TableGen Fundamentals

TableGen supports BCPL style "//" comments, which run to the end of the line, and it also supports nestable "/* */" comments.

TableGen files are strongly typed, in a simple (but complete) type-system. These types are used to perform automatic conversions, check for errors, and to help interface designers constrain the input that they allow. Every value definition is required to have an associated type.

TableGen supports a mixture of very low-level types (such as bit) and very high-level types (such as dag). This flexibility is what allows it to describe a wide range of information conveniently and compactly. The TableGen types are:

bit

A 'bit' is a boolean value that can hold either 0 or 1.

int

The 'int' type represents a simple 32-bit integer value, such as 5.

string

The 'string' type represents an ordered sequence of characters of arbitrary length.

bits<n>

A 'bits' type is an arbitrary, but fixed, size integer that is broken up into individual bits. This type is useful because it can handle some bits being defined while others are undefined.

list<ty>

This type represents a list whose elements are some other type. The contained type is arbitrary: it can even be another list type.

Class type

Specifying a class name in a type context means that the defined value must be a subclass of the specified class. This is useful in conjunction with the list type, for example, to constrain the elements of the list to a common base class (e.g., a list<Register> can only contain definitions derived from the "Register" class).

dag

This type represents a nestable directed graph of elements.

code

This represents a big hunk of text. This is lexically distinct from string values because it doesn't require escapeing double quotes and other common characters that occur in code.

To date, these types have been sufficient for describing things that TableGen has been used for, but it is straight-forward to extend this list if needed.

TableGen allows for a pretty reasonable number of different expression forms when building up values. These forms allow the TableGen file to be written in a natural syntax and flavor for the application. The current expression forms supported include:

?

uninitialized field

0b1001011

binary integer value

07654321

octal integer value (indicated by a leading 0)

7

decimal integer value

0x7F

hexadecimal integer value

"foo"

string value

[{ ... }]

code fragment

[ X, Y, Z ]<type>

list value. <type> is the type of the list element and is usually optional. In rare cases, TableGen is unable to deduce the element type in which case the user must specify it explicitly.

{ a, b, c }

initializer for a "bits<3>" value

value

value reference

value{17}

access to one bit of a value

value{15-17}

access to multiple bits of a value

DEF

reference to a record definition

CLASS<val list>

reference to a new anonymous definition of CLASS with the specified template arguments.

X.Y

reference to the subfield of a value

list[4-7,17,2-3]

A slice of the 'list' list, including elements 4,5,6,7,17,2, and 3 from it. Elements may be included multiple times.

(DEF a, b)

a dag value. The first element is required to be a record definition, the remaining elements in the list may be arbitrary other values, including nested `dag' values.

!strconcat(a, b)

A string value that is the result of concatenating the 'a' and 'b' strings.

!cast<type>(a)

A symbol of type type obtained by looking up the string 'a' in the symbol table. If the type of 'a' does not match type, TableGen aborts with an error. !cast<string> is a special case in that the argument must be an object defined by a 'def' construct.

!subst(a, b, c)

If 'a' and 'b' are of string type or are symbol references, substitute 'b' for 'a' in 'c.' This operation is analogous to $(subst) in GNU make.

!foreach(a, b, c)

For each member 'b' of dag or list 'a' apply operator 'c.' 'b' is a dummy variable that should be declared as a member variable of an instantiated class. This operation is analogous to $(foreach) in GNU make.

!head(a)

The first element of list 'a.'

!tail(a)

The 2nd-N elements of list 'a.'

!empty(a)

An integer {0,1} indicating whether list 'a' is empty.

!if(a,b,c)

'b' if the result of 'int' or 'bit' operator 'a' is nonzero, 'c' otherwise.

!eq(a,b)

'bit 1' if string a is equal to string b, 0 otherwise. This only operates on string, int and bit objects. Use !cast<string> to compare other types of objects.

Note that all of the values have rules specifying how they convert to values for different types. These rules allow you to assign a value like "7" to a "bits<4>" value, for example.

Value definitions define named entries in records. A value must be defined before it can be referred to as the operand for another value definition or before the value is reset with a let expression. A value is defined by specifying a TableGen type and a name. If an initial value is available, it may be specified after the type with an equal sign. Value definitions require terminating semicolons.

A record-level let expression is used to change the value of a value definition in a record. This is primarily useful when a superclass defines a value that a derived class or definition wants to override. Let expressions consist of the 'let' keyword followed by a value name, an equal sign ("="), and a new value. For example, a new class could be added to the example above, redefining the V field for all of its subclasses:

class D : C { let V = 0; }
def Z : D;

In this case, the Z definition will have a zero value for its "V" value, despite the fact that it derives (indirectly) from the C class, because the D class overrode its value.

