What is the rationale for this undefined behavior?

Question

Warning[...]: undefined behavior: the order of volatile accesses is undefined in this statement x.cpp xxx

Why this line is undefined behavior?

  case 2:
    Vdda = 3.3 * (*VREFINT_CAL) / ADC_DR->DATA;

Where the declarations/initializations are:

volatile short const *VREFINT_CAL = (short *) 0x1FFFF7BA;

and

volatile STRUCT_ADC_DR *ADC_DR = (STRUCT_ADC_DR*) 0x40012440;

defined by:

typedef struct
{
  unsigned DATA         : 16;
  unsigned              : 16;
} STRUCT_ADC_DR;

Is it because the compiler isn't sure about the volatile elements could act diferent in the order they get accessed? (What is the case)

But shouldn't it be ensured that the calculation gets performed from left to right as the operators have the same priority?

Answer 1

volatile implies to the compiler that you are reading something that is not an ordinary memory address, like an I/O port. For two such reads, it is very likely that you will want those reads to happen in a certain order.

In both C and C++, the evaluation order of operands is not defined. If it helps you, think of the division as a function call:

Vdda = 3.3 * divide(*VREFINT_CAL, ADC_DR->DATA);

The point is now, that for volatile, where it's likely that the order is important, you might not want to leave this decision to the compiler. So it warns about it.

To get rid of the warning, simply make the order explicit by introducing additional sequence points to your code. For instance:

short const x = *VREFINT_CAL;
unsigned const y = ADC_DR->DATA;
Vdda = 3.3 * x / y;