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High-level language constructs can introduce substantial run-time overhead if we naively translate each construct independently into machine code. This chapter discusses how to eliminate many of these inefficiencies. Elimination of unnecessary instructions in object code, or the replacement of one sequence of instructions by a faster sequence of instructions that does the same thing is usually called "code improvement" or "code optimization."
Local code optimization (code improvement within a basic block) was intro-duced in Section 8.5. This chapter deals with global code optimization, where improvements take into account what happens across basic blocks. We begin in Section 9.1 with a discussion of the principal opportunities for code improve-ment.
Most global optimizations are based on data-flow analyses, which are algo-rithms to gather information about a program. The results of data-flow analyses all have the same form: for each instruction in the program, they specify some property that must hold every time that instruction is executed. The analyses differ in the properties they compute. For example, a constant-propagation analysis computes, for each point in the program, and for each variable used by the program, whether that variable has a unique constant value at that point. This information may be used to replace variable references by constant values, for instance. As another example, a liveness analysis determines, for each point in the program, whether the value held by a particular variable at that point is sure to be overwritten before it is read. If so, we do not need to preserve that value, either in a register or in a memory location.
We introduce data-flow analysis in Section 9.2, including several important examples of the kind of information we gather globally and then use to improve the code. Section 9.3 introduces the general idea of a data-flow framework, of which the data-flow analyses in Section 9.2 are special cases. We can use essentially the same algorithms for all these instances of data-flow analysis, and we can measure the performance of these algorithms and show their correctness on all instances, as well. Section 9.4 is an example of the general framework that does more powerful analysis than the earlier examples. Then, in Section 9.5 we consider a powerful technique, called "partial redundancy elimination," for optimizing the placement of each expression evaluation in the program. The solution to this problem requires the solution of a variety of different data-flow problems.
In Section 9.6 we take up the discovery and analysis of loops in programs.
The identification of loops leads to another family of algorithms for solving data-flow problems that is based on the hierarchical structure of the loops of a well-formed ("reducible") program. This approach to data-flow analysis is covered in Section 9.7. Finally, Section 9.8 uses hierarchical analysis to eliminate induction variables (essentially, variables that count the number of iterations around a loop). This code improvement is one of the most important we can make for programs written in commonly used programming languages.
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