# 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

```mermaid
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