一个编译器中的运算符优先级不同，为什么？

Question

I'm porting my game to Android which has around 200k lines of code and encountered a very strange bug, after 5 hours of digging I cracked it down to this line:

// background info
short m = ((dwMask << 4) | dwMask) << ASTAR_OFFSET_VIS;
short zm = (~ASTAR_MASK_DISCOVERED) | (~dwMask);
short *b = (short*)tilecache.GetDataPtr() + index;

// unexpected behavior
*b++ = (*b&zm) | m;

// works
*b = ((*b)&zm) | m; b++;

My guess is this compiler (GCC ARM) treats the ++ operator differently than all other compilers I've built this game on, which seems a little crazy, but not unbelievable. Previously the game has been built for Windows, Mac, iOS, and Windows CE and all processed the top version fine.

I think it's evaluating (*b&zm) | m then incrementing the pointer b++ then doing the assignment, because data is shifted left in my array each evaluation.

I have located numerous places in my code that use this type of syntax and changed them, but I want to make sure the problem is what I'm thinking it is, and if this is a compiler option I could switch to make it the same as the other compilers I use? In case I have other syntax like this in my code elsewhere.

Answer 1

The order in which arguments to functions or operators (including built-in operators) are evaluated is unspecified. Some compilers will evaluate the expressions from left to right, others evaluate them from right left. For some operators, eg, the comma, the ternary, and the logic operators the order in which the arguments are evaluated and when side-effects happen is specified: the first operand is evaluated first.

This is unrelated to operator precedence which determines in which order the operators are evaluated. It is also unspecified when side-effects happen other than that they happen at the end of a full expression or after evaluating the first argument of one of the special operators.