TableGen permits the definition of parameterized classes as well as normal concrete classes. Parameterized TableGen classes specify a list of variable bindings (which may optionally have defaults) that are bound when used. Here is a simple example:

class FPFormat<bits<3> val> {
  bits<3> Value = val;
}
def NotFP      : FPFormat<0>;
def ZeroArgFP  : FPFormat<1>;
def OneArgFP   : FPFormat<2>;
def OneArgFPRW : FPFormat<3>;
def TwoArgFP   : FPFormat<4>;
def CompareFP  : FPFormat<5>;
def CondMovFP  : FPFormat<6>;
def SpecialFP  : FPFormat<7>;

In this case, template arguments are used as a space efficient way to specify a list of "enumeration values", each with a "Value" field set to the specified integer.

The more esoteric forms of TableGen expressions are useful in conjunction with template arguments. As an example:

class ModRefVal<bits<2> val> {
  bits<2> Value = val;
}

def None   : ModRefVal<0>;
def Mod    : ModRefVal<1>;
def Ref    : ModRefVal<2>;
def ModRef : ModRefVal<3>;

class Value<ModRefVal MR> {
  // Decode some information into a more convenient format, while providing
  // a nice interface to the user of the "Value" class.
  bit isMod = MR.Value{0};
  bit isRef = MR.Value{1};

  // other stuff...
}

// Example uses
def bork : Value<Mod>;
def zork : Value<Ref>;
def hork : Value<ModRef>;

This is obviously a contrived example, but it shows how template arguments can be used to decouple the interface provided to the user of the class from the actual internal data representation expected by the class. In this case, running tblgen on the example prints the following definitions:

def bork {      // Value
  bit isMod = 1;
  bit isRef = 0;
}
def hork {      // Value
  bit isMod = 1;
  bit isRef = 1;
}
def zork {      // Value
  bit isMod = 0;
  bit isRef = 1;
}

This shows that TableGen was able to dig into the argument and extract a piece of information that was requested by the designer of the "Value" class. For more realistic examples, please see existing users of TableGen, such as the X86 backend.

While classes with template arguments are a good way to factor commonality between two instances of a definition, multiclasses allow a convenient notation for defining multiple definitions at once (instances of implicitly constructed classes). For example, consider an 3-address instruction set whose instructions come in two forms: "reg = reg op reg" and "reg = reg op imm" (e.g. SPARC). In this case, you'd like to specify in one place that this commonality exists, then in a separate place indicate what all the ops are.

Here is an example TableGen fragment that shows this idea:

def ops;
def GPR;
def Imm;
class inst<int opc, string asmstr, dag operandlist>;

multiclass ri_inst<int opc, string asmstr> {
  def _rr : inst<opc, !strconcat(asmstr, " $dst, $src1, $src2"),
                 (ops GPR:$dst, GPR:$src1, GPR:$src2)>;
  def _ri : inst<opc, !strconcat(asmstr, " $dst, $src1, $src2"),
                 (ops GPR:$dst, GPR:$src1, Imm:$src2)>;
}

// Instantiations of the ri_inst multiclass.
defm ADD : ri_inst<0b111, "add">;
defm SUB : ri_inst<0b101, "sub">;
defm MUL : ri_inst<0b100, "mul">;
...

The name of the resultant definitions has the multidef fragment names appended to them, so this defines ADD_rr, ADD_ri, SUB_rr, etc. A defm may inherit from multiple multiclasses, instantiating definitions from each multiclass. Using a multiclass this way is exactly equivalent to instantiating the classes multiple times yourself, e.g. by writing:

def ops;
def GPR;
def Imm;
class inst<int opc, string asmstr, dag operandlist>;

class rrinst<int opc, string asmstr>
  : inst<opc, !strconcat(asmstr, " $dst, $src1, $src2"),
         (ops GPR:$dst, GPR:$src1, GPR:$src2)>;

class riinst<int opc, string asmstr>
  : inst<opc, !strconcat(asmstr, " $dst, $src1, $src2"),
         (ops GPR:$dst, GPR:$src1, Imm:$src2)>;

// Instantiations of the ri_inst multiclass.
def ADD_rr : rrinst<0b111, "add">;
def ADD_ri : riinst<0b111, "add">;
def SUB_rr : rrinst<0b101, "sub">;
def SUB_ri : riinst<0b101, "sub">;
def MUL_rr : rrinst<0b100, "mul">;
def MUL_ri : riinst<0b100, "mul">;
...

