Storage for uninitialized objects (PR 24626)

[Robert Dewar]

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!