Basic blocks and flow graphs in Compiler Design - Step-by-Step Execution

Concept Flow - Basic blocks and flow graphs

Start: Code Sequence

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Identify Leaders

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Form Basic Blocks

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Create Nodes for Each Block

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Analyze Control Flow

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Draw Edges Between Blocks

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Result: Flow Graph

The process starts by finding leaders in code, grouping instructions into basic blocks, then connecting these blocks based on control flow to form a flow graph.

Execution Sample

Compiler Design

1: a = 5
2: if a > 0 goto 4
3: a = a - 1
4: print(a)

This code is divided into basic blocks and connected to form a flow graph showing possible execution paths.

Analysis Table

Step	Action	Details	Basic Blocks Formed	Flow Graph Edges
1	Identify Leaders	Line 1 is first instruction, line 3 follows jump at 2, line 4 is target of jump	Leaders: 1,3,4	None yet
2	Form Basic Blocks	Block1: lines 1-2; Block2: line 3; Block3: line 4	Block1: [1,2], Block2: [3], Block3: [4]	None yet
3	Create Nodes	Each block becomes a node	Nodes: B1, B2, B3	None yet
4	Analyze Control Flow	From B1: if condition true -> B3; else -> B2	Same blocks	Edges: B1->B3 (if true), B1->B2 (if false)
5	Add Edge	From B2 to B3 (sequential flow)	Same blocks	Edges: B1->B3, B1->B2, B2->B3
6	Flow Graph Complete	All blocks connected based on control flow	3 blocks	Edges: B1->B3, B1->B2, B2->B3

💡 All basic blocks identified and connected; flow graph represents program control flow.

State Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	Final
Leaders	[]	[1,3,4]	[1,3,4]	[1,3,4]	[1,3,4]
Basic Blocks	[]	[]	[B1:1-2, B2:3, B3:4]	[B1:1-2, B2:3, B3:4]	[B1:1-2, B2:3, B3:4]
Flow Graph Edges	[]	[]	[]	[B1->B3, B1->B2]	[B1->B3, B1->B2, B2->B3]

Key Insights - 3 Insights

Why is the first instruction always a leader?

Because execution always starts at the first instruction, so it must begin a basic block (see execution_table step 1).

Why do we create edges from one block to another?

Edges represent possible jumps or sequential flow between blocks, showing how control moves (see execution_table steps 4 and 5).

Can a basic block have multiple entry points?

No, by definition a basic block has only one entry point to keep execution linear without jumps inside (see variable_tracker Basic Blocks).

Visual Quiz - 3 Questions

Look at the execution_table at step 1, which lines are identified as leaders?

A Lines 1 and 3

B Lines 2 and 3

C Lines 1, 3 and 4

D Lines 2 and 4

💡 Hint

Check the 'Details' and 'Basic Blocks Formed' columns at step 1 in execution_table.

At which step are edges between basic blocks first added to the flow graph?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look at the 'Flow Graph Edges' column in execution_table to see when edges appear.

If the jump target was line 3 instead of line 4, how would the leaders change?

A Leaders would be lines 2 and 4

B Leaders would be lines 1 and 3

C Leaders would be lines 1 and 4

D Leaders would be lines 2 and 3

💡 Hint

Leaders include the first instruction and any jump targets (see variable_tracker Leaders).

Concept Snapshot

Basic blocks are sequences of instructions with one entry and one exit.
Leaders mark starts of blocks: first instruction, jump targets, and instructions after jumps.
Flow graphs connect blocks with edges showing possible control flow.
Used in compilers to analyze and optimize code execution paths.

Full Transcript

Basic blocks and flow graphs help organize code into manageable chunks for analysis. First, leaders are identified as the first instruction, jump targets, and instructions following jumps. These leaders mark the start of basic blocks, which are sequences of instructions executed linearly without jumps inside. Each basic block becomes a node in the flow graph. Edges between nodes represent possible control flow paths, such as jumps or sequential execution. This structure helps compilers understand program flow and optimize code effectively.