[mlir] Support unranked memrefs, return and call ops in bare-ptr calling convention

[dcaballe]

Summary

The bare-ptr calling convention lowers memref-typed values to LLVM
bare pointers at the boundaries of a function call. So far, we only
had support for ranked memref types and function arguments in this
convention. This patch adds support for unranked memref types and
'return' and 'call' ops. This mostly covers all the general use cases!

Some of these changes are also aimed at aligning the bare-ptr calling
convention code with the latest changes in the default calling convention
and reducing the amount of customization code needed. For example, the
'convertCallingConventionType' API is an attempt to abstract away the
conversion customization on types that are sensitive to the calling convention
and move those customizations from the conversion patterns to the type
converter. This is paving the way to isolate and even remove more calling
convention customizations in the future. For example, in follow-up
patches we should be able to unify 'barePtrFuncArgTypeConverter' and
'structFuncArgTypeConverter' into a single class. We could also create
independent type converters, one for each calling convention. Having
all these use cases working first would help us make sure that we preserve
the functionality along the improvements/refactoring.

[ftynse]

As a general observation, do you folks actually want the bare-pointer convention, or is it only there to attach noalias argument attribute? Maybe we should consider investing into expressing aliasing information in MLIR rather than keep the maintenance burden.

mlir/lib/Conversion/StandardToLLVM/StandardToLLVM.cpp

376

I understand this code has been here before, but I am confused by the comment. The "shape can be computed statically" sounds like the code expects a static shape with sizes/strides/offset known at compile time. But getStridesAndOffset does _not_ guarantee that, all the results can be kDynamicStride...

378–381

isStrided does only the check

424

Nit: elide trivial braces

442

Could this rather create an LLVMIntegerType of the appropriate bitwidth? Then this function wouldn't need to be placed in the type converter.

1245

Nit: invert the condition + continue

1249–1251

Being able to pass an optional list of "skip these users" to replaceOp sounds useful indeed.

1258

Is there actually some guarantee that the type has static shape? Otherwise the call will just assert and fail on valid IR.

2200

Nit: the code around this uses unsigned

3453

Typo: one value

[dcaballe]

• Refactor code to a utility function.
• Replace i64 type with index type.
• Address Alex's feedback.

Thanks!

[dcaballe]

Thanks, Alex!

As a general observation, do you folks actually want the bare-pointer convention, or is it only there to attach noalias argument attribute? Maybe we should consider investing into expressing aliasing information in MLIR rather than keep the maintenance burden.

The noalias issue is one problem but I think we also need to model some kind of memref that is lowered to a bare pointer (e.g., if we want to pass a pointer to an external function). Creating specific dialects to cover each and every one of these special case doesn't scale well. I thought unranked memrefs had this goal but now I see that in the default calling convention they are lowered to another descriptor that also contains a field for the rank. I've also seen some memcopies being generated as part of the unranked memref lowering. I'm a bit confused. Do you know about the current use cases for unranked memrefs and if it's going to evolve somehow?

If we progressively have all the pieces needed, I'll be definitely happy to transition the bare pointer calling convention to whatever we see fit!

mlir/lib/Conversion/StandardToLLVM/StandardToLLVM.cpp

376

The "shape can be computed statically" sounds like the code expects a static shape with sizes/strides/offset known at compile time

Yes, that's actually the case (at least for know). I think the check was suggested in the previous review. I added the comment because it took me a while to understand the purpose. Would it make more sense to replace it with something like 'hasStaticShape'?

442

I can do that but we have other utilities doing similar transformation in this class: promoteOperands and promoteOneMemRefDescriptor. Would it make sense to keep all the type/descriptor conversion utilities here for consistency?

1258

Yes, that's what the check in convertMemRefToBarePtr is for. We only support static shapes in this convention.

[ftynse]

The noalias issue is one problem but I think we also need to model some kind of memref that is lowered to a bare pointer (e.g., if we want to pass a pointer to an external function). Creating specific dialects to cover each and every one of these special case doesn't scale well.

What are those cases specifically? I don't see a problem with passing a memref descriptor or its unpacked form to an external function, other than being careful and using the proper ABI. Memref is not (just) a pointer by design, it also contains the rank/offset/sizes/strides information that we would just drop on the floor by treating memrefs as pointers. The closest analogy of a pointer is memref<? x type> or, equivalently memref<? x type, offset:0, strides:[1]>.

I thought unranked memrefs had this goal but now I see that in the default calling convention they are lowered to another descriptor that also contains a field for the rank.

Unranked memrefs were originally intended to move the rank-dispatch from function signatures into the function. Instead of having print_2d_memref_f32 and print_3d_memref_f32 with different signatures, we have just one print_memref_f32, which does switch(rank) internally.

How do you propose to support memrefs with dynamic rank ("unranked" is a terminology abuse IMO) if you don't store the rank?

I've also seen some memcopies being generated as part of the unranked memref lowering.

We need to allocate space to store shape/stride information of unknown-at-compile-time length. Inside a function, the allocation happens on stack to avoid expensive dynamic allocation, but if we need to return the descriptor, the allocation must outlive the function so it cannot be on stack anymore, hence a new allocation+copy (and copy+free on the caller's side). The memcpy we insert is an LLVM intrinsic, so it may just generate code for it rather than call the library.

Do you know about the current use cases for unranked memrefs and if it's going to evolve somehow?

Mostly API trick I described above. TF kernel generator (see recent ODS) also uses these combined with an external shape dialect to implement rank-polymorphic pointwise operations.

Nothing is likely to evolve unless there are concrete proposals or, at least, use cases that need to be addressed ;)

mlir/lib/Conversion/StandardToLLVM/StandardToLLVM.cpp

376

It depends on what are the actual requirements for the memref to be convertible to a bare pointer. hasStaticShape only checks that the shape is static, but does not guarantee that so are the offset and strides, or even that the memref has a strided form (it can have an arbitrary layout map). So if there some expectation of storage contiguity and zero offset, this code should check exactly for those. I'd work back from the indexing scheme.

