Thank you for the explanation. Given that it would appear that the only time the compiler may attempt to optimize the allocation/use of an un-initialized variable, is when it knows for certain it hasn't been initialized; might it be preferable to then assign it a deterministic cheap value such as 0 which it may then presume during optimization, given all remaining alternatives would seem to have less desirable potential consequences?
> From: Ian Lance Taylor <ian@airs.com> >. Paul Schlie <[EMAIL PROTECTED]> writes: >> 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; > > No, the ISO C standard is clear that an uninitialized variable may be > set to a trap representation (6.7.8, 3.17.2). So in this case x might > be a signalling NaN, and executing sin (x) might well terminate the > program with a floating point exception. That would be a valid > translation of the program by the compiler, and would even be > reasonable and appropriate in a non-optimized compilation. > >> (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.) > > No, the ISO C standard is clear that the value of an uninitialized > variable is indeterminate. It may be a trap representation, or it may > be an unspecified value. In the latter case it must have the correct > type. But there is no other restriction on it, and the standard > specifically says (3.17.3) it "imposes no requirements on which value > is chosen in any instance." > > Ian
