SPIN variable shadowing suprise (flexspin)

__deets__ · 2021-03-14 16:23

I just scratched my head over a rather stupid mistake that I would have hoped the compiler to prevent me from making.

I pass the address of an instruction buffer to another cog. I called it mux (it's the MUX-register of an ADS1256 that I move through to acquire data). The expectation is that this buffer does not change during the other cogs operation, to avoid a needless copy.

I then observed changes in this buffer that where puzzling to me. Until I found that I also created a local variable of the same name

Excerpts from the code below:

  INSTRUCTION_BUFFER_SIZE = 2
  BUFFER_SIZE = 100
var
  BYTE mux[INSTRUCTION_BUFFER_SIZE]
  LONG read
  LONG write
  LONG buffer[BUFFER_SIZE * 2]
obj
  term : "jm_fullduplexserial"
  adc : "buffered_ads1256"

pub main | a, b, c, channel, timestamp, mux, value, rpos, wpos
  term.start(RX1, TX1, %0000, BR_TERM)
  read := BUFFER_SIZE
  adc.start(ADC_SDI, ADC_SDO, ADC_CLK, ADC_CS, ADC_DRDY, ADC_PDWN, @read)

  pinlow(TRIGGER)
  pinhigh(TRIGGER)
  adc.wreg(adc.DRATE, %00000011) ' 2.5SPS
  a := adc.rreg(adc.DRATE, 1)
  'term.fstr1(string("ADC DRATE \%%8.8b\r\n"), a) ', b, c)

  mux[0] := (0 << 4) | (1 << 3) ' AIN-channel and AINCOM
  mux[1] := (1 << 4) | (1 << 3) ' AIN-channel and AINCOM
  adc.start_continuous_read(@mux, INSTRUCTION_BUFFER_SIZE)
  repeat
    timestamp, mux, value := adc.read_continuous
    term.fstr3(string("ICDAC %8.8x MUX 0x%2.2x: %8.8x\r\n"), timestamp, mux, value)
    'term.fstr2(string("\r\nrpos: %d wpos: %d\r\n\r\n"), rpos, wpos)
    term.fstr3(string("MUX0 0x%2.2x MUX1 0x%2.2x, %d\r\n"), instruction_buffer[0], instruction_buffer[1], adc.available)

The problem is easily solved of course, I just renamed the buffer to instruction_buffer and that's it.

However given that I'm not allowed to e.g. have variable and method of the same name, I wonder if this would be worth an error? Or at least warning, if it would break existing code.

ersmith · 2021-03-14 16:36

If you pass -Wall on the flexspin command line you'll get a warning for this.

__deets__ · 2021-03-14 17:33

Neat! Found quite a few other questionable things I actually had wondered about (using multiple return values for example w/o declaring them).

SPIN variable shadowing suprise (flexspin)

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