> Ian Lance Taylor writes: > > gcc assumes that if a variable is uninitialized, it can use any value > whatsoever for any use of the variable. gcc does not assume that all > uses of an uninitialized variable must have the same value. > > It is of course possible to write a correct program which uses an > uninitialized variable. However, I believe that such a program must > never examine the variable in any way whatsoever, so I don't agree > that it is possible to detect the difference in values. Even > comparing the uninitialized variable to itself is undefined behaviour. > Given that, I think that gcc's current approach will generate the most > efficient code for correct programs which have uninitialized > variables.
- I believe that it is a grave mistake to conclude that a well defined semantic operation on an indeterminate value, has undefined semantics. As this is simply an erroneous conclusion. As a simple example, although x may be indeterminate -1 < sin(x) < +1 is unconditionally true, as must be tan(x) = sin(x)/cos(x), and x^x = 0; So although neither the compiler (nor program) may know what the value of x may be until run-time (unless it is chosen to be given a default initialized value of convenience by the compiler); regardless of that value, it must be utilized in a semantically consistent manor. (as a general rule, optimization should never alter observable semantics of a program within it's control; so if GCC chooses not to assign a default value to an otherwise un-initialized variable; it must assume it may be any single value, not any multiple value; although should feel free to complain loudly about it in either case.)
