Upwards exposed uses

In compiler theory, upwards exposed uses or reachable uses[1] are all uses of a variable that are reachable from a point in the program. A use of variable is a point or statement where that variable is referenced (read) but not changed. A use of a variable A is reachable from a point p if there exists a control-flow path in the control-flow graph from p to the use with not definition of A on the path.

A reachable uses analysis is a data-flow analysis[1] to calculate all reachable uses of a program. It is very similar to the liveness analysis. A variable is live in a point in the program if it has one or more reachable uses. Compared to the liveness analysis, reachable uses analysis provides additional information where the variable is used.

## Uses

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Upwards exposed uses occurs in the copy propagation stage of program compilation.[2] During the copy propagation stage, instances of a target are replaced with assignments to their values. During this process, it is necessary for the compiler to understand which instances of a target are being accessed so that appropriate substitution may occur, related to the concept of reaching definition in reaching analysis.[3] This is done with the purpose of simplifying code before execution: if the number of upwards exposed uses of an assignment is zero, it does not contribute to the end result of the code and can be safely removed.[1] This is also useful for improving code security during the compilation stages.[4]

## Example

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Consider the following pseudocode:

x = 1
y = z

if False:
x = 0
else:
x = y + 2

It is safe to assume that line 5 will never occur, as demonstrated by the number of upwards exposed uses for this point being zero. This can therefore be simplified:

y = z
x = z + 2

This leads to a result that is less complex to compile and more efficient to run.[2] This also meets the definition of reaching definition: In this context, upwards flow analysis was the technique used to demonstrate the needs for reaching definition. Additional techniques allow for more complex analysis of more deeply intertwined or complex control flow problems, such as those with various forms of loops.[4]

## See also

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* Dead code elimination
* Reaching definition
* Live-variable analysis

## References

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1. ^ a b c Harrold, Mary Jean (Fall 2009). "Basic Analysis" (PDF). Georgia Tech - College of Computing. CS 6340: Software Analysis and Testing. Atlanta, Georgia, USA: Georgia Institute of Technology. Archived (PDF) from the original on 2020-06-20. Retrieved 2020-06-12.
2. ^ a b Aho, Alfred Vaino; Lam, Monica Sin-Ling; Sethi, Ravi; Ullman, Jeffrey David (2006). Compilers: Principles, Techniques, and Tools (2 ed.). Boston, Massachusetts, USA: Addison-Wesley. ISBN 0-321-48681-1. OCLC 70775643.
3. ^ Kore, Aamod (2020). "Upwards Exposed Uses". Toronto, Ontario, Canada: Department of Computer Science, University of Toronto. Archived from the original on 2020-06-20. Retrieved 2020-06-12.
4. ^ a b Bergeretti, Jean-Francois; Carré, Bernard A. (1985-01-02). "Information-Flow and Data-Flow Analysis of while-Programs". ACM Transactions on Programming Languages and Systems. 7 (1). Association for Computing Machinery: 37–61\. doi:10.1145/2363.2366. S2CID 19682896.

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Compiler optimizations

Basic block|

* Peephole optimization
* Local value numbering

Loop|

* Automatic parallelization
* Automatic vectorization
* Induction variable
* Loop fusion
* Loop-invariant code motion
* Loop inversion
* Loop interchange
* Loop nest optimization
* Loop splitting
* Loop unrolling
* Loop unswitching
* Software pipelining
* Strength reduction

Data-flow
analysis|

* Available expression
* Common subexpression elimination
* Constant folding
* Dead store elimination
* Induction variable recognition and elimination
* Live-variable analysis
* Upwards exposed uses
* Use-define chain
* Reaching definitions

SSA-based|

* Global value numbering
* Sparse conditional constant propagation

Code generation|

* Instruction scheduling
* Instruction selection
* Register allocation
* Rematerialization

Functional|

* Deforestation
* Tail-call elimination

Global|

* Interprocedural optimization

Other|

* Bounds-checking elimination
* Compile-time function execution
* Dead-code elimination
* Expression templates
* Inline expansion
* Jump threading
* Partial evaluation
* Profile-guided optimization

Static analysis|

* Alias analysis
* Array-access analysis
* Control-flow analysis
* Data-flow analysis
* Dependence analysis
* Escape analysis
* Pointer analysis
* Shape analysis
* Value range analysis

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