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Abstract interpretation operates on a flowchart representation of the function called a '''control flow graph''' or '''CFG'''. Here is the CFG for the example function:
[[Image:treehydra-cfg.png]]
It looks like a pretty normal flowchart. Some features to note:
* All possible control transitions are shown as explicit edges.
* Statements always have exactly one operation and at most one assignment. Temporary variables are added if necessary. This will really simplify our job: instead of handling every kind of C++ statement, we have only have to analyze these simple statements.
A little terminology:
* Each node is called a '''basic block'''. The defining feature of a basic block is that there is a single entry point and a single exit--basic blocks are straight-line code.
* In different sources, statements may be called ''statements'', ''instructions'', ''triples'' (of a destination and up to 2 operands), ''quadruples'' or ''quads'' (counting the operator), or even ''tuples''. We'll call them ''instructions'', as that's the name used in Treehydra code.
* A '''program point''' is the (notional) point just before or after each instruction. The function has a well-defined state at program points, so our analysis facts will always refer to program points. E.g., at the program point just before 't1 := k + m', k is constant with value 2.
=Concrete Interpretation=
