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Concept
Basic block
Summary
In compiler construction, 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:
One entry point, meaning that no code within it is the destination of a jump instruction anywhere in the program.
One exit point, meaning that only the last instruction can cause the program to begin executing code in a different basic block.
Under these circumstances, whenever the first instruction in a basic block is executed, the rest of the instructions are necessarily executed exactly once and 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:
The instruction in each position dominates (always executes before) all those in later positions.
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 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 that 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.
Official source
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