Archives March 2023

Another often-used statement is multiclass. A multiclass allows you to define multiple records at once. Let’s expand the example to show why this can be useful.
The definition of an add instruction is very simplistic. In reality, a CPU often has several add instructions. A common variant is that one instruction has two register operands while another instruction has one register operand and an immediate operand, which is a small number. Assume that for the instruction having an immediate operand, the designer of the instruction set decided to mark them with i as a suffix. So, we end up with the add and addi instructions. Further, assume that the opcodes differ by 1. Many arithmetic and logical instructions follow this scheme; therefore, you want the definition to be as compact as possible.
The first challenge is that you need to manipulate values. There is a limited number of operators that you can use to modify a value. For example, to produce the sum of 1 and the value of the field opcode, you write:

!add(opcode, 1)

Such an expression is best used as an argument for a class. Testing a field value and then changing it based on the found value is generally not possible because it requires dynamic statements that are not available. Always remember that all calculations are done while the records are constructed!
In a similar way, strings can be concatenated:

!strconcat(mnemonic,”i”)

Because all operators begin with an exclamation mark (!), they are also called bang operators. You find a full list of bang operators in the Programmer’s Reference: https://llvm.org/docs/TableGen/ProgRef.htmlappendix-a-bang-operators.
Now, you can define a multiclass. The Inst class serves again as the base:

class Inst {
string Mnemonic = mnemonic;
int Opcode = opcode;
}

The definition of a multiclass is a bit more involved, so let’s do it in steps:

1. The definition of a multiclass uses a similar syntax to classes. The new multiclass is named InstWithImm and has two parameters, mnemonic and opcode:

multiclass InstWithImm {

2. First, you define an instruction with two register operands. As in a normal record definition, you use the def keyword to define the record, and you use the Inst class to create the record content. You also need to define an empty name. We will explain later why this is necessary: def “”: Inst;
3. Next, you define an instruction with the immediate operand. You derive the values for the mnemonic and the opcode from the parameters of the multiclass, using bang operators. The record is named I: def I: Inst;
4. That is all; the class body can be closed, like so:

}

To instantiate the records, you must use the defm keyword:

defm ADD : InstWithImm<“add”, 0xA0>;

These statements result in the following:

1. The Inst<“add”, 0xA0> record is instantiated. The name of the record is the concatenation of the name following the defm keyword and of the name following def inside the multiclass statement, which results in the name ADD.
2. The Inst<“addi”, 0xA1> record is instantiated and, following the same scheme, is given the name ADDI.
   Let’s verify this claim with llvm-tblgen:

$ llvm-tblgen –print-records inst.td
————- Classes —————–
class Inst {
string Mnemonic = Inst:mnemonic;
int Opcode = Inst:opcode;
}
————- Defs —————–
def ADD { // Inst
string Mnemonic = “add”;
int Opcode = 160;
}
def ADDI { // Inst
string Mnemonic = “addi”;
int Opcode = 161;
}

Using a multiclass, it is very easy to generate multiple records at once. This feature is used very often!
A record does not need to have a name. Anonymous records are perfectly fine. Omitting the name is all you need to do to define an anonymous record. The name of a record generated by a multiclass is made up of two names, and both names must be given to create a named record. If you omit the name after defm, then only anonymous records are created. Similarly, if the def inside the multiclass is not followed by a name, an anonymous record is created. This is the reason why the first definition in the multiclass example used the empty name “”: without it, the record would be anonymous.