ULP Advisor > Rule 13.1 Count down in loops

ULP Advisor - Rule Table ULP 1.1 Ensure LPM usage ULP 2.1 Leverage timer module for delay loops ULP 3.1 Use ISRs instead of flag polling ULP 4.1 Terminate unused GPIOs ULP 5.1 Avoid processing-intensive operations: modulo, divide. ULP 5.2 Avoid processing-intensive operations: floating point ULP 5.3 Avoid processing-intensive operations: (s)printf() ULP 6.1 Avoid multiplication on devices without hardware multiplier ULP 6.2 Use MATHLIB for complex math operations ULP 6.3 Use Low Energy Accelerator (LEA) software library ULP 7.1 Use local instead of global variables where possible ULP 8.1 Use 'static' & 'const' modifiers for local variables ULP 9.1 Use pass by reference for large variables ULP 10.1 Minimize function calls from within ISRs ULP 11.1 Use lower bits for loop program control flow ULP 11.2 Use lower bits for port bit-banging ULP 12.1 Use DMA for large memcpy() calls ULP 12.1b Use DMA for potentially large memcpy() calls ULP 12.2 Use DMA for repetitive transfer ULP 13.1 Count down in loops ULP 14.1 Use unsigned variables for indexing ULP 15.1 Use bit-masks instead of bit-fields Let us know what you think! Feedback, suggestions & comments are welcome @ ULPAdvisorFeedback@list.ti.com

What it means

In MSP430 assembly code, a conditional branch based on comparing a variable/register against a non-zero value requires two instructions: compare and branch. However, when branching & comparing against zero, a specific instruction, BNE, can be used to perform both actions. This also holds true for a branch statement in C. Hence a counting down loop can reduce one instruction for each iteration of the loop when compared to a loop counting up.

Risks, Severity

A counting-up loop consumes one extra instruction for every iteration of the loop.

Why it is happening

A loop with an index counting up is detected in the code.

Remedy

• Use a loop that counts down whenever possible.
• Ensure that -o2 optimization level is selected in the compiler, or greater implements are included in the project settings to enable optimization for counting down loops.

Code Example

   int i;
   P1OUT |= 0x01;                            // Set P1.0 LED on
   for (i = 5000; i>0; i--)               // Count down loop
   // In instead of: (i = 0; i <5000; i++)
   {
     /* Execute your application code */
   }

See the rest of the code examples for all MSP430 devices here!

More Resources

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