Re: Enumeration representation clause surprise.

[Mike Silva <snarflemike@yahoo.com>]

On Jun 13, 3:21 am, Keith Thompson <ks...@mib.org> wrote:
> Mike Silva <snarflem...@yahoo.com> writes:
> > On Jun 12, 2:41 pm, Markus Schöpflin <nos...@nospam.org> wrote:
> >> Mike Silva schrieb:
> [...]
> >> > Are you saying now that there's no problem after all?
>
> >> Not at all. The memory layout of the variables is not what I expect it
> >> to be, because the compiler silently biased those values. The compiler
> >> is perfectly aware of this, as it makes up for the biasing when
> >> explicitly querying the value, but this doesn't help at all because I'm
> >> interested in the exact bitwise representation.
>
> >> Admittedly, there is an inconsistency in what I'm asking the compiler to
> >> do, but that's the whole point: I do not want this inconsistency to be
> >> silently 'fixed' by the compiler, I want to get noted about my error to
> >> get a chance and fix it myself.
>
> > I wonder then if this is an error at all.  IANALL, but if the program
> > makes available your specified values when queried with the proper
> > mechanism (in this case, unchecked conversion), then I don't see what
> > difference it makes how it stores the representations in raw memory.
>
> > I'd welcome other comments on the validity or non-validity of this
> > view.
>
> If it doesn't make any difference how the representations are stored
> in raw memory, then you wouldn't use a representation clause; you'd
> let the compiler make any decisions about representation, subject to
> the constraint that the code has to produce the right answers.
>
> But if you *do* provide a representation clause, then the
> representation clearly *does* matter.  And if the compiler can't
> satisfy the requested representation (e.g., because two representation
> clauses contradict each other), then the compiler should reject the
> compilation unit.
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>
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So that's why I'm wondering why compiler writers ever introduced
biased representations in the first place.  I can't believe that the
problem with this approach has only been noticed in 2008.  Aren't
biased representations fundamentally incompatible with "the
representation clearly *does* matter"?