ARM: Efficient C for ARM: Optimising

by David Thomas on

Optimising

  • The goal is to write C code which compiles into perfect, efficient, ARM code which:
    • Runs as fast as possible, and
    • Is as small as possible.
  • Often these goals are contradictory.
  • This process is called optimisation.
  • Optimisation is not the same as fixing, reworking or refactoring your code.
  • Optimisation takes time and may reduce readability.

Remarks

Optimisation is an iterative process, often involving much discovery.

When To Optimise

  • Only optimise heavily-used routines.
  • Discover heavily-used routines by:
    • Inspection
    • Using a profiler
  • The act of discovery is often surprising; sometimes embarrassing:

He was calling strlen() in a loop.

Remarks

It might be worth optimising big routines for space.

Inspection is important: just reading the code can show up a multitude of problems.

Sometimes it’s valuable to re-visit code you’ve written earlier. A fresh perspective can make a difference to how you view the code.

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