Type Systems for Optimizing Stack-based Code

Presented by:- Nebiyou Enbakom

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IntroductionDead Code EliminationStore/Load+ EliminationRelated WorkConclusionReview

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Optimizing bytecode directly offers challenges not present in high orintermediate-level program optimizations. The following reasons can be outlined

Expressions and statements are not explicit. In a naive approach, a reconstruction of expression trees from instructions would be required for many optimizations.

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Related instructions are not necessarily next to each other. A value could be put on the stack, a number of other instructions executed and only then the value used and popped. This means that related instructions, for example those that put a value on a stack, and those that consume it, can be arbitrarily far apart. Links between them need to be found during the analysis.

A single expression can span several different basic blocks. The Java Virtual Machine specification does not require zero stack depth at control flow junctions, so an expression used in a basic block can be partially computed in other basic blocks.

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The analyses and optimizations in this paper address dead stores load-pop pairs duplicating loads and store-load pairs, which are typical optimization situations in stack-based code.

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Dead code elimination Dead Code elimination optimization

removes program statements that do not affect the values of variables that are live at the end of the program.

For example the program x := z + y could be compiled into

0, load z 1, load y 2, add 3, store x 4.

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If the analysis shows that x is dead, then in the intermediate code, the assignment to x can be deleted. In the stack-based code however, not only the store instruction on line 3, but also lines 0-2 should be deleted.

In stack-based code optimizations statements and expression can span several basic blocks.

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THANK YOU!

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