NaplesPU Clang Documentation

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This section shows how the Clang frontend is modified to match NaplesPU requirements concerning intrinsic recognition and target registration.

Builtins Definition

When implementing support for a new target, a frontend implementation must be able to recognise target-specific instructions, also called builtins. Clang provides a simple mechanism to add to frontend the capability to identify builtins, described in the file Builtins.def. It looks like a database for builtin functions, in which each entry is of the following form:

BUILTIN(<name of the builtin>, <types>, <function attributes>)

Referring to the above schema, the first value is the function name, the second one is the concatenation of types for the result value and for each argument, the latter is the concatenation of codes, each one is a function attribute. For example, the following is a builtin definition:

BUILTIN(__builtin_npu_flush, vi, n)

In this case, the function __builtin_npu_flush has got only one argument of int type and it is a void return function. n attribute means that it is a nothrow function. To get a full list of attributes and types you may check the just cited Builtins.def

At this point, builtins are defined but cannot be recognised by frontend tools. To realise this behaviour it is necessary to collect all builtin definitions in a .def file and then include it in the TargetBuiltins header file as shown below: 

namespace NaplesPU {
    enum {
      LastTIBuiltin = clang::Builtin::FirstTSBuiltin-1,
      #define BUILTIN(ID, TYPE, ATTRS) BI##ID,
      #include "clang/Basic/BuiltinsNaplesPU.def"
      LastTSBuiltin
    };
  }

Defining Target Features

In order to provide target informations, the LLVM team provides a class named TargetInfo. It contains information fields, each of them is a potentially supported feature. To be able to use this class, it is required to inherit it by creating a specialized TargetInfo class, defining the target features and register it as new supported target.

The following code sample is extracted by NaplesPU.h in clang/lib/Basic/Targets. 

class NaplesPUTargetInfo : public TargetInfo {
  static const char *const GCCRegNames[];
  static const Builtin::Info BuiltinInfo[];
 public:
 NaplesPUTargetInfo(const llvm::Triple &Triple) : TargetInfo(Triple) {
    BigEndian = false;
    IntWidth = IntAlign = 32;
    ...
    resetDataLayout("e-m:e-p:32:32-i32:32:32-f32:32:32");
  }

Referring to the code sample above, the DataLayout field describes the target in terms of its features. Each of them is separated by a - character. By looking at the example above, the first feature defines the endianness as little. The other ones are related to size and alignment of pointers, integers and floats respectively. They should be read as follows:

<feature_code>:size:alignment

More about the data layout parsing can be found in the DataLayout documentation.

The TargetInfo class implementation provides two static variables, GCCRegNames and BuiltinInfo, of array type. The first contains the list of the registers' names supported by the target platform, while the latter is the definition of the target builtin format. 

const char *const NaplesPUTargetInfo::GCCRegNames[] = {
  "s0",  "s1",  "s2",  "s3",  "s4",  "s5",  "s6",  "s7",
  "s8",  "s9",  "s10", "s11", "s12", "s13", "s14", "s15",
  "s16",  "s17",  "s18",  "s19",  "s20",  "s21",  "s22",  "s23",
  "s24",  "s25",  "s26", "s27",  "s28",  "s29",  "s30", "s31", 
  "s32",  "s33",  "s34",  "s35",  "s36",  "s37",  "s38",  "s39",
  "s40",  "s41",  "s42",  "s43",  "s44",  "s45",  "s46",  "s47",
  "s48",  "s49",  "s50",  "s51",  "s52",  "s53",  "s54",  "s55",
  "s56",  "s57",  "TR",  "RM",  "FP", "SP", "RA", "PC",
  "v0",  "v1",  "v2",  "v3",  "v4",  "v5",  "v6",  "v7",
  "v8",  "v9",  "v10", "v11", "v12", "v13", "v14", "v15",
  "v16",  "v17",  "v18",  "v19",  "v20",  "v21",  "v22",  "v23",
  "v24",  "v25",  "v26", "v27", "v28", "v29", "v30", "v31",
  "v32",  "v33",  "v34",  "v35",  "v36",  "v37",  "v38",  "v39",
  "v40",  "v41",  "v42",  "v43",  "v44",  "v45",  "v46",  "v47",
  "v48",  "v49",  "v50",  "v51",  "v52",  "v53",  "v54",  "v55",
  "v56",  "v57",  "v58",  "v59",  "v60",  "v61",  "v62",  "v63"
};

Another implemented method is validateAsmConstraint. This function is used to validate the constraint used for inputs, outputs and clobbers for inline assembly code declarations. Recall that the inline assembly code is the way to embed asm code into an HLL source file.

