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Problem with optimisation of volatiles

Richard Roebuck

Hi there All,

I have a problem which seemed to be defying explanation, but I have come up with a theory. Could I possibly have some feedback on whether the following is likely, plausible, possible, untrue or downright rubbish?

If one reads the contents of a CAN or ADC chip register at a particular address, then the label volatile is placed upon that address to prevent the compiler optimising out repeat readings of the address. If one reads the contents of the address into a variable, then the compiler would automatically treat the contents of this variable with similar care.

Is it possible that there has been an oversight with statements where the contents of a variable depend on the contents of a volatile by way of an if statement, ie...

normal_var=volatile_var;

...is correctly optimised, but...

normal_var=voltile_var;
if (normal_var=0x00)
   {
   another_normal_var+=1;
   }

...is not correctly optimised all of the time, dependant on the surrounding code, unless normal_var itself is declared to be volatile?

For info - am using optimisation level...

OPTIMIZE(3,SPEED)

...and am using version...

C166 COMPILER V4.11

Any thoughts, or is any or all of the above thoughts and understanding way off the mark?


Yours (grateful for any input),


Richard.

Parents
  • 0 in reply to Mik Kleshov

    Volatility is not contagious. A variable not explicitly declared as volatile will be treated however the optimizer treats it, regardless of where its value came from.

    (This is a good thing, as it allows you to sample a volatile register into a variable, and then carry that one sample around without having to worry that it will be re-read.)

Reply
  • 0 in reply to Mik Kleshov

    Volatility is not contagious. A variable not explicitly declared as volatile will be treated however the optimizer treats it, regardless of where its value came from.

    (This is a good thing, as it allows you to sample a volatile register into a variable, and then carry that one sample around without having to worry that it will be re-read.)

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