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Optimization

Optimization

Optimization is a series of actions taken by the compiler on your program's code generation process to reduce:

• Number of instructions (code space optimization)

• Memory access time (time space optimization)

• Power consumption

By default, the compiler doesn't invoke any optimization on your program (i.e., Optimization level 0; -O0). Optimization can be enabled by using the compiler optimization flags.

Optimization Levels (GCC Compiler Flags)

You need to work with different optimization levels to find out what works best for your situation.

-O0

• No optimization (default)

• Not recommended for productions if you have limited CODE and RAM space

• Has fastest compilation time

• Debugging friendly and used during development

• Code that works with -O0 optimization level may not work with higher optimization levels. Code needs to be verified again when it is compiled with different optimization level.

• Note that many other compilers do substantial optimization even if "no optimization" is specified. With gcc, it is very unusual to use -O0 for production if execution time is of any concern, since -O0 means (almost) no optimization. This difference between gcc and other compilers should be kept in mind when doing performance comparisons.

-O1

• Moderate optimization

• Optimizes reasonably well (to decrease memory access time and code space) but does not degrade compilation time significantly

-O2

• Full optimization

• Generates highly optimized code and has the slowest compilation time

• Not debugging friendly

-O3

• Full optimization as in -O2 + some more aggressive optimization steps taken by the compiler

• Has slowest compilation time

• May cause bugs in the program

• Not debugging friendly

-Os

• Optimizes space usage (code and data) of resulting program.

Effect of Optimization

• No optimization (-O0)

  xxxxxxxxxx
  4
  1
  arm-none-eabi-size   pin_read.elf 
  2
     text    data     bss     dec     hex filename
  3
      820       8    1568    2396     95c pin_read.elf
  4
  Finished building: default.size.stdout
  

  text size = 820

• -01

  xxxxxxxxxx
  5
  1
  arm-none-eabi-size   pin_read.elf 
  2
  arm-none-eabi-objdump -h -S  pin_read.elf  > "pin_read.list"
  3
     text    data     bss     dec     hex filename
  4
      724       8    1568    2300     8fc pin_read.elf
  5
  Finished building: default.size.stdout
  

  text size = 724

• -03

  xxxxxxxxxx
  5
  1
  arm-none-eabi-size   pin_read.elf 
  2
  arm-none-eabi-objdump -h -S  pin_read.elf  > "pin_read.list"
  3
     text    data     bss     dec     hex filename
  4
      700       8    1568    2276     8e4 pin_read.elf
  5
  Finished building: default.size.stdout
  

  text size = 700