Come Again, What's the Cost and Overhead?

I've been implementing object-based stuff in C for embedded firmware with passing an index parameter to an array of structs, like I tried to explain here. Yesterday I remembered an article about pointer-less C in Embedded System Design and thought I will give it a couple minutes more. The article gives some ideas how to make pointers safer to use and talks about the cost and benefit of using pointers. I'm not saying anything about the value of article itself, but I got curious. I took our simple switch debounce filter routine which is used a lot, and which is typically the only one that actually implements multiple objects at run time. It uses the me index as parameter for two functions. I quickly modified the code so that both functions take in a pointer to a struct instead of simple unsigned char. I compiled both versions for Microchip 16F876A (8-bit uC) and got surprised. The pointer version used over 30% less code memory than the index version, reducing the ROM image from above 300 words to below 200 words. I then compiled the source files to give me the assy listing, and below are the results that I found for a simple function that just sets some members of a struct. Because of werd mechanism of indirect addressing in PIC micros, the pointer is actually passed just as a 8-bit register. Word of caution; we have experienced problems when passing pointers and enabling optimization with Hi-Tech compilar for PIC micros, that is another reason why we have avoided this.

The lesson; while I do believe in intuition for great solutions, it is worth to challenge your beliefs every once in a while.

Pointer version:

    30  0764             _Cbutton_construct
    31                   ;button_ptr.c: 27: me->debounceCounter = 0;
    32                          
    33  0764  0084       movwf fsr
    34                            
    35  0765  1383       bcf 3,7
    36  0766  0180       clrf 0
    37                   ;button_ptr.c: 28: me->level = !1;
    38                          
    39                   ; _me assigned to ?a_Cbutton_construct+0
    40  0000             _Cbutton_construct$me set 
                         ?a_Cbutton_construct+0
    41                   ;_me stored from w
    42  0767  1283       bcf status,5
    43  0768  1303       bcf status,6 ;carry unused
    44  0769  00A8       movwf (((?a_Cbutton_construct+0)))
    45  076A  0A28       incf (((?a_Cbutton_construct+0))),w
    46  076B  0084       movwf fsr
    47                            
    48  076C  1383       bcf 3,7
    49  076D  0180       clrf 0

Index version:

    30  06F6             _Cbutton_construct
    31                   ;button.C: 27: theButton[meIndex].
                         debounceCounter = 0;
    32                          
    33                   global _theButton
    34                   ; _meIndex assigned to ?a_Cbutton_construct+0
    35  0000             _Cbutton_construct$meIndex set ?a_
                         Cbutton_construct+0
    36                   ;_meIndex stored from w
    37  06F6  1283       bcf status,5
    38  06F7  1303       bcf status,6 ;carry unused
    39  06F8  00A8       movwf (((?a_Cbutton_construct+0)))
    40  06F9  0828       movf (((?a_Cbutton_construct+0))),w
    41  06FA  00F0       movwf btemp
    42  06FB  1003       bcf status,0
    43  06FC  0DF0       rlf btemp
    44  06FD  1003       bcf status,0
    45  06FE  0D70       rlf btemp,w
    46  06FF  3E20       addlw ((_theButton))
    47  0700  0084       movwf fsr
    48                            
    49  0701  1383       bcf 3,7
    50  0702  0180       clrf 0
    51                   ;button.C: 28: theButton[meIndex].level = !1;
    52                          
    53  0703  0828       movf (((?a_Cbutton_construct+0))),w
    54  0704  00F0       movwf btemp
    55  0705  1003       bcf status,0
    56  0706  0DF0       rlf btemp
    57  0707  1003       bcf status,0
    58  0708  0D70       rlf btemp,w
    59  0709  00F0       movwf btemp
    60  070A  0A70       incf btemp,w
    61  070B  3E20       addlw ((_theButton))
    62  070C  0084       movwf fsr
    63                            
    64  070D  1383       bcf 3,7
    65  070E  0180       clrf 0