program-with-loop

Backlinks

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Source: samek-embedded
See also: General considerations for non-linear control flow

Code and instructions

Code

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10
int main()
{
  int counter = 0;
  
  while (counter < 10) {
  ++counter;
  }
  
  return 0;
}

Disassembly

Comparison

Original structure	Optimised structure

In the compiled code, the execution of the while loop is different than that given by the code.
However, the effect of the loop on the calculation remains the same.
The compiler proposes this order of instructions for increased efficiency, e.g. the BLT.Ninstruction steps back to ++counter instead of forwards, which allows the PC register to naturally increment without having to use another branch instruction.

Compare


Before
After

CMP modifies the APSR (Application Program Status Register)

N bit (negative) is set, because R0 - #10 is negative