseOptionalLLVMKeyword<LLVM::Visibility, int64_t>(
+                              parser, result, LLVM::Visibility::Default)));
+
+  // Parse optional UnnamedAddr, default to None.
+  result.addAttribute(getUnnamedAddrAttrName(result.name),
+                      parser.getBuilder().getI64IntegerAttr(
+                          parseOptionalLLVMKeyword<UnnamedAddr, int64_t>(
+                              parser, result, LLVM::UnnamedAddr::None)));
+
+  if (succeeded(parser.parseOptionalKeyword("thread_local")))
+    result.addAttribute(getThreadLocal_AttrName(result.name),
+                        parser.getBuilder().getUnitAttr());
+
+  StringAttr name;
+  if (parser.parseSymbolName(name, getSymNameAttrName(result.name),
+                             result.attributes))
+    return failure();
+
+  if (succeeded(parser.parseOptionalKeyword("comdat"))) {
+    SymbolRefAttr comdat;
+    if (parser.parseLParen() || parser.parseAttribute(comdat) ||
+        parser.parseRParen())
+      return failure();
+
+    result.addAttribute(getComdatAttrName(result.name), comdat);
+  }
+
+  SmallVector<Type, 1> types;
+  if (parser.parseOptionalAttrDict(result.attributes) ||
+      parser.parseOptionalColonTypeList(types))
+    return failure();
+
+  if (types.size() > 1)
+    return parser.emitError(parser.getNameLoc(), "expected zero or one type");
+
+  Region &initRegion = *result.addRegion();
+  if (parser.parseRegion(initRegion).failed())
+    return failure();
+
+  result.addAttribute(getAliasTypeAttrName(result.name),
+                      TypeAttr::get(types[0]));
+  return success();
+}
+
+LogicalResult AliasOp::verify() {
+  bool validType = isCompatibleOuterType(getType())
+                       ? !llvm::isa<LLVMVoidType, LLVMTokenType,
+                                    LLVMMetadataType, LLVMLabelType>(getType())
+                       : llvm::isa<PointerElementTypeInterface>(getType());
+  if (!validType)
+    return emitOpError(
+        "expects type to be a valid element type for an LLVM global");
+  if ((*this)->getParentOp() && !satisfiesLLVMModule((*this)->getParentOp()))
+    return emitOpError("must appear at the module level");
+
+  if (getLinkage() == Linkage::Appending) {
+    if (!llvm::isa<LLVMArrayType>(getType())) {
+      return emitOpError() << "expected array type for '"
+                           << stringifyLinkage(Linkage::Appending)
+                           << "' linkage";
+    }
+  }
+
+  if (failed(verifyComdat(*this, getComdat())))
+    return failure();
+
+  return success();
+}
+
+LogicalResult AliasOp::verifyRegions() {
+  if (Block *b = getInitializerBlock()) {
+    ReturnOp ret = cast<ReturnOp>(b->getTerminator());
+    if (ret.operand_type_begin() == ret.operand_type_end())
+      return emitOpError("initializer region cannot return void");
+
+    for (Operation &op : *b) {
+      auto iface = dyn_cast<MemoryEffectOpInterface>(op);
+      if (!iface || !iface.hasNoEffect())
+        return op.emitError()
+               << "ops with side effects not allowed in aliases initializers";
+    }
+  }
+
+  return success();
+}
+
 //===----------------------------------------------------------------------===//
 // ShuffleVectorOp
 //===----------------------------------------------------------------------===//