[Libclc-dev] [PATCH] Add definition for M_PI_F v2

[Jeroen Ketema]

I think you can reasonably argue that the compiler/hardware is faulty in case they do slightly different things. They have to follow the IEE standard here.

Jeroen

On 18 Mar 2014, at 22:59, Tom Stellard <tom at stellard.net> wrote:

> On Tue, Mar 11, 2014 at 10:21:17PM +0000, Jeroen Ketema wrote:
>> 
>> The form of type-punning Tom uses is perfectly legal in OpenCL 1.2: see section 6.2.4.1.
>> 
>> I don’t see point of using it though. Specifying a float directly with a sufficient number of digits seems a much clearer solution.
>> 
> 
> Maybe I'm being overly cautious, but with the hexadecimal format,
> I know exactly what value I'm getting, but with the floating point
> format, I worry that different compilers / hardware might do slightly
> different things.
> 
> -Tom
> 
>> Jeroen
>> 
>> On 11 Mar 2014, at 18:15, Jan Vesely <jan.vesely at rutgers.edu> wrote:
>> 
>>> On Tue, 2014-03-11 at 10:50 -0700, Matt Arsenault wrote:
>>>> On 03/11/2014 08:38 AM, Tom Stellard wrote:
>>>>> v2:
>>>>>  - Use a hexadecimal constant.
>>>>> ---
>>>>> generic/include/clc/float/definitions.h | 11 +++++++++++
>>>>> 1 file changed, 11 insertions(+)
>>>>> 
>>>>> diff --git a/generic/include/clc/float/definitions.h b/generic/include/clc/float/definitions.h
>>>>> index e6ef1d8..a6c947f 100644
>>>>> --- a/generic/include/clc/float/definitions.h
>>>>> +++ b/generic/include/clc/float/definitions.h
>>>>> @@ -9,6 +9,17 @@
>>>>> #define FLT_MIN 	0x1.0p-126f
>>>>> #define FLT_EPSILON 	0x1.0p-23f
>>>>> 
>>>>> +_CLC_INLINE static float m_pi_f() {
>>>>> +  union {
>>>>> +    unsigned i;
>>>>> +    float f;
>>>>> +  } pi;
>>>>> +  pi.i = 0x40490fdb;
>>>>> +  return pi.f;
>>>>> +}
>>>>> +
>>>>> +#define M_PI_F m_pi_f()
>>>>> +
>>>>> #ifdef cl_khr_fp64
>>>>> 
>>>>> #define DBL_DIG 	15
>>>> I think this is technically undefined behavior that everyone violates. 
>>> 
>>> type-punning is afaik legal in c99 (implementation defined in c89),
>>> "man gcc" includes an example same as this use-case
>>> 
>>> quick search gives this:
>>> http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_283.htm
>>> 
>>> although I'd ask the same question as Jeroen,
>>> 
>>> what's wrong with 3.14159274101257f ?
>>> 
>>> regards,
>>> 
>>>> If you want the hex constant, you could use the format that comes out of 
>>>> the %a format specifier:
>>>> 
>>>>    float cast = M_PI;
>>>>    printf("M_PI  %a\n", M_PI);
>>>>    printf("M_PI cast float: %a\n", cast);
>>>> 
>>>> Gives me:
>>>> 
>>>> M_PI  0x1.921fb54442d18p+1
>>>> M_PI cast float: 0x1.921fb6p+1
>>>> 
>>>> 
>>>> 
>>>> 
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>>> 
>>> -- 
>>> Jan Vesely <jan.vesely at rutgers.edu>
>>> _______________________________________________
>>> Libclc-dev mailing list
>>> Libclc-dev at pcc.me.uk
>>> http://www.pcc.me.uk/cgi-bin/mailman/listinfo/libclc-dev
>> 
>> 
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