[llvm-dev] double to unsigned char cast

[James Y Knight via llvm-dev]

I'd also note that -fsanitize=undefined will detect the error at runtime.

On Fri, May 25, 2018 at 4:38 AM Hans Wennborg via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On Tue, May 22, 2018 at 6:17 PM, Jay K via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > This seems disappointing.
> >
> > Casting any value to an integral type should yield a value within the
> range
> > of that integral type.
>
> As Tim pointed out, that's not what the standard says though.
>
> Trunk Clang (I'm not sure what version your Xcode Clang is based on)
> has a flag to disable this optimization:
> -fno-strict-float-cast-overflow
>
>  - Hans
>
>
> > The value can be anything, but it should be within range.
> >
> > i.e. it should be as if I anded it with (type)~(type)0.
> >
> >
> > i.e. int i = (signed char)x;
> >
> > assert(i >= -128 && i <= 127);
> >
> >
> > int i = (unsigned char)x;
> >
> > assert(i >= 0 && i <= 255);
> >
> >
> > seem very reasonable, assuming there is no "trap"  before the assert.
> >
> >  - Jay
> >
> >
> >
> > ________________________________
> > From: Tim Northover <t.p.northover at gmail.com>
> > Sent: Thursday, April 5, 2018 4:41 PM
> > To: Jay K
> > Cc: llvm-dev at lists.llvm.org; lewurm at gmail.com
> > Subject: Re: [llvm-dev] double to unsigned char cast
> >
> > Hi Jay,
> >
> > On 5 April 2018 at 07:49, Jay K via llvm-dev <llvm-dev at lists.llvm.org>
> > wrote:
> >> wrt
> >>
> >>
> https://github.com/mono/mono/commit/fb91fce5d339bb9ffe507588f5bc1d8d6f244d9b
> >
> >>
> >>
> >> This doesn't seem right to me.
> >
> > Nevertheless, it's what the standard says. C99 6.3.1.4: When a finite
> > value of real floating type is converted to an integer type other than
> > _Bool, the fractional part is discarded (i.e. the value is truncated
> > toward zero). If the value of the integral part cannot be represented
> > by the integer type, the behavior is undefined.
> >
> > By definition any double you're allowed to convert to an unsigned char
> > doesn't need the "and".
> >
> > Cheers.
> >
> > Tim.
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
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