A defm can also be used inside a multiclass providing several levels of multiclass instanciations.

class Instruction<bits<4> opc, string Name> {
  bits<4> opcode = opc;
  string name = Name;
}

multiclass basic_r<bits<4> opc> {
  def rr : Instruction<opc, "rr">;
  def rm : Instruction<opc, "rm">;
}

multiclass basic_s<bits<4> opc> {
  defm SS : basic_r<opc>;
  defm SD : basic_r<opc>;
  def X : Instruction<opc, "x">;
}

multiclass basic_p<bits<4> opc> {
  defm PS : basic_r<opc>;
  defm PD : basic_r<opc>;
  def Y : Instruction<opc, "y">;
}

defm ADD : basic_s<0xf>, basic_p<0xf>;
...

// Results
def ADDPDrm { ...
def ADDPDrr { ...
def ADDPSrm { ...
def ADDPSrr { ...
def ADDSDrm { ...
def ADDSDrr { ...
def ADDY { ...
def ADDX { ...

defm declarations can inherit from classes too, the rule to follow is that the class list must start after the last multiclass, and there must be at least one multiclass before them.

class XD { bits<4> Prefix = 11; }
class XS { bits<4> Prefix = 12; }

class I<bits<4> op> {
  bits<4> opcode = op;
}

multiclass R {
  def rr : I<4>;
  def rm : I<2>;
}

multiclass Y {
  defm SS : R, XD;
  defm SD : R, XS;
}

defm Instr : Y;

// Results
def InstrSDrm {
  bits<4> opcode = { 0, 0, 1, 0 };
  bits<4> Prefix = { 1, 1, 0, 0 };
}
...
def InstrSSrr {
  bits<4> opcode = { 0, 1, 0, 0 };
  bits<4> Prefix = { 1, 0, 1, 1 };
}

TableGen supports the 'include' token, which textually substitutes the specified file in place of the include directive. The filename should be specified as a double quoted string immediately after the 'include' keyword. Example:

include "foo.td"

"Let" expressions at file scope are similar to "let" expressions within a record, except they can specify a value binding for multiple records at a time, and may be useful in certain other cases. File-scope let expressions are really just another way that TableGen allows the end-user to factor out commonality from the records.

File-scope "let" expressions take a comma-separated list of bindings to apply, and one or more records to bind the values in. Here are some examples:

let isTerminator = 1, isReturn = 1, isBarrier = 1, hasCtrlDep = 1 in
  def RET : I<0xC3, RawFrm, (outs), (ins), "ret", [(X86retflag 0)]>;

let isCall = 1 in
  // All calls clobber the non-callee saved registers...
  let Defs = [EAX, ECX, EDX, FP0, FP1, FP2, FP3, FP4, FP5, FP6, ST0,
              MM0, MM1, MM2, MM3, MM4, MM5, MM6, MM7,
              XMM0, XMM1, XMM2, XMM3, XMM4, XMM5, XMM6, XMM7, EFLAGS] in {
    def CALLpcrel32 : Ii32<0xE8, RawFrm, (outs), (ins i32imm:$dst,variable_ops),
                           "call\t${dst:call}", []>;
    def CALL32r     : I<0xFF, MRM2r, (outs), (ins GR32:$dst, variable_ops),
                        "call\t{*}$dst", [(X86call GR32:$dst)]>;
    def CALL32m     : I<0xFF, MRM2m, (outs), (ins i32mem:$dst, variable_ops),
                        "call\t{*}$dst", []>;
  }

File-scope "let" expressions are often useful when a couple of definitions need to be added to several records, and the records do not otherwise need to be opened, as in the case with the CALL* instructions above.

It's also possible to use "let" expressions inside multiclasses, providing more ways to factor out commonality from the records, specially if using several levels of multiclass instanciations. This also avoids the need of using "let" expressions within subsequent records inside a multiclass.

multiclass basic_r<bits<4> opc> {
  let Predicates = [HasSSE2] in {
    def rr : Instruction<opc, "rr">;
    def rm : Instruction<opc, "rm">;
  }
  let Predicates = [HasSSE3] in
    def rx : Instruction<opc, "rx">;
}

multiclass basic_ss<bits<4> opc> {
  let IsDouble = 0 in
    defm SS : basic_r<opc>;

  let IsDouble = 1 in
    defm SD : basic_r<opc>;
}

defm ADD : basic_ss<0xf>;