Basic block

In syntax analysis, a basic block is a straight-line code sequence with no branches in except to the entry and no branches out except at the exit. This restricted form makes a basic block highly amenable to analysis.Compilers usually decompose programs into their basic blocks as a first step in the analysis process. Basic blocks form the vertices or nodes in a control flow graph.

The code in a basic block has:

Under these circumstances, whenever the first instruction in a basic block is executed, the rest of the instructions are necessarily executed exactly once, in order.

The code may be source code, assembly code or some other sequence of instructions.

More formally, a sequence of instructions forms a basic block if:

This definition is more general than the intuitive one in some ways. For example, it allows unconditional jumps to labels not targeted by other jumps. This definition embodies the properties that make basic blocks easy to work with when constructing an algorithm.

The blocks to which control may transfer after reaching the end of a block are called that block's successors, while the blocks from which control may have come when entering a block are called that block's predecessors. The start of a basic block may be jumped to from more than one location.

The algorithm for generating basic blocks from a listing of code is simple: the analyser scans over the code, marking block boundaries, which are instructions which may either begin or end a block because they either transfer control or accept control from another point. Then, the listing is simply "cut" at each of these points, and basic blocks remain.

Note that this method does not always generate maximal basic blocks, by the formal definition, but they are usually sufficient (maximal basic blocks are basic blocks which cannot be extended by including adjacent blocks without violating the definition of a basic block).

Input: A sequence of instructions (mostly three-address code).
Output: A list of basic blocks with each three-address statement in exactly one block.

Step 1. Identify the leaders in the code. Leaders are instructions which come under any of the following 3 categories :

Step 2. Starting from a leader, the set of all following instructions until and not including the next leader is the basic block corresponding to the starting leader.

...
Wikipedia