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Introduction toMachine-Independent OptimizationsPart 1

Y.N. Srikant

Department of Computer ScienceIndian Institute of Science

Bangalore 560 012

NPTEL Course on Compiler Design

Y.N. Srikant Introduction to Optimizations

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Outline of the Lecture

What is code optimization?

Types of code optimizationsIllustrations of code optimizations

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Machine-independent Code Optimization

Intermediate code generation process introduces many

inefcienciesExtra copies of variables, using variables instead ofconstants, repeated evaluation of expressions, etc.

Code optimization removes such inefciencies andimproves codeImprovement may be time, space, or power consumptionIt changes the structure of programs, sometimes of beyondrecognition

Inlines functions, unrolls loops, eliminates some

programmer-dened variables, etc.Code optimization consists of a bunch of heuristics andpercentage of improvement depends on programs (may bezero also)

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Examples of Machine-Independant Optimizations

Global common sub-expression eliminationCopy propagationConstant propagation and constant foldingLoop invariant code motion

Induction variable elimination and strength reductionPartial redundancy eliminationLoop unrollingFunction inlining

Tail recursion removalVectorization and ConcurrentizationLoop interchange, and loop blocking

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Data-ow Analysis

Code optimization needs information about the programwhich expressions are being recomputed in a function?Which expressions are partially redundant?which denitions reach a point?Which copies and constants can be propagated? Etc.

All such information is gathered through data-ow analysis

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Bubble Sort Running Example

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GCSE Conceptual Example

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GCSE on Running Example - 1

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GCSE on Running Example - 2

Y.N. Srikant Introduction to Optimizations

C P i R i E l

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Copy Propagation on Running Example

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GCSE d C P i R i E l

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GCSE and Copy Propagation on Running Example

Y.N. Srikant Introduction to Optimizations

C t t P ti d F ldi E l

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Constant Propagation and Folding Example

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L I i t C d ti E l

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Loop Invariant Code motion Example

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Strength Reduction

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Strength Reduction

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Induction Variable Elimination

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Induction Variable Elimination

Y.N. Srikant Introduction to Optimizations

Induction Variable Elimination and Strength Reduction

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Induction Variable Elimination and Strength Reduction

Y.N. Srikant Introduction to Optimizations

Partial Redundancy Elimination

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Partial Redundancy Elimination

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Unrolling a For-loop

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Unrolling a For-loop

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