442

Okay, I take the consistency argument here. In a longer term, we may consider factoring out all these utilities.

1258

Yeah, except that the check does not provide the guarantees required here.

[dcaballe]

Thanks, Alex! Sorry, I misunderstood what an unranked memref is modeling. Let me remove that from the patch since it's incorrect and only leave the support for call and return ops. We can add support for unranked memrefs when needed.

mlir/lib/Conversion/StandardToLLVM/StandardToLLVM.cpp

376

Ok! I'm restricting this to memrefs with only static information, including shape, strides and offset. Note that there is no other specific assumption since the memref descriptor is recreated inside the function with all the static information.

[dcaballe]

• Removed support for unranked memrefs
• Addressed feedback

[ftynse]

Please update the commit description to what it actually does now before landing.

mlir/include/mlir/Conversion/StandardToLLVM/ConvertStandardToLLVM.h

91

Typo: they -> the

mlir/lib/Conversion/StandardToLLVM/StandardToLLVM.cpp

392–393

Nit: prefer the early-return pattern for this

2762

Nit: the code actually extracts the aligned pointer (which is the right thing to do).

[dcaballe]

Thanks, Alex. Will do.

mlir/include/mlir/Conversion/StandardToLLVM/ConvertStandardToLLVM.h

 class IntegerType;
 class LLVMContext;
 class Module;
 class Type;
 } // namespace llvm
 
 namespace mlir {
 
+class BaseMemRefType;
 class ComplexType;
 class LLVMTypeConverter;
 class UnrankedMemRefType;
 
 namespace LLVM {
 class LLVMDialect;
 class LLVMType;
 class LLVMPointerType;
public:
 
   /// Convert a function type.  The arguments and results are converted one by
   /// one and results are packed into a wrapped LLVM IR structure type. `result`
   /// is populated with argument mapping.
   LLVM::LLVMType convertFunctionSignature(FunctionType type, bool isVariadic,
                                           SignatureConversion &result);
 
   /// Convert a non-empty list of types to be returned from a function into a
-  /// supported LLVM IR type.  In particular, if more than one values is
+  /// supported LLVM IR type.  In particular, if more than one value is
   /// returned, create an LLVM IR structure type with elements that correspond
   /// to each of the MLIR types converted with `convertType`.
   Type packFunctionResults(ArrayRef<Type> types);
 
+  /// Convert a type in the context of the default or bare pointer calling
+  /// convention. Calling convention sensitive types, such as MemRefType and
+  /// UnrankedMemRefType, are converted following the specific rules for the
+  /// calling convention. Calling convention independent types are converted
+  /// following the default LLVM type conversions.
+  Type convertCallingConventionType(Type type);
+
+  /// Promote the bare pointers in 'values' that resulted from memrefs to
+  /// descriptors. 'stdTypes' holds they types of 'values' before the conversion
+  /// to the LLVM-IR dialect (i.e., MemRefType, or any other Standard type).
+  void promoteBarePtrsToDescriptors(ConversionPatternRewriter &rewriter,
+                                    Location loc, ArrayRef<Type> stdTypes,
+                                    SmallVectorImpl<Value> &values);
+
   /// Returns the MLIR context.
   MLIRContext &getContext();
 
-
   /// Returns the LLVM dialect.
   LLVM::LLVMDialect *getDialect() { return llvmDialect; }
 
   const LowerToLLVMOptions &getOptions() const { return options; }
 
   /// Promote the LLVM representation of all operands including promoting MemRef
   /// descriptors to stack and use pointers to struct to avoid the complexity
   /// of the platform-specific C/C++ ABI lowering related to struct argument
private:
   /// !llvm<"i8*"> (type-erased pointer).
   /// These types can be recomposed to a unranked memref descriptor struct.
   SmallVector<Type, 2> convertUnrankedMemRefSignature();
 
   // Convert an unranked memref type to an LLVM type that captures the
   // runtime rank and a pointer to the static ranked memref desc
   Type convertUnrankedMemRefType(UnrankedMemRefType type);
 
+  /// Convert a memref type to a bare pointer to the memref element type.
+  Type convertMemRefToBarePtr(BaseMemRefType type);
+
   // Convert a 1D vector type into an LLVM vector type.
   Type convertVectorType(VectorType type);
 
   /// Options for customizing the llvm lowering.
   LowerToLLVMOptions options;
 };
 
 /// Helper class to produce LLVM dialect operations extracting or inserting

mlir/lib/Conversion/StandardToLLVM/StandardToLLVM.cpp

LogicalResult mlir::structFuncArgTypeConverter(LLVMTypeConverter &converter,
   }
   auto converted = converter.convertType(type);
   if (!converted)
     return failure();
   result.push_back(converted);
   return success();
 }
 
-/// Convert a MemRef type to a bare pointer to the MemRef element type.
-static Type convertMemRefTypeToBarePtr(LLVMTypeConverter &converter,
-                                       MemRefType type) {
-  int64_t offset;
-  SmallVector<int64_t, 4> strides;
-  if (failed(getStridesAndOffset(type, strides, offset)))
-    return {};
-
-  LLVM::LLVMType elementType =
-      unwrap(converter.convertType(type.getElementType()));
-  if (!elementType)
-    return {};
-  return elementType.getPointerTo(type.getMemorySpace());
-}
-
 /// Callback to convert function argument types. It converts MemRef function
 /// arguments to bare pointers to the MemRef element type.
 LogicalResult mlir::barePtrFuncArgTypeConverter(LLVMTypeConverter &converter,
                                                 Type type,
                                                 SmallVectorImpl<Type> &result) {
-  // TODO: Add support for unranked memref.
-  if (auto memrefTy = type.dyn_cast<MemRefType>()) {
-    auto llvmTy = convertMemRefTypeToBarePtr(converter, memrefTy);
-    if (!llvmTy)
-      return failure();
-
-    result.push_back(llvmTy);
-    return success();
-  }
-
-  auto llvmTy = converter.convertType(type);
+  auto llvmTy = converter.convertCallingConventionType(type);
   if (!llvmTy)
     return failure();
 
   result.push_back(llvmTy);
   return success();
 }
 
 /// Create an LLVMTypeConverter using default LowerToLLVMOptions.
SmallVector<Type, 2> LLVMTypeConverter::convertUnrankedMemRefSignature() {
   return {getIndexType(), LLVM::LLVMType::getInt8PtrTy(&getContext())};
 }
 
