Compile error with gcc 4.8.2

[patburke]

Using gcc 4.8.2 on Ubuntu 14.04.

src/wren_vm.c: In function ‘runInterpreter’:
src/wren_vm.c:384:41: error: operation on ‘fiber->stackTop’ may be undefined [-Werror=sequence-point]
   #define PUSH(value)  (*fiber->stackTop++ = value)
                                         ^
src/wren_vm.c:562:23: note: in expansion of macro ‘PUSH’
     CASE_CODE(DUP):   PUSH(PEEK()); DISPATCH();
                       ^

[MarcoLizza]

Did notice it too, in the Travis CI output. Quite odd.

[kmarekspartz]

Here's those two macros expanded:

(*fiber->stackTop++ = (*(fiber->stackTop - 1)))

Might it be undefined due to the ++? Perhaps these should be multi-line macros to make the intent clearer / more defined.

[munificent]

Yeah, my mistake. It's fixed now. :)

For some reason, clang doesn't complain about it, so I didn't catch this locally.

[munificent]

@zeckalpha I think it might be the =? Either way, the result of the expansion is an expression where there isn't a defined sequence point between accessing and setting stackTop.

[MarcoLizza]

Neither MSC (with the default settings), nor Cppcheck complains about it.

It is due to the post increment operator, @zeckalpha is right.

The evaluation order of the subexpressions is not defined (and mostly compiler dependant), and the post increment side effect could affect the outcome.

(l-value subexpressions are not necessarily evaluated after the r-value ones)