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

, 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. Unlike human-readable source code, bytecodes are compact numeric codes, constants, and references (normal ...

used by

optimizing compiler An optimizing compiler is a compiler designed to generate code that is optimized in aspects such as minimizing program execution time, memory usage, storage size, and power consumption. Optimization is generally implemented as a sequence of op ...

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, 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 (computer science), expression results to a limited number of processor registers. Register allocation can happen over a basic bloc ...

. 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 (ANF).

Examples

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

assembly language In computing, assembly language (alternatively 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 bet ...

. 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 mathematics, a quadratic equation () is an equation that can be rearranged in standard form as
ax^2 + bx + c = 0\,,
where the variable (mathematics), variable  represents an unknown number, and , , and  represent known numbers, where . (If  and ...
.
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, 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 ...

. 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