 // Function types are converted to LLVM Function types by recursively converting
 // argument and result types.  If MLIR Function has zero results, the LLVM
 // Function has one VoidType result.  If MLIR Function has more than one result,
 // they are into an LLVM StructType in their order of appearance.
 LLVM::LLVMType LLVMTypeConverter::convertFunctionSignature(
-    FunctionType type, bool isVariadic,
+    FunctionType funcTy, bool isVariadic,
     LLVMTypeConverter::SignatureConversion &result) {
   // Select the argument converter depending on the calling convetion.
   auto funcArgConverter = options.useBarePtrCallConv
                               ? barePtrFuncArgTypeConverter
                               : structFuncArgTypeConverter;
   // Convert argument types one by one and check for errors.
-  for (auto &en : llvm::enumerate(type.getInputs())) {
+  for (auto &en : llvm::enumerate(funcTy.getInputs())) {
     Type type = en.value();
     SmallVector<Type, 8> converted;
     if (failed(funcArgConverter(*this, type, converted)))
       return {};
     result.addInputs(en.index(), converted);
   }
 
   SmallVector<LLVM::LLVMType, 8> argTypes;
   argTypes.reserve(llvm::size(result.getConvertedTypes()));
   for (Type type : result.getConvertedTypes())
     argTypes.push_back(unwrap(type));
 
   // If function does not return anything, create the void result type,
   // if it returns on element, convert it, otherwise pack the result types into
   // a struct.
   LLVM::LLVMType resultType =
-      type.getNumResults() == 0
+      funcTy.getNumResults() == 0
           ? LLVM::LLVMType::getVoidTy(&getContext())
-          : unwrap(packFunctionResults(type.getResults()));
+          : unwrap(packFunctionResults(funcTy.getResults()));
   if (!resultType)
     return {};
   return LLVM::LLVMType::getFunctionTy(resultType, argTypes, isVariadic);
 }
 
 /// Converts the function type to a C-compatible format, in particular using
 /// pointers to memref descriptors for arguments.
 LLVM::LLVMType

 static constexpr unsigned kPtrInUnrankedMemRefDescriptor = 1;
 
 Type LLVMTypeConverter::convertUnrankedMemRefType(UnrankedMemRefType type) {
   auto rankTy = getIndexType();
   auto ptrTy = LLVM::LLVMType::getInt8PtrTy(&getContext());
   return LLVM::LLVMType::getStructTy(rankTy, ptrTy);
 }
 
+/// Convert a memref type to a bare pointer to the memref element type.
+Type LLVMTypeConverter::convertMemRefToBarePtr(BaseMemRefType type) {
+  if (auto memrefTy = type.dyn_cast<MemRefType>()) {
+    // Check that the memref has static shape, strides and offset. Otherwise, it
+    // cannot be lowered to a bare pointer.
+    if (!memrefTy.hasStaticShape())
+      return {};
+
+    int64_t offset = 0;
+    SmallVector<int64_t, 4> strides;
+    if (failed(getStridesAndOffset(memrefTy, strides, offset)))
+      return {};
+
+    for (int64_t stride : strides)
+      if (ShapedType::isDynamicStrideOrOffset(stride))
+        return {};
+
+    if (ShapedType::isDynamicStrideOrOffset(offset))
+      return {};
+  } else {
+    // Unranked memref is not supported in the bare pointer calling convention.
+    return {};
+  }
+
+  LLVM::LLVMType elementType = unwrap(convertType(type.getElementType()));
+  if (!elementType)
+    return {};
+  return elementType.getPointerTo(type.getMemorySpace());
+}
+
 // Convert an n-D vector type to an LLVM vector type via (n-1)-D array type when
 // n > 1.
 // For example, `vector<4 x f32>` converts to `!llvm.type<"<4 x float>">` and
 // `vector<4 x 8 x 16 f32>` converts to `!llvm<"[4 x [8 x <16 x float>]]">`.
 Type LLVMTypeConverter::convertVectorType(VectorType type) {
   auto elementType = unwrap(convertType(type.getElementType()));
   if (!elementType)
     return {};
   auto vectorType =
       LLVM::LLVMType::getVectorTy(elementType, type.getShape().back());
   auto shape = type.getShape();
   for (int i = shape.size() - 2; i >= 0; --i)
     vectorType = LLVM::LLVMType::getArrayTy(vectorType, shape[i]);
   return vectorType;
 }
 
