Global Data Flow Analysis in Compiler Optimization: Enhancing Program Performance

Unlike local analysis (which only examines individual program statements or blocks), global analysis considers the entire control flow graph (CFG), providing a more complete understanding of the program’s behavior. This broader perspective is essential for performing many advanced compiler optimizations. After knowing the concepts of optimizations in basic blocks and loops, in this module we will study about how to perform optimization by means of data flow analysis. We shall discuss the need and usage of data- flow analysis from a code optimization perspective.

Data flow graph¶

Whenever the vulnerable module Z_DYN_CODE is scanned as part of its compilation unit, its vulnerable character is detected and uniquely identified by the red source code lines. If I integrate modules from other developers, departments or companies, I have to rely on someone else’s decision on whether a detected finding is considered critical or not. A call of a procedure with x as a parameter or a procedure that can access x because x is in the scope of the procedure.

• GenS is theset of definitions “generated” by S while killS is the set of definitionsthat never reach the end of S.
• It would generally be unacceptable to use anunsafe policy — one that sped up the code at the expense of changing what theprogram computes.
• In addition, a “point” can also be the position above the first statement and following the last statement.
• If variable x is not put in INJB orOUTp9], then there is no path from the beginning or end of block B, respectively, along which x might be used.
• Many data-flow problemscan be solved by synthesized translation to compute gen and kill.
• For thecase of reaching definitions, then, we call a set of definitions safe orconservative if the estimate is a superset of the true set of reachingdefinitions.

Using local data flow¶

It forms the foundation for a wide variety of compiler optimizations and program verification techniques. A program’s control-flow graph (CFG) is used to determine those parts of a program to which a particular value assigned to a variable might propagate. The information gathered is often used by compilers when optimizing a program. Other commonly used data-flow analyses include live variable analysis, available expressions, constant propagation, and very busy expressions, each serving a distinct purpose in compiler optimization passes. Global data flow analysis is a technique used in compiler optimization to gather information about how variables and data are used throughout an entire program.

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The first example shows a scenario in which the input parameter iv_param of a function module is not directly provided to the dynamic code part. A data flow analysis detects that the value of iv_param is assigned to lv_param and lv_param is used as input in the dynamic code. We assume that any graph-theoretic path in the flow graph is also an execution path, i.e., a path that is executed when the program is run with least one possible input. When we compare the computed gen with the “true” gen we discover that the true gen is always a subset of the computed gen. on the other hand, the true kill is always a superset of the computed kill.

• In this code, at line 3 the initial assignment is useless and x +1 expression can be simplified as 7.
• Since x + y is never in OUT ENTRY , andit is never generated along the path in question, we can show by induction onthe length of the path that x + y is eventually removed from I N ‘ s and O U T ‘ s along that path.
• One could expect that the problem in the function module Z_DYN_CODE was detected in the past since a function module that is called must have been checked already separately before (e.g. because it was transported before into the QA and/or production system).
• Pi is the end of some block and pi+1 is the beginning of a successor block.
• This can be guaranteedby imposing constraints on the combination of the value domain of the states, the transfer functions and the join operation.

The Market leading solution, Onapsis C4CA and other tools in the market follow different approaches with regard to this data flow analysis and the resulting finding management. While some tools only start a local data flow analysis, C4CA optionally executes a global data flow analysis. If the control paths are evident from the syntax, then data-flow equations can be set up and solved in a syntax directed manner. When programs can contain goto statements or even the more disciplined break and continue statements, the approach we have taken must be modified to take the actual control paths into account. As discussed already, a definition of a variable ‘x’ is a state ment that assigns or may assign a value to ‘x’.

Basic principles

The data flow graph is computed using classes to model the program elements that represent the graph’s nodes.The flow of data between the nodes is modeled using predicates to compute the graph’s edges. The live variable analysis calculates for each program point the variables that may be potentially read afterwards before their next write update. The result is typically used bydead programmer skills code elimination to remove statements that assign to a variable whose value is not used afterwards.

Note the steps must be done in the correct order, as x could be the same as y orz. After we reach the end of the block,S is the set of generated SQL and Data Analyst/BI Analyst job expressionsfor the block. The set of killed expressions is all expressions, say y + z, such that either y or zis defined in the block, and y + z isnot generated by the block. An important use for live-variable information is register allocationfor basic blocks. After a value is computed in a register, and presumably used within ablock, it is not necessary to store that value if it is dead at the end of theblock.