llvm dev - Oct 2015 - [compiler-rt] Undefined negation in float emulation functions

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.

- Steve

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

On 26/10/15 02:54, Joerg Sonnenberger wrote:
> 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.
Chris, were you suggesting a cast (`a = -(unsigned)a`)? Otherwise, like Steve, I
don't understand the difference between
this proposal and Steve's code above.