+/// Convert a type in the context of the default or bare pointer calling
+/// convention. Calling convention sensitive types, such as MemRefType and
+/// UnrankedMemRefType, are converted following the specific rules for the
+/// calling convention. Calling convention independent types are converted
+/// following the default LLVM type conversions.
+Type LLVMTypeConverter::convertCallingConventionType(Type type) {
+  if (options.useBarePtrCallConv)
+    if (auto memrefTy = type.dyn_cast<BaseMemRefType>())
+      return convertMemRefToBarePtr(memrefTy);
+
+  return convertType(type);
+}
+
+/// Promote the bare pointers in 'values' that resulted from memrefs to
+/// descriptors. 'stdTypes' holds they types of 'values' before the conversion
+/// to the LLVM-IR dialect (i.e., MemRefType, or any other Standard type).
+void LLVMTypeConverter::promoteBarePtrsToDescriptors(
+    ConversionPatternRewriter &rewriter, Location loc, ArrayRef<Type> stdTypes,
+    SmallVectorImpl<Value> &values) {
+  assert(stdTypes.size() == values.size() &&
+         "The number of types and values doesn't match");
+  for (unsigned i = 0, end = values.size(); i < end; ++i) {
+    Type stdTy = stdTypes[i];
+    if (auto memrefTy = stdTy.dyn_cast<MemRefType>())
+      values[i] = MemRefDescriptor::fromStaticShape(rewriter, loc, *this,
+                                                    memrefTy, values[i]);
+    else
+      llvm_unreachable("Unranked memrefs are not supported");
+  }
+}
+
 ConvertToLLVMPattern::ConvertToLLVMPattern(StringRef rootOpName,
                                            MLIRContext *context,
                                            LLVMTypeConverter &typeConverter,
                                            PatternBenefit benefit)
     : ConversionPattern(rootOpName, benefit, typeConverter, context),
       typeConverter(typeConverter) {}
 
 /*============================================================================*/
static void wrapExternalFunction(OpBuilder &builder, Location loc,
   builder.create<LLVM::ReturnOp>(loc, call.getResults());
 }
 
 namespace {
 
 struct FuncOpConversionBase : public ConvertOpToLLVMPattern<FuncOp> {
 protected:
   using ConvertOpToLLVMPattern<FuncOp>::ConvertOpToLLVMPattern;
-  using UnsignedTypePair = std::pair<unsigned, Type>;
-
-  // Gather the positions and types of memref-typed arguments in a given
-  // FunctionType.
-  void getMemRefArgIndicesAndTypes(
-      FunctionType type, SmallVectorImpl<UnsignedTypePair> &argsInfo) const {
-    argsInfo.reserve(type.getNumInputs());
-    for (auto en : llvm::enumerate(type.getInputs())) {
-      if (en.value().isa<MemRefType, UnrankedMemRefType>())
-        argsInfo.push_back({en.index(), en.value()});
-    }
-  }
 
   // Convert input FuncOp to LLVMFuncOp by using the LLVMTypeConverter provided
   // to this legalization pattern.
   LLVM::LLVMFuncOp
   convertFuncOpToLLVMFuncOp(FuncOp funcOp,
                             ConversionPatternRewriter &rewriter) const {
     // Convert the original function arguments. They are converted using the
     // LLVMTypeConverter provided to this legalization pattern.

 struct BarePtrFuncOpConversion : public FuncOpConversionBase {
   using FuncOpConversionBase::FuncOpConversionBase;
 
   LogicalResult
   matchAndRewrite(Operation *op, ArrayRef<Value> operands,
                   ConversionPatternRewriter &rewriter) const override {
     auto funcOp = cast<FuncOp>(op);
 
-    // Store the positions and type of memref-typed arguments so that we can
-    // promote them to MemRef descriptor structs at the beginning of the
-    // function.
-    SmallVector<UnsignedTypePair, 4> promotedArgsInfo;
-    getMemRefArgIndicesAndTypes(funcOp.getType(), promotedArgsInfo);
+    // Store the type of memref-typed arguments before the conversion so that we
+    // can promote them to MemRef descriptor at the beginning of the function.
+    SmallVector<Type, 8> oldArgTypes =
+        llvm::to_vector<8>(funcOp.getType().getInputs());
 
     auto newFuncOp = convertFuncOpToLLVMFuncOp(funcOp, rewriter);
     if (!newFuncOp)
       return failure();
     if (newFuncOp.getBody().empty()) {
       rewriter.eraseOp(op);
       return success();
     }
 
-    // Promote bare pointers from MemRef arguments to a MemRef descriptor struct
-    // at the beginning of the function so that all the MemRefs in the function
-    // have a uniform representation.
-    Block *firstBlock = &newFuncOp.getBody().front();
-    rewriter.setInsertionPoint(firstBlock, firstBlock->begin());
-    auto funcLoc = funcOp.getLoc();
-    for (const auto &argInfo : promotedArgsInfo) {
-      // TODO: Add support for unranked MemRefs.
-      if (auto memrefType = argInfo.second.dyn_cast<MemRefType>()) {
-        // Replace argument with a placeholder (undef), promote argument to a
-        // MemRef descriptor and replace placeholder with the last instruction
-        // of the MemRef descriptor. The placeholder is needed to avoid
-        // replacing argument uses in the MemRef descriptor instructions.
-        BlockArgument arg = firstBlock->getArgument(argInfo.first);
-        Value placeHolder =
-            rewriter.create<LLVM::UndefOp>(funcLoc, arg.getType());
-        rewriter.replaceUsesOfBlockArgument(arg, placeHolder);
-        auto desc = MemRefDescriptor::fromStaticShape(
-            rewriter, funcLoc, typeConverter, memrefType, arg);
-        rewriter.replaceOp(placeHolder.getDefiningOp(), {desc});
-      }
+    // Promote bare pointers from memref arguments to memref descriptors at the
+    // beginning of the function so that all the memrefs in the function have a
+    // uniform representation.
+    Block *entryBlock = &newFuncOp.getBody().front();
+    auto blockArgs = entryBlock->getArguments();
+    assert(blockArgs.size() == oldArgTypes.size() &&
+           "The number of arguments and types doesn't match");
+
+    OpBuilder::InsertionGuard guard(rewriter);
+    rewriter.setInsertionPointToStart(entryBlock);
+    for (auto it : llvm::zip(blockArgs, oldArgTypes)) {
+      BlockArgument arg = std::get<0>(it);
+      Type argTy = std::get<1>(it);
+
+      // Unranked memrefs are not supported in the bare pointer calling
+      // convention. We should have bailed out before in the presence of
+      // unranked memrefs.
+      assert(!argTy.isa<UnrankedMemRefType>() &&
+             "Unranked memref is not supported");
+      auto memrefTy = argTy.dyn_cast<MemRefType>();
+      if (!memrefTy)
+        continue;
+
+      // Replace barePtr with a placeholder (undef), promote barePtr to a ranked
+      // or unranked memref descriptor and replace placeholder with the last
+      // instruction of the memref descriptor.
+      // TODO: The placeholder is needed to avoid replacing barePtr uses in the
+      // MemRef descriptor instructions. We may want to have a utility in the
+      // rewriter to properly handle this use case.
+      Location loc = op->getLoc();
+      auto placeholder = rewriter.create<LLVM::UndefOp>(loc, memrefTy);
+      rewriter.replaceUsesOfBlockArgument(arg, placeholder);
+
+      Value desc = MemRefDescriptor::fromStaticShape(
+          rewriter, loc, typeConverter, memrefTy, arg);
+      rewriter.replaceOp(placeholder, {desc});
     }
 
