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Re: Is it Okay for GCC to do the following simplifications with "-ffast-math" flag


On Wed, May 6, 2015 at 2:56 AM,  <msebor@redhat.com> wrote:
> On 05/05/2015 08:27 AM, Renlin Li wrote:
>>
>> Hi all,
>>
>> For the following illustrative code,
>>
>> double f1(int x) { return (double)(float)x; } --> return (double)x;
>> int f2(double x) { return (int)(float)x; } --> return (int)x;
>>
>> Is it Okay for the compiler to do the simplifications shown above with
>> fast-match enabled?
>
>
> Such a transformation would yield different results
> for integers that are exactly representable in double
> but not in float. For example, the smallest positive
> integer with such a property in IEEE 754, 16,777,217,
> converts to 16,777,216 in float. I'm not a math expert
> but such a result would seem unexpected even with
> -ffast-math.

Yeah, such changes would be not welcome with -ffast-math.

Richard.

> Martin
>
>>
>> Regards,
>> Renlin Li
>>
>


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