Dataflow Analysis

Optimization means improving resource utilization not changing what the program computes. Resource utilization means many things:

Execution time
Code size
Network messages sent.

Basic Block (BB)

A BB is a maximum sequence of instructions with no labels, no jumps All instructions in a BB has fixed control flow.

Control Flow Graph

flowchart TD
    Entry --> A
    A[BB1] --> B[BB2]
    A --> C[BB3]
    B --> D[BB4]
    C --> D
    D --> E[BB5]
    E --> G[BB7]
    E --> F[BB6]
    G --> Exit
    F --> Exit

Local Optimization

Algebraic Simplification

x := x + 0 -> x := 0 y := y ** 2 -> y := y * y x := x * 8 -> x := x << 3 x := x * 15 -> t := x << 4; x := t - x

Constant Folding

x := 2 + 2 -> x := 4 if 2 < 0 jump L -> nop if 2 > 0 jump L -> jump L

But Constant folding can be dangerous on cross-compilation (in precision).

Unreachable Code

Dead Code Elimination

Global Optimization

It is not actually global but in control graph.

In Basic Block, there are a few instructions (4-8). So only local optimization, it is not quite optimizable. There are many cases where the optimization can be performed through the entire CFG.