A calculation the compiler may remove
Consider `double x = 5.0; double y = 3.0; x = (x * 1.5) - y; printf("x is %g\n", x);`. The output is `x is 4.5`. If `x` is never used again, the compiler may compute the result during compilation and replace the calculation with `printf("x is 4.5\n");`. The multiplication and subtraction need not occur in the executable: their intermediate results are not observed. section 5.1
Optimization changes how a program is carried out, not what its required observable behavior is. A C compiler need not follow every written step literally; it must produce an executable consistent with the abstract state machine’s observable states as execution proceeds. Those states include output and the contents of relevant variables. section 5.1 section 5.1.4
Combining additions
Suppose `x += 5;` is followed by work that does not use `x`, then by `x += 7;`. The compiler may be able to use a single `x += 12;`, placed before or after that intervening work. This is not permitted merely because the final arithmetic result looks right: printing or otherwise using the intermediate value would make the change observable. section 5.1.4
TAKEAWAY: Type determines optimization opportunities.
- For an unsigned type, the combined addition remains consistent with the two separate additions even if the value wraps. section 5.1.4
- For signed integer or floating-point types, an intermediate overflow may raise an exception and terminate execution. If the compiler cannot rule that out, it cannot discard the intermediate operation on the strength of the final value alone. section 5.1.4
A control-flow trap
After `setjmp`, a `longjmp` can return control to the saved position by an exceptional path. In a parser diagnostic, optimization can make a changed nesting-depth value appear as `0` rather than the expected `3`: the compiler analyzed the ordinary switch paths and assumed the value could not change. Objects modified across `longjmp` and needed on that path must be `volatile`; `unsigned volatile depth = 0;` is the shown repair. section 19.5