To be correctly recognised, 'naplespu has to be added in the llvm::Triple namespace, by defining it in Triple.h: 

enum ArchType {
    UnknownArch,
    arm,            // ARM (little endian): arm, armv.*, xscale
    ...
    naplespu,       // ADDING NaplesPU TARGET

Once the custom target is declared, it can be allocated by frontend. To enable this operation, it is necessary to define a link between the clang function AllocateTarget and the TargetInfo object constructor in clang/lib/Basic/Targets.cpp: 

static TargetInfo *AllocateTarget(const llvm::Triple &Triple, const TargetOptions &Opts) {
  switch (Triple.getArch()) {
  ...
  case llvm::Triple::naplespu:
    return new NaplesPUTargetInfo(Triple);

Mapping Builtins on LLVM Intrinsics

The next step in adding frontend support for the custom target is to create a mapping between the defined builtins and the corresponding LLVM Intrinsics, that are used to build the intermediate representation.

LLVM provides a simple way to define the intrinsics by using the TableGen language. Let's take a look on the Intrinsics class definition showed below. 

class Intrinsic<list<LLVMType> ret_types,
                list<LLVMType> param_types = [],
                list<IntrinsicProperty> intr_properties = [],
                string name = "" > : SDPatternOperator {
...
}

As shown, the Intrinsic class has several arguments:

  • ret_types is the field containing the list of return types of the defined function. It is a list of LLVMType objects, each of them is described in Intrinsics.td. However, it is possible to define a custom type by deriving it from the defined ones. For example:

def my_dummy_type : LLVMType<v16i32>

The LLVMType object has a single argument that is a ValueType object. It is defined in ValueTypes.td. For example the following is the definition of v16i32: 

//ValueType <Size, Value>
def v16i32 : ValueType<512, 42>;
  • param_types is a list of parameter types, each one must be a LLVMType object.
  • properties is an array of IntrinsicProperty, used to describe the behaviour of the intrinsic. For example IntrNoMem is the property to tell that the intrinsic does not access to memory or have side effects.
  • name is the string name of the intrinsic, prefixes included.

Each intrinsic must be defined as an object of the just cited class, and each definition should be of the following format:

def int_[name] : Intrinsic <...>

where name is the intrinsic name excluded of the prefixes. For example, the following one is a correct definition: 

    def int_npu_write_control_reg : Intrinsic<[], 
        [llvm_i32_ty, llvm_i32_ty], [],
        "llvm.npu.__builtin_npu_write_control_reg">

In order to enable the frontend to generate the IR for our custom platform, it is needed to define a function to map builtins on the intrinsics. This function has to be defined in CodeGenFunction header and then implemented by switching among builtins and returning the associate Intrinsic object. 

llvm::Value *CodeGenFunction::EmitNaplesPUBuiltinExpr(unsigned BuiltinID, const CallExpr *E) {
  ...
  SmallVector<Value*, 2> Ops;
  for (unsigned i = 0; i < E->getNumArgs(); i++){
        Ops.push_back(EmitScalarExpr(E->getArg(i)));
  }
  llvm::Function *F;
  ...
  case MyTarget::BI__builtin_mytarget_ctzv8i64:
    F = CGM.getIntrinsic(Intrinsic::npu_ctzv8i64);
        break;
..
  return Builder.CreateCall(F, Ops, "");
}


When clang tries to convert a builtin in an intrinsic, it calls a method, that is EmitTargetArchBuiltinExpr, that switches among the different target to get the proper function to call. This method has got a static implementation in CGBuiltin.cpp 

static Value *EmitTargetArchBuiltinExpr(CodeGenFunction *CGF, unsigned BuiltinID, const CallExpr *E, llvm::Triple::ArchType Arch) {
  switch (Arch) {
  ...
  case llvm::Triple::naplespu:
    return CGF->EmitNaplesPUBuiltinExpr(BuiltinID, E);

Toolchain properties definition

In order to define the features of a toolchain, LLVM abstracts it through the ToolChain class. It is possible to define a new toolchain by using the inheritance mechanism and then specialising methods and attributes. 

class NaplesPUToolChain : public ToolChain {
   NaplesPUToolChain(const Driver &D, const llvm::Triple &Triple, const llvm::opt::ArgList &Args);
    ...
}

For example, if it is required to disable standard include directories, it is necessary to override the addClangTargetOptions function as follows: 

void NaplesPUToolChain::addClangTargetOptions(const ArgList &DriverArgs,
                                  ArgStringList &CC1Args) const {
  CC1Args.push_back("-nostdsysteminc");
 ...
}

clang/lib/Driver/Driver.cpp has been modified to be able to instance a toolchain object as follows: 

const ToolChain &Driver::getToolChain(const ArgList &Args,
                                      const llvm::Triple &Target) const {
 case llvm::Triple::naplespu:
        TC = llvm::make_unique<toolchains::NaplesPUToolChain>(*this, Target, Args);
        break;
}

clang/lib/Driver/ToolChains/NaplesPU.h also contains the class definition for the Linker tool. 

class NaplesPULinker : public Tools {
   ConstructJob(...);
    ...
}
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