[mlir] Support llvm.readnone attribute for all FunctionOpInterface ops.

The attribute is translated into LLVM's function attribute 'readnone'.
There is no explicit verification regarding conflicting 'readnone'
and function attributes from 'passthrough', though, LLVM would assert
if they are incompatible during LLVM IR creation.

Differential Revision: https://reviews.llvm.org/D131457

mlir/include/mlir/Dialect/LLVMIR/LLVMOpBase.td

@@ -66,6 +66,12 @@ def LLVM_Dialect : Dialect {
     /// Returns `true` if the given type is compatible with the LLVM dialect.
     static bool isCompatibleType(Type);
 
+    /// Name of the attribute mapped to LLVM's 'readnone' function attribute.
+    /// It is allowed on any FunctionOpInterface operations.
+    static StringRef getReadnoneAttrName() {
+      return "llvm.readnone";
+    }
+
   private:
     /// A cache storing compatible LLVM types that have been verified. This
     /// can save us lots of verification time if there are many occurrences

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

@@ -2683,6 +2683,17 @@ LogicalResult LLVMDialect::verifyOperationAttribute(Operation *op,
              << "' to be a `loopopts` attribute";
   }
 
+  if (attr.getName() == LLVMDialect::getReadnoneAttrName()) {
+    const auto attrName = LLVMDialect::getReadnoneAttrName();
+    if (!isa<FunctionOpInterface>(op))
+      return op->emitOpError()
+             << "'" << attrName
+             << "' is permitted only on FunctionOpInterface operations";
+    if (!attr.getValue().isa<UnitAttr>())
+      return op->emitOpError()
+             << "expected '" << attrName << "' to be a unit attribute";
+  }
+
   if (attr.getName() == LLVMDialect::getStructAttrsAttrName()) {
     return op->emitOpError()
            << "'" << LLVM::LLVMDialect::getStructAttrsAttrName()

mlir/lib/Target/LLVMIR/ConvertFromLLVMIR.cpp

@@ -168,6 +168,10 @@ class Importer {
   /// Imports `f` into the current module.
   LogicalResult processFunction(llvm::Function *f);
 
+  /// Converts function attributes of LLVM Function \p f
+  /// into LLVM dialect attributes of LLVMFuncOp \p funcOp.
+  void processFunctionAttributes(llvm::Function *f, LLVMFuncOp funcOp);
+
   /// Imports GV as a GlobalOp, creating it if it doesn't exist.
   GlobalOp processGlobal(llvm::GlobalVariable *gv);
 
@@ -1157,6 +1161,15 @@ FlatSymbolRefAttr Importer::getPersonalityAsAttr(llvm::Function *f) {
   return FlatSymbolRefAttr();
 }
 
+void Importer::processFunctionAttributes(llvm::Function *func,
+                                         LLVMFuncOp funcOp) {
+  auto addNamedUnitAttr = [&](StringRef name) {
+    return funcOp->setAttr(name, UnitAttr::get(context));
+  };
+  if (func->hasFnAttribute(llvm::Attribute::ReadNone))
+    addNamedUnitAttr(LLVMDialect::getReadnoneAttrName());
+}
+
 LogicalResult Importer::processFunction(llvm::Function *f) {
   blocks.clear();
   instMap.clear();
@@ -1191,6 +1204,9 @@ LogicalResult Importer::processFunction(llvm::Function *f) {
   if (f->hasGC())
     fop.setGarbageCollectorAttr(b.getStringAttr(f->getGC()));
 
+  // Handle Function attributes.
+  processFunctionAttributes(f, fop);
+
   if (f->isDeclaration())
     return success();
 

mlir/lib/Target/LLVMIR/ModuleTranslation.cpp

@@ -927,6 +927,9 @@ LogicalResult ModuleTranslation::convertFunctionSignatures() {
     mapFunction(function.getName(), llvmFunc);
     addRuntimePreemptionSpecifier(function.getDsoLocal(), llvmFunc);
 
+    if (function->getAttrOfType<UnitAttr>(LLVMDialect::getReadnoneAttrName()))
+      llvmFunc->addFnAttr(llvm::Attribute::ReadNone);
+
     // Forward the pass-through attributes to LLVM.
     if (failed(forwardPassthroughAttributes(
             function.getLoc(), function.getPassthrough(), llvmFunc)))

mlir/test/Dialect/LLVMIR/func.mlir

@@ -277,3 +277,21 @@ module {
   "llvm.func"() ({
   }) {sym_name = "generic_unknown_calling_convention", CConv = #llvm.cconv<cc_12>, function_type = !llvm.func<i64 (i64, i64)>} : () -> ()
 }
+
+// -----
+
+module {
+  // expected-error@+3 {{'llvm.readnone' is permitted only on FunctionOpInterface operations}}
+  "llvm.func"() ({
+  ^bb0:
+    llvm.return {llvm.readnone}
+  }) {sym_name = "readnone_return", function_type = !llvm.func<void ()>} : () -> ()
+}
+
+// -----
+
+module {
+  // expected-error@+1 {{op expected 'llvm.readnone' to be a unit attribute}}
+  "llvm.func"() ({
+  }) {sym_name = "readnone_func", llvm.readnone = true, function_type = !llvm.func<void ()>} : () -> ()
+}

mlir/test/Target/LLVMIR/Import/function_attributes.ll

@@ -0,0 +1,17 @@
+; RUN: mlir-translate -import-llvm -split-input-file %s | FileCheck %s
+
+// -----
+
+; CHECK: llvm.func @readnone_attr() attributes {llvm.readnone}
+declare void @readnone_attr() #0
+attributes #0 = { readnone }
+
+// -----
+
+; CHECK: llvm.func @readnone_attr() attributes {llvm.readnone} {
+; CHECK:   llvm.return
+; CHECK: }
+define void @readnone_attr() readnone {
+entry:
+  ret void
+}

mlir/test/Target/LLVMIR/llvmir.mlir

@@ -1937,3 +1937,11 @@ llvm.func @vararg_function(%arg0: i32, ...) {
   // CHECK: ret void
   llvm.return
 }
+
+// -----
+
+// Function attributes: readnone
+
+// CHECK: declare void @readnone_function() #[[ATTR:[0-9]+]]
+// CHECK: attributes #[[ATTR]] = { readnone }
+llvm.func @readnone_function() attributes {llvm.readnone}