Sequential I2C read from MPU6050 with PIC16F18877 different from expected

Question

To save time and reading I copied a code block from a program found on the internet to determine the X angle of inclination.

int Ay = (((int)I2C_Read(0)<<8) | (int)I2C_Read(0));
int Az = (((int)I2C_Read(0)<<8) | (int)I2C_Read(1));

During the executing I could not get the expected results. Comparing the code with a similar code block in an Arduino sketch showed a good approach. The Arduino sketch worked well but the pic program behaved strange. Finally I decided to hook up a logic analyzer and noticed the incoming byte before last had the NACK instead of the last one. I decided to change the code into:

AcY = (int)I2C_Read(0)<<8;
AcY +=(int)I2C_Read(0);
AcZ = (int)I2C_Read(0)<<8; 
AcZ +=(int)I2C_Read(1);

Now the results are correct. This brings me to the conclusion that the executing order is different depending on the environment. It would be nice to know if my conclusion is correct and is it possible to change the behavior of the compiler (XC8).

Answer 1

Order of evaluation is not defined by standard. It is up to the compiler how it implements this. Refer to the compiler manual if it can be changed. I highly doubt it is changeable. Besides, it would be just easier to write code that does not require certain assumptions about the compiler or its configuration.