In computing, a basic block is a straight-line piece of code without any jumps or jump targets in the middle; jump targets, if any, start a block, and jumps end a block. Basic blocks are usually the basic unit to which compiler optimizations are applied in compiler theory. Basic blocks form the vertices or nodes in a control flow graph.

More formally, we say a sequence of instructions forms a basic block if the instruction in each position dominates, or always executes before, all those in later positions, and no other instruction executes between two instructions in the sequence. 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 algorithm for generating basic blocks from a listing of code is simple: you scan 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.

Instructions that end a basic block include

• Unconditional and conditional branches, both direct and indirect.
• Returns to a calling procedure
• Instructions which may throw an exception
• Function calls can be at the end of a basic block if they may not return, such as functions which throw exceptions or special calls like C's longjmp and exit.

Instructions which begin a new basic block include

• Procedure entry points
• Targets of jumps or branches
• "Fall-through" instructions following some conditional branches
• Instructions following ones that throw exceptions
• Exception handlers

Note that, because control can never pass through the end of a basic block, some block boundaries may have to be modified after finding the basic blocks. In particular, fall-through conditional branches must be changed to two-way branches, and function calls throwing exceptions must have unconditional jumps added after them. Doing these may require adding labels to the beginning of other blocks.

See also: control flow graph