     rewriter.eraseOp(op);
     return success();
   }
 };
 
 //////////////// Support for Lowering operations on n-D vectors ////////////////
if (numResults < 2) {
       results.reserve(numResults);
       for (unsigned i = 0; i < numResults; ++i) {
         auto type = this->typeConverter.convertType(op->getResult(i).getType());
         results.push_back(rewriter.create<LLVM::ExtractValueOp>(
             op->getLoc(), type, newOp.getOperation()->getResult(0),
             rewriter.getI64ArrayAttr(i)));
       }
     }
-    if (failed(copyUnrankedDescriptors(
-            rewriter, op->getLoc(), this->typeConverter, op->getResultTypes(),
-            results, /*toDynamic=*/false)))
+
+    if (this->typeConverter.getOptions().useBarePtrCallConv) {
+      // For the bare-ptr calling convention, promote memref results to
+      // descriptors.
+      assert(results.size() == resultTypes.size() &&
+             "The number of arguments and types doesn't match");
+      this->typeConverter.promoteBarePtrsToDescriptors(rewriter, op->getLoc(),
+                                                       resultTypes, results);
+    } else if (failed(copyUnrankedDescriptors(rewriter, op->getLoc(),
+                                              this->typeConverter, resultTypes,
+                                              results,
+                                              /*toDynamic=*/false))) {
       return failure();
-    rewriter.replaceOp(op, results);
+    }
 
+    rewriter.replaceOp(op, results);
     return success();
   }
 };
 
 struct CallOpLowering : public CallOpInterfaceLowering<CallOp> {
   using Super::Super;
 };
 
 struct CallIndirectOpLowering : public CallOpInterfaceLowering<CallIndirectOp> {
   using Super::Super;
 };
 
 // A `dealloc` is converted into a call to `free` on the underlying data buffer.
 // The memref descriptor being an SSA value, there is no need to clean it up

 // Emit `UndefOp` followed by `InsertValueOp`s to create such structure if
 // necessary before returning it
 struct ReturnOpLowering : public ConvertOpToLLVMPattern<ReturnOp> {
   using ConvertOpToLLVMPattern<ReturnOp>::ConvertOpToLLVMPattern;
 
   LogicalResult
   matchAndRewrite(Operation *op, ArrayRef<Value> operands,
                   ConversionPatternRewriter &rewriter) const override {
+    Location loc = op->getLoc();
     unsigned numArguments = op->getNumOperands();
-    auto updatedOperands = llvm::to_vector<4>(operands);
-    copyUnrankedDescriptors(rewriter, op->getLoc(), typeConverter,
+    SmallVector<Value, 4> updatedOperands;
+
+    if (typeConverter.getOptions().useBarePtrCallConv) {
+      // For the bare-ptr calling convention, extract the allocated pointer to
+      // be returned from the memref descriptor.
+      for (auto it : llvm::zip(op->getOperands(), operands)) {
+        Type oldTy = std::get<0>(it).getType();
+        Value newOperand = std::get<1>(it);
+        if (oldTy.isa<MemRefType>()) {
+          MemRefDescriptor memrefDesc(newOperand);
+          newOperand = memrefDesc.alignedPtr(rewriter, loc);
+        } else if (oldTy.isa<UnrankedMemRefType>()) {
+          // Unranked memref is not supported in the bare pointer calling
+          // convention.
+          return failure();
+        }
+        updatedOperands.push_back(newOperand);
+      }
+    } else {
+      updatedOperands = llvm::to_vector<4>(operands);
+      copyUnrankedDescriptors(rewriter, loc, typeConverter,
                               op->getOperands().getTypes(), updatedOperands,
                               /*toDynamic=*/true);
+    }
 
     // If ReturnOp has 0 or 1 operand, create it and return immediately.
     if (numArguments == 0) {
       rewriter.replaceOpWithNewOp<LLVM::ReturnOp>(
           op, ArrayRef<Type>(), ArrayRef<Value>(), op->getAttrs());
       return success();
     }
     if (numArguments == 1) {
       rewriter.replaceOpWithNewOp<LLVM::ReturnOp>(
           op, ArrayRef<Type>(), updatedOperands, op->getAttrs());
       return success();
     }
 
     // Otherwise, we need to pack the arguments into an LLVM struct type before
     // returning.
     auto packedType = typeConverter.packFunctionResults(
         llvm::to_vector<4>(op->getOperandTypes()));
 
