computer science Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to Applied science, practical discipli ...

, three-address code (often abbreviated to TAC or 3AC) is an

intermediate code Bytecode (also called portable code or p-code) is a form of instruction set designed for efficient execution by a software Interpreter (computing), interpreter. Unlike Human-readable code, human-readable source code, bytecodes are compact nume ...

used by

optimizing compiler In computing, an optimizing compiler is a compiler that tries to minimize or maximize some attributes of an executable computer program. Common requirements are to minimize a program's execution time, memory footprint, storage size, and power cons ...

s to aid in the implementation of code-improving transformations. Each TAC instruction has at most three operands and is typically a combination of assignment and a binary operator. For example, t1 := t2 + t3. The name derives from the use of three operands in these statements even though instructions with fewer operands may occur. Since three-address code is used as an intermediate language within compilers, the operands will most likely not be concrete memory addresses or

processor registers A processor register is a quickly accessible location available to a computer's Processor (computing), processor. Registers usually consist of a small amount of fast Computer storage, storage, although some registers have specific hardware functi ...

, but rather symbolic addresses that will be translated into actual addresses during

register allocation In compiler optimization, register allocation is the process of assigning local automatic variables and expression results to a limited number of processor registers. Register allocation can happen over a basic block (''local register allocation' ...

. It is also not uncommon that operand names are numbered sequentially since three-address code is typically generated by the compiler. A refinement of three-address code is

A-normal form In computer science, A-normal form (abbreviated ANF) is an Intermediate language, intermediate representation of program (computer science), programs in functional compilers. In ANF, all argument (computer science), arguments to a function (comput ...

(ANF).

Examples

In three-address code, this would be broken down into several separate instructions. These instructions translate more easily to

assembly language In computer programming, assembly language (or assembler language, or symbolic machine code), often referred to simply as Assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence be ...

. It is also easier to detect common sub-expressions for shortening the code. In the following example, one calculation is composed of several smaller ones:

# Calculate one solution to the Quadratic equation 

In algebra, a quadratic equation () is any equation that can be rearranged in standard form as 
ax^2 + bx + c = 0\,,
where  represents an unknown (mathematics), unknown value, and , , and  represent known numbers, where . (If  and  then the equati ...
.
x = (-b + sqrt(b^2 - 4*a*c)) / (2*a)

t1 := b * b
t2 := 4 * a
t3 := t2 * c
t4 := t1 - t3
t5 := sqrt(t4)
t6 := 0 - b
t7 := t5 + t6
t8 := 2 * a
t9 := t7 / t8
x := t9

Three-address code may have conditional and unconditional jumps and methods of accessing memory. It may also have methods of calling functions, or it may reduce these to jumps. In this way, three-address code may be useful in

control-flow analysis In computer science, control-flow analysis (CFA) is a static-code-analysis technique for determining the control flow of a program. The control flow is expressed as a control-flow graph (CFG). For both functional programming languages and object- ...

. In the following C-like example, a loop stores the squares of the numbers between 0 and 9: ... for (i = 0; i < 10; ++i) ...

     t1 := 0                ; initialize i
L1:  if t1 >= 10 goto L2    ; conditional jump
     t2 := t1 * t1          ; square of i
     t3 := t1 * 4           ; word-align address
     t4 := b + t3           ; address to store i*i
     *t4 := t2              ; store through pointer
     t1 := t1 + 1           ; increase i
     goto L1                ; repeat loop
L2:

References

Compiler construction Articles with example C code

Examples

See also

References