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Re: New gcc 4.0.0 warnings seem spurious
Vincent Lefevre writes:
> On 2005-04-27 03:37:15 -0700, Zack Weinberg wrote:
> > Vincent Lefevre <vincent+gcc@vinc17.org> writes:
> > > However it is correct to store any integer to an unsigned variable,
> > > even if the original value cannot be represented.
> >
> > If that operation occurs at runtime it has a well-defined result.
>
> And gcc 4 doesn't output a diagnostic in this case?
>
> > If that operation occurs at compile time, as part of an initializer
> > expression, the overflow must be diagnosed (C99 6.6p4 - constraint
> > violation).
>
> Before the conversion, the value is representable in the type of
> the expression, and after the conversion (which is well-defined),
> it is still representable in the (new) type of the expression.
> 6.7.8#11 mentions the possible conversion. So, I disagree here.
>
> > > The fact that they are not considered as constant expressions,
> > > is it due to the fact that the environment is allowed to modify
> > > them?
> >
> > No (and in fact the environment is not allowed to do that). It is
> > because the set of things that are allowed in constant expressions are
> > explicitly listed in C99 6.6, and read-only variables aren't one of
> > the things on the list.
>
> The only two constraints in 6.6 are:
>
> [#3] Constant expressions shall not contain assignment,
> increment, decrement, function-call, or comma operators,
> except when they are contained within a subexpression that
> is not evaluated.86)
>
> [#4] Each constant expression shall evaluate to a constant
> that is in the range of representable values for its type.
>
> #3 doesn't include variables. #4 is OK if one considers that the
> value cannot be modified.
>
> #6 adds other requirements, but this is out of the scope of the
> given diagnostic (which complained about an expression not being a
> constant -- not because it wasn't an integer constant expression).
6.7.8 Para 4. All the expressions for an initializer for an object
that has static storage duration shall be constant expressions ...
6.6 Para 6. An integer constant expression shall have integer type
and shall only have operands that are inetger constants, enumeration
constants, character constants, sizeof expressions whose results are
integer constants, and floating-point constans that are the
immediate operands of casts ...
> Couldn't the expression fall into #10 with some implementations?
Probably, but that would be yet another gcc-local extension.
Andrew.