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

[Jeroen Ketema]

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.

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
>> 
>> 
>> 
>> 
>> _______________________________________________
>> Libclc-dev mailing list
>> Libclc-dev at pcc.me.uk
>> http://www.pcc.me.uk/cgi-bin/mailman/listinfo/libclc-dev
> 
> -- 
> 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