[mlir][LLVM] Fix crash when using opaque pointers in function signatures

[zero9178]

Summary

Using opaque pointers in function signatures leads to an attempt to recursively convert all types, including sub types in LLVM types. In the case of LLVM pointers, it may not have a subtype aka element type if it is opaque which would then lead to a null pointer dereference.

mlir/lib/Conversion/LLVMCommon/TypeConverter.cpp

LLVMTypeConverter::LLVMTypeConverter(MLIRContext *ctx,
   addConversion([](Type type) {
     return LLVM::isCompatibleType(type) ? llvm::Optional<Type>(type)
                                         : llvm::None;
   });
 
   // LLVM container types may (recursively) contain other types that must be
   // converted even when the outer type is compatible.
   addConversion([&](LLVM::LLVMPointerType type) -> llvm::Optional<Type> {
+    if (type.isOpaque())
+      return type;
     if (auto pointee = convertType(type.getElementType()))
       return LLVM::LLVMPointerType::get(pointee, type.getAddressSpace());
     return llvm::None;
   });
   addConversion([&](LLVM::LLVMStructType type, SmallVectorImpl<Type> &results,
                     ArrayRef<Type> callStack) -> llvm::Optional<LogicalResult> {
     // Fastpath for types that won't be converted by this callback anyway.
     if (LLVM::isCompatibleType(type)) {

mlir/test/Conversion/FuncToLLVM/convert-types.mlir

 // RUN: mlir-opt -test-convert-call-op %s | FileCheck %s
 
 // CHECK-LABEL: @ptr
 // CHECK: !llvm.ptr<i42>
 func.func private @ptr() -> !llvm.ptr<!test.smpla>
 
+// CHECK-LABEL: @opaque_ptr
+// CHECK: !llvm.ptr
+// CHECK-NOT: <
+func.func private @opaque_ptr() -> !llvm.ptr
+
 // CHECK-LABEL: @ptr_ptr()
 // CHECK: !llvm.ptr<ptr<i42>> 
 func.func private @ptr_ptr() -> !llvm.ptr<!llvm.ptr<!test.smpla>>
 
 // CHECK-LABEL: @struct_ptr()
 // CHECK: !llvm.struct<(ptr<i42>)> 
 func.func private @struct_ptr() -> !llvm.struct<(ptr<!test.smpla>)>