-    Value packed = rewriter.create<LLVM::UndefOp>(op->getLoc(), packedType);
+    Value packed = rewriter.create<LLVM::UndefOp>(loc, packedType);
     for (unsigned i = 0; i < numArguments; ++i) {
       packed = rewriter.create<LLVM::InsertValueOp>(
-          op->getLoc(), packedType, packed, updatedOperands[i],
+          loc, packedType, packed, updatedOperands[i],
           rewriter.getI64ArrayAttr(i));
     }
     rewriter.replaceOpWithNewOp<LLVM::ReturnOp>(op, ArrayRef<Type>(), packed,
                                                 op->getAttrs());
     return success();
   }
 };
 

 
 void mlir::populateStdToLLVMConversionPatterns(
     LLVMTypeConverter &converter, OwningRewritePatternList &patterns) {
   populateStdToLLVMFuncOpConversionPattern(converter, patterns);
   populateStdToLLVMNonMemoryConversionPatterns(converter, patterns);
   populateStdToLLVMMemoryConversionPatterns(converter, patterns);
 }
 
-// Create an LLVM IR structure type if there is more than one result.
+/// Convert a non-empty list of types to be returned from a function into a
+/// supported LLVM IR type.  In particular, if more than one value is returned,
+/// create an LLVM IR structure type with elements that correspond to each of
+/// the MLIR types converted with `convertType`.
 Type LLVMTypeConverter::packFunctionResults(ArrayRef<Type> types) {
   assert(!types.empty() && "expected non-empty list of type");
 
   if (types.size() == 1)
-    return convertType(types.front());
+    return convertCallingConventionType(types.front());
 
   SmallVector<LLVM::LLVMType, 8> resultTypes;
   resultTypes.reserve(types.size());
   for (auto t : types) {
-    auto converted = convertType(t).dyn_cast<LLVM::LLVMType>();
+    auto converted = convertCallingConventionType(t).dyn_cast<LLVM::LLVMType>();
     if (!converted)
       return {};
     resultTypes.push_back(converted);
   }
 
   return LLVM::LLVMType::getStructTy(&getContext(), resultTypes);
 }
 
SmallVector<Value, 4> LLVMTypeConverter::promoteOperands(Location loc,
                                                          ValueRange operands,
                                                          OpBuilder &builder) {
   SmallVector<Value, 4> promotedOperands;
   promotedOperands.reserve(operands.size());
   for (auto it : llvm::zip(opOperands, operands)) {
     auto operand = std::get<0>(it);
     auto llvmOperand = std::get<1>(it);
 
+    if (options.useBarePtrCallConv) {
+      // For the bare-ptr calling convention, we only have to extract the
+      // aligned pointer of a memref.
+      if (auto memrefType = operand.getType().dyn_cast<MemRefType>()) {
+        MemRefDescriptor desc(llvmOperand);
+        llvmOperand = desc.alignedPtr(builder, loc);
+      } else if (operand.getType().isa<UnrankedMemRefType>()) {
+        llvm_unreachable("Unranked memrefs are not supported");
+      }
+    } else {
       if (operand.getType().isa<UnrankedMemRefType>()) {
         UnrankedMemRefDescriptor::unpack(builder, loc, llvmOperand,
                                          promotedOperands);
         continue;
       }
       if (auto memrefType = operand.getType().dyn_cast<MemRefType>()) {
         MemRefDescriptor::unpack(builder, loc, llvmOperand,
                                  operand.getType().cast<MemRefType>(),
                                  promotedOperands);
         continue;
       }
+    }
 
     promotedOperands.push_back(llvmOperand);
   }
   return promotedOperands;
 }
 
 namespace {
 /// A pass converting MLIR operations into the LLVM IR dialect.

mlir/test/Conversion/StandardToLLVM/convert-static-memref-ops.mlir

 // RUN: mlir-opt -convert-std-to-llvm %s | FileCheck %s
 // RUN: mlir-opt -convert-std-to-llvm='use-bare-ptr-memref-call-conv=1' -split-input-file %s | FileCheck %s --check-prefix=BAREPTR
 
 // BAREPTR-LABEL: func @check_noalias
 // BAREPTR-SAME: %{{.*}}: !llvm.ptr<float> {llvm.noalias = true}, %{{.*}}: !llvm.ptr<float> {llvm.noalias = true}
 func @check_noalias(%static : memref<2xf32> {llvm.noalias = true}, %other : memref<2xf32> {llvm.noalias = true}) {
     return
 }
 
 // -----
 
 // CHECK-LABEL: func @check_static_return
 // CHECK-COUNT-2: !llvm.ptr<float>
 // CHECK-COUNT-5: !llvm.i64
 // CHECK-SAME: -> !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
 // BAREPTR-LABEL: func @check_static_return
-// BAREPTR-SAME: (%[[arg:.*]]: !llvm.ptr<float>) -> !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)> {
+// BAREPTR-SAME: (%[[arg:.*]]: !llvm.ptr<float>) -> !llvm.ptr<float> {
 func @check_static_return(%static : memref<32x18xf32>) -> memref<32x18xf32> {
 // CHECK:  llvm.return %{{.*}} : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
 
