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Subject: Re: [boost] [Safe Numerics] floating point conversions
From: John Maddock (jz.maddock_at_[hidden])
Date: 2017-03-12 18:23:41

On 12/03/2017 16:10, Robert Ramey via Boost wrote:
> There's another issue here as well. I've tried to shy a way from
> making the presumption that all arithmetic is two's complement. When
> I think about the cases you cite, I then have to evaluate them in
> light of this view. Going down this path leads me to a chamber with
> mulitple exits - all alike. When I find myself here, I've reverted to
> the text of the standard - which is more restrictive. (negative
> shift? for me no problem - but the standard says it's undefined. The
> real question for me here is should I
> a) insist that all code be standards conforming and not engage in
> undefined behavior

No argument from me there, as far as I know I haven't suggested
otherwise (though I wouldn't necessarily bet against it!)


> b) permit code which is in common usage (like shifting negative
> numbers) but is undefined by the standard.
> I've tended toward the former for a couple of reasons. Undefined
> behavior is non-portable - even though it's likely portable in the 3
> or 4 compilers which occupy the current market.
> I've become aware that compiler writers are using undefined bevavior
> to optimize out related code - apparently without letting use know.
> If this is true, we're laying a whole new minefield.
> Sooooo - I've been inclined to disallow behavior which makes sense and
> strictly follow the standard - because it seems that's the only way to
> make a guarantee - no unexpected behavior. Also - it's the only way I
> and avoid having to make "hidden" decisions inside the code which
> create the kind of uncertainty which is cause of the real problems
> that I'm trying to address.
> Of course it's not made easier by the fact that the standard also
> changes in subtle ways in what it permits.
> I want to be able to write code which does what it says it does -
> that's my goal. But it seems that I'm frustated by good intentions of
> others. The road to hell IS paved by good intentions - at least in
> this case.
> Robert Ramey
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