Intermediate Code Generation #

Intermediate Code Generation is a crucial phase of the compilation process where the compiler translates the source code, which has passed lexical, syntax, and semantic analysis, into an intermediate representation that makes it easier to apply optimizations and generate the final machine code.

Intermediate Code Generation is the fourth phase of the compilation process. Once the source code has passed lexical, syntax, and semantic analysis, the compiler generates an intermediate representation of the code. This intermediate representation (IR) lies between the high-level source code and the low-level machine code, making it easier for the compiler to perform optimizations and generate the final machine code.

There are several types of intermediate representations, with the most common being:

• Three-address code (TAC): In three-address code, each instruction operates on at most three operands, usually representing assignments, arithmetic operations, or control flow. TAC resembles assembly language but uses an infinite set of temporary variables (t1, t2, t3, …).

• Abstract syntax tree (AST): As generated during the parsing phase, the AST can also serve as an intermediate representation, with additional annotations from semantic analysis, such as symbol table information and type information.

• Static Single Assignment (SSA): In SSA form, each variable is assigned exactly once, and every variable is defined before it is used. This form makes it easier to reason about the program’s data flow, simplifying optimizations like dead code elimination and constant propagation.

• Bytecode: Some compilers, particularly those targeting virtual machines (e.g., Java, .NET), generate an intermediate bytecode representation. Bytecode is a low-level, platform-agnostic representation that can be executed by a virtual machine or further compiled to native machine code.

Let’s consider an example of intermediate code generation using three-address code. Given the following high-level C-like code:

int main() {
    int a = 10;
    int b = 20;
    int c = a + b * 2;
    return c;
}

A possible three-address code representation could be:

t1 = 20
t2 = 2
t3 = t1 * t2
t4 = 10
t5 = t4 + t3
c = t5
return c

In this example, the compiler has translated the high-level code into a sequence of simpler instructions, each involving at most three operands. This intermediate representation makes it easier for the compiler to apply various optimizations before generating the final machine code.

After generating the intermediate code, the compiler may perform several optimization passes to improve the code’s efficiency, such as constant folding, dead code elimination, loop optimizations, or function inlining. These optimizations help reduce the code’s size, improve its runtime performance, or lower its power consumption.