 // BAREPTR: %[[udf:.*]] = llvm.mlir.undef : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
-// BAREPTR-NEXT: %[[base:.*]] = llvm.insertvalue %[[arg]], %[[udf]][0] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
-// BAREPTR-NEXT: %[[aligned:.*]] = llvm.insertvalue %[[arg]], %[[base]][1] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
+// BAREPTR-NEXT: %[[base0:.*]] = llvm.insertvalue %[[arg]], %[[udf]][0] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
+// BAREPTR-NEXT: %[[aligned:.*]] = llvm.insertvalue %[[arg]], %[[base0]][1] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
 // BAREPTR-NEXT: %[[val0:.*]] = llvm.mlir.constant(0 : index) : !llvm.i64
 // BAREPTR-NEXT: %[[ins0:.*]] = llvm.insertvalue %[[val0]], %[[aligned]][2] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
 // BAREPTR-NEXT: %[[val1:.*]] = llvm.mlir.constant(32 : index) : !llvm.i64
 // BAREPTR-NEXT: %[[ins1:.*]] = llvm.insertvalue %[[val1]], %[[ins0]][3, 0] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
 // BAREPTR-NEXT: %[[val2:.*]] = llvm.mlir.constant(18 : index) : !llvm.i64
 // BAREPTR-NEXT: %[[ins2:.*]] = llvm.insertvalue %[[val2]], %[[ins1]][4, 0] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
 // BAREPTR-NEXT: %[[val3:.*]] = llvm.mlir.constant(18 : index) : !llvm.i64
 // BAREPTR-NEXT: %[[ins3:.*]] = llvm.insertvalue %[[val3]], %[[ins2]][3, 1] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
 // BAREPTR-NEXT: %[[val4:.*]] = llvm.mlir.constant(1 : index) : !llvm.i64
 // BAREPTR-NEXT: %[[ins4:.*]] = llvm.insertvalue %[[val4]], %[[ins3]][4, 1] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
-// BAREPTR-NEXT: llvm.return %[[ins4]] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
+// BAREPTR-NEXT: %[[base1:.*]] = llvm.extractvalue %[[ins4]][1] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
+// BAREPTR-NEXT: llvm.return %[[base1]] : !llvm.ptr<float>
   return %static : memref<32x18xf32>
 }
 
 // -----
 
 // CHECK-LABEL: func @check_static_return_with_offset
 // CHECK-COUNT-2: !llvm.ptr<float>
 // CHECK-COUNT-5: !llvm.i64
 // CHECK-SAME: -> !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
 // BAREPTR-LABEL: func @check_static_return_with_offset
-// BAREPTR-SAME: (%[[arg:.*]]: !llvm.ptr<float>) -> !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)> {
+// BAREPTR-SAME: (%[[arg:.*]]: !llvm.ptr<float>) -> !llvm.ptr<float> {
 func @check_static_return_with_offset(%static : memref<32x18xf32, offset:7, strides:[22,1]>) -> memref<32x18xf32, offset:7, strides:[22,1]> {
 // CHECK:  llvm.return %{{.*}} : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
 
 // BAREPTR: %[[udf:.*]] = llvm.mlir.undef : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
-// BAREPTR-NEXT: %[[base:.*]] = llvm.insertvalue %[[arg]], %[[udf]][0] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
-// BAREPTR-NEXT: %[[aligned:.*]] = llvm.insertvalue %[[arg]], %[[base]][1] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
+// BAREPTR-NEXT: %[[base0:.*]] = llvm.insertvalue %[[arg]], %[[udf]][0] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
+// BAREPTR-NEXT: %[[aligned:.*]] = llvm.insertvalue %[[arg]], %[[base0]][1] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
 // BAREPTR-NEXT: %[[val0:.*]] = llvm.mlir.constant(7 : index) : !llvm.i64
 // BAREPTR-NEXT: %[[ins0:.*]] = llvm.insertvalue %[[val0]], %[[aligned]][2] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
 // BAREPTR-NEXT: %[[val1:.*]] = llvm.mlir.constant(32 : index) : !llvm.i64
 // BAREPTR-NEXT: %[[ins1:.*]] = llvm.insertvalue %[[val1]], %[[ins0]][3, 0] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
 // BAREPTR-NEXT: %[[val2:.*]] = llvm.mlir.constant(22 : index) : !llvm.i64
 // BAREPTR-NEXT: %[[ins2:.*]] = llvm.insertvalue %[[val2]], %[[ins1]][4, 0] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
 // BAREPTR-NEXT: %[[val3:.*]] = llvm.mlir.constant(18 : index) : !llvm.i64
 // BAREPTR-NEXT: %[[ins3:.*]] = llvm.insertvalue %[[val3]], %[[ins2]][3, 1] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
 // BAREPTR-NEXT: %[[val4:.*]] = llvm.mlir.constant(1 : index) : !llvm.i64
 // BAREPTR-NEXT: %[[ins4:.*]] = llvm.insertvalue %[[val4]], %[[ins3]][4, 1] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
-// BAREPTR-NEXT: llvm.return %[[ins4]] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
+// BAREPTR-NEXT: %[[base1:.*]] = llvm.extractvalue %[[ins4]][1] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)>
+// BAREPTR-NEXT: llvm.return %[[base1]] : !llvm.ptr<float>
   return %static : memref<32x18xf32, offset:7, strides:[22,1]>
 }
 
 // -----
 
 // CHECK-LABEL: func @zero_d_alloc() -> !llvm.struct<(ptr<float>, ptr<float>, i64)> {
-// BAREPTR-LABEL: func @zero_d_alloc() -> !llvm.struct<(ptr<float>, ptr<float>, i64)> {
+// BAREPTR-LABEL: func @zero_d_alloc() -> !llvm.ptr<float> {
 func @zero_d_alloc() -> memref<f32> {
 // CHECK-NEXT:  llvm.mlir.constant(1 : index) : !llvm.i64
 // CHECK-NEXT:  %[[null:.*]] = llvm.mlir.null : !llvm.ptr<float>
 // CHECK-NEXT:  %[[one:.*]] = llvm.mlir.constant(1 : index) : !llvm.i64
 // CHECK-NEXT:  %[[gep:.*]] = llvm.getelementptr %[[null]][%[[one]]] : (!llvm.ptr<float>, !llvm.i64) -> !llvm.ptr<float>
 // CHECK-NEXT:  %[[sizeof:.*]] = llvm.ptrtoint %[[gep]] : !llvm.ptr<float> to !llvm.i64
 // CHECK-NEXT:  llvm.mul %{{.*}}, %[[sizeof]] : !llvm.i64
 // CHECK-NEXT:  %[[one_1:.*]] = llvm.mlir.constant(1 : index) : !llvm.i64

