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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 Richard Roebuck

    In Mike's message is says...

    You are suggesting that the compiler might optimize it...Well, it's not going to happen because ADC is volatile.

    ...which I took to mean that Mike was suggesting it would be untouched by the optimiser.

    Wrong. Words taken out of context.
    I still suggest that you come up with a real-world example of C code and possibly undesirable optimizations by the compiler on that code. Then there would be a basis for discussion. If it's not possible to think of such an example, then the problem does not exist.

    Regards,
    - mike

Reply
  • 0 in reply to Richard Roebuck

    In Mike's message is says...

    You are suggesting that the compiler might optimize it...Well, it's not going to happen because ADC is volatile.

    ...which I took to mean that Mike was suggesting it would be untouched by the optimiser.

    Wrong. Words taken out of context.
    I still suggest that you come up with a real-world example of C code and possibly undesirable optimizations by the compiler on that code. Then there would be a basis for discussion. If it's not possible to think of such an example, then the problem does not exist.

    Regards,
    - mike

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