Paul Schlie wrote:
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.
Well the standards committee made the grave mistake if it is one, so if
you want to challenge this, take it up with the standards committee
As this is simply an erroneous conclusion.
No, it is an exactly correct conclusion of the standard
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;
Nonsense, in this case x is a floating-point number, and it is not at all
true that sin(signalling NaN) is anything at all. So here, even from a
pragmatic point of view, this is wrong, and of course sin(x) is entirely
undefined in the standard if x is uninitialized.
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.
When a variable is uninitialized, it is not the case that it has
some value that is defined, it is undefined, and any reference
that reads the value of such a variable is undefined. A
compiler that deleted the system disk when you did this
would be impolite, but still conforming.
(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, that is not the way the language is defined. Sorry!
