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Department of Computer Science and Technology


Course pages 2020–21

Optimising Compilers

Principal lecturer: Dr Timothy Jones
Taken by: Part II CST 50%, Part II CST 75%
Hours: 16
Suggested hours of supervisions: 4
Prerequisites: Compiler Construction
Past exam questions


The aims of this course are to introduce the principles of program optimisation and related issues in decompilation. The course will cover optimisations of programs at the abstract syntax, flowgraph and target-code level. It will also examine how related techniques can be used in the process of decompilation.


  • Introduction and motivation. Outline of an optimising compiler. Optimisation partitioned: analysis shows a property holds which enables a transformation. The flow graph; representation of programming concepts including argument and result passing. The phase-order problem.
  • Kinds of optimisation. Local optimisation: peephole optimisation, instruction scheduling. Global optimisation: common sub-expressions, code motion. Interprocedural optimisation. The call graph.
  • Classical dataflow analysis. Graph algorithms, live and avail sets. Register allocation by register colouring. Common sub-expression elimination. Spilling to memory; treatment of CSE-introduced temporaries. Data flow anomalies. Static Single Assignment (SSA) form.
  • Higher-level optimisations. Abstract interpretation, Strictness analysis. Constraint-based analysis, Control flow analysis for lambda-calculus. Rule-based inference of program properties, Types and effect systems. Points-to and alias analysis.
  • Target-dependent optimisations. Instruction selection. Instruction scheduling and its phase-order problem.
  • Decompilation. Legal/ethical issues. Some basic ideas, control flow and type reconstruction.


At the end of the course students should

  • be able to explain program analyses as dataflow equations on a flowgraph;
  • know various techniques for high-level optimisation of programs at the abstract syntax level;
  • understand how code may be re-scheduled to improve execution speed;
  • know the basic ideas of decompilation.

Recommended reading

* Nielson, F., Nielson, H.R. and Hankin, C.L. (1999). Principles of program analysis. Springer. Good on part A and part B.
Appel, A. (1997). Modern compiler implementation in Java/C/ML (3 editions).
Muchnick, S. (1997). Advanced compiler design and implementation. Morgan Kaufmann.
Wilhelm, R. (1995). Compiler design. Addison-Wesley.
Aho, A.V., Sethi, R. and Ullman, J.D. (2007). Compilers: principles, techniques and tools. Addison-Wesley (2nd ed.).