ULP Advisor > Rule 15.1 Use bit-masks instead of bit-fields

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

It is common practice in embedded C code to store multiple bits of information into a single variable via the use of bit-fields or bit-mask. The advantage of bit-mask over bit-field is that several bit-masks can be combined into one single bit-mask, allowing the compiler to issue a single instruction to access the variable. On the other hand, the compiler cannot roll consecutive bit-field instructions into one instruction. This results in additional code execution and consequently draws more power to execute the same amount of effective code.

Risks, Severity

Additional instructions might be executed, depending on the width and/or the number of bits of the bit-field being accessed/modified. The effect increases when the instructions in question are part of a loop/function.

Why it is happening

Consecutive instructions accessing the volatile bit-field variable(s) (such as hardware registers) cannot be combined by the compiler and hence requires more power & energy to execute.

Remedy

Use bit-mask instead of bit-field.

Code Example

 #define FLAG_1     1
 #define FLAG_2     2
 #define FLAG_3     4
 #define FLAG_4     8
    
    
 void functionA(unsigned char variable)
 { 
   unsigned int i, variable=FLAG_1;
   if ( variable & FLAG_1)
   {
    /* Execute application code */
   }
   
   if ( variable & FLAG_2)
   {
    /* Execute application code */
   }
 
   if ( variable & FLAG_4)
   {
    /* Execute application code */
   }      
 } 
     
 void main(void)
 {
   unsigned int variable1 = FLAG_4;
   /* Execute application code */
   variable1|= FLAG_1 | FLAG_2 | FLAG_3;      
   functionA(variable1);
 }

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

More Resources

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