[llvm-dev] [compiler-rt] Undefined negation in float emulation functions

[Joerg Sonnenberger via llvm-dev]

On Sun, Oct 25, 2015 at 06:46:48AM -0400, Steve Canon via llvm-dev wrote:
> On Oct 24, 2015, at 6:02 PM, Chris Lattner <clattner at apple.com> wrote:
> > 
> > 
> >> On Oct 23, 2015, at 7:43 PM, Matthew Fernandez via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> >>> On 21/10/15 00:15, Stephen Canon wrote:
> >>> Yup, this is UB.  If you want to propose a patch, I would do something like the following:
> >>> 
> >>>    rep_t sign = 0;
> >>>    unsigned int aAbs = a;
> >>>    if (a < 0) {
> >>>        sign = signBit;
> >>>        aAbs = -aAbs;
> >>>    }
> >>>    // Now use aAbs instead of a.
> > 
> > FWIW, another way to avoid the UB is to use an unsigned value.
> 
> I'm confused, that's exactly what this does.

At least in one place it is negating the signed variable, not creating a
new unsigned variable.

Joerg