52 lines
981 B
Markdown
52 lines
981 B
Markdown
# 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 |