[llvm-dev] fptosi undefined behaviour

[mats petersson via llvm-dev]

Resending to "everyone":

What's wrong with checking

short foo(double d)
{
    if (trunc(d) > MAX_SHORT || trunc(d) < MIN_SHORT) return 0; else return
short(d);
}

If you don't care about the one in difference, you could do `abs(trunc(d))
< MAX_SHORT;` to avoid the check against min.

That's what you want, right?

--
Mats

On 28 January 2016 at 11:17, Simon Byrne via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Thank Tom and Tim for your responses.
>
> If the behaviour is truly undefined as Tom says, would it be possible
> to get checked intrinsics for this?
>
> -Simon
>
> On 22 January 2016 at 20:24, Tim Northover <t.p.northover at gmail.com>
> wrote:
> > On 22 January 2016 at 12:20, Tom Stellard via llvm-dev
> > <llvm-dev at lists.llvm.org> wrote:
> >>> 1) I realise this is a somewhat silly question, but is this still
> >>> acceptable "undefined behaviour"?
> >>
> >> Yes, it is.
> >
> > I always thought these out-of-range instructions did produce an
> > "undef" rather than allowing fully-general undefined behaviour
> > (otherwise we couldn't speculate them, for a start).
> >
> > If so, I think the code ought to be valid: %1 is *some* i16
> > bitpattern, which means %2 cannot be completely unconstrained and
> > should never be equal to %0.
> >
> > Cheers.
> >
> > Tim.
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