 // BAREPTR-NEXT:  llvm.insertvalue %[[c0]], %{{.*}}[2] : !llvm.struct<(ptr<float>, ptr<float>, i64, array<1 x i64>, array<1 x i64>)>
   %0 = alloc() {alignment = 8} : memref<42xf32>
   return %0 : memref<42xf32>
 }
 
 // -----
 
 // CHECK-LABEL: func @static_alloc() -> !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)> {
-// BAREPTR-LABEL: func @static_alloc() -> !llvm.struct<(ptr<float>, ptr<float>, i64, array<2 x i64>, array<2 x i64>)> {
+// BAREPTR-LABEL: func @static_alloc() -> !llvm.ptr<float> {
 func @static_alloc() -> memref<32x18xf32> {
 //      CHECK:  %[[sz1:.*]] = llvm.mlir.constant(32 : index) : !llvm.i64
 // CHECK-NEXT:  %[[sz2:.*]] = llvm.mlir.constant(18 : index) : !llvm.i64
 // CHECK-NEXT:  %[[num_elems:.*]] = llvm.mul %0, %1 : !llvm.i64
 // CHECK-NEXT:  %[[null:.*]] = llvm.mlir.null : !llvm.ptr<float>
 // CHECK-NEXT:  %[[one:.*]] = llvm.mlir.constant(1 : index) : !llvm.i64
 // CHECK-NEXT:  %[[gep:.*]] = llvm.getelementptr %[[null]][%[[one]]] : (!llvm.ptr<float>, !llvm.i64) -> !llvm.ptr<float>
 // CHECK-NEXT:  %[[sizeof:.*]] = llvm.ptrtoint %[[gep]] : !llvm.ptr<float> to !llvm.i64
// BAREPTR:  llvm.mlir.constant(13 : index) : !llvm.i64
   %c3 = constant 3 : index
   %3 = dim %static, %c3 : memref<42x32x15x13x27xf32>
 // CHECK:  llvm.mlir.constant(27 : index) : !llvm.i64
 // BAREPTR:  llvm.mlir.constant(27 : index) : !llvm.i64
   %c4 = constant 4 : index
   %4 = dim %static, %c4 : memref<42x32x15x13x27xf32>
   return
 }
+
+// -----
+
+// BAREPTR: llvm.func @foo(!llvm.ptr<i8>) -> !llvm.ptr<i8>
+func @foo(memref<10xi8>) -> memref<20xi8>
+
+// BAREPTR-LABEL: func @check_memref_func_call
+// BAREPTR-SAME:    %[[in:.*]]: !llvm.ptr<i8>) -> !llvm.ptr<i8>
+func @check_memref_func_call(%in : memref<10xi8>) -> memref<20xi8> {
+  // BAREPTR:         %[[inDesc:.*]] = llvm.insertvalue %{{.*}}, %{{.*}}[4, 0]
+  // BAREPTR-NEXT:    %[[barePtr:.*]] = llvm.extractvalue %[[inDesc]][1] : !llvm.struct<(ptr<i8>, ptr<i8>, i64, array<1 x i64>, array<1 x i64>)>
+  // BAREPTR-NEXT:    %[[call:.*]] = llvm.call @foo(%[[barePtr]]) : (!llvm.ptr<i8>) -> !llvm.ptr<i8>
+  // BAREPTR-NEXT:    %[[desc0:.*]] = llvm.mlir.undef : !llvm.struct<(ptr<i8>, ptr<i8>, i64, array<1 x i64>, array<1 x i64>)>
+  // BAREPTR-NEXT:    %[[desc1:.*]] = llvm.insertvalue %[[call]], %[[desc0]][0] : !llvm.struct<(ptr<i8>, ptr<i8>, i64, array<1 x i64>, array<1 x i64>)>
+  // BAREPTR-NEXT:    %[[desc2:.*]] = llvm.insertvalue %[[call]], %[[desc1]][1] : !llvm.struct<(ptr<i8>, ptr<i8>, i64, array<1 x i64>, array<1 x i64>)>
+  // BAREPTR-NEXT:    %[[c0:.*]] = llvm.mlir.constant(0 : index) : !llvm.i64
+  // BAREPTR-NEXT:    %[[desc4:.*]] = llvm.insertvalue %[[c0]], %[[desc2]][2] : !llvm.struct<(ptr<i8>, ptr<i8>, i64, array<1 x i64>, array<1 x i64>)>
+  // BAREPTR-NEXT:    %[[c20:.*]] = llvm.mlir.constant(20 : index) : !llvm.i64
+  // BAREPTR-NEXT:    %[[desc6:.*]] = llvm.insertvalue %[[c20]], %[[desc4]][3, 0] : !llvm.struct<(ptr<i8>, ptr<i8>, i64, array<1 x i64>, array<1 x i64>)>
+  // BAREPTR-NEXT:    %[[c1:.*]] = llvm.mlir.constant(1 : index) : !llvm.i64
+  // BAREPTR-NEXT:    %[[outDesc:.*]] = llvm.insertvalue %[[c1]], %[[desc6]][4, 0] : !llvm.struct<(ptr<i8>, ptr<i8>, i64, array<1 x i64>, array<1 x i64>)>
+  %res = call @foo(%in) : (memref<10xi8>) -> (memref<20xi8>)
+  // BAREPTR-NEXT:    %[[res:.*]] = llvm.extractvalue %[[outDesc]][1] : !llvm.struct<(ptr<i8>, ptr<i8>, i64, array<1 x i64>, array<1 x i64>)>
+  // BAREPTR-NEXT:    llvm.return %[[res]] : !llvm.ptr<i8>
+  return %res : memref<20xi8>
+}