[Libclc-dev] [PATCH v2 16/16] native_powr: Switch implementation to native_exp2 and native_log2

[Jeroen Ketema via Libclc-dev]

Hi Jan,

Thanks for the clarification. Looking back at the code

const __CLC_GENTYPE zero = (__CLC_GENTYPE)0.0f;
const __CLC_GENTYPE res = native_exp2(y * native_log2(x));
const __CLC_INTN condnan = isless(x, zero);
return select(res, (__CLC_GENTYPE)NAN, condnan);

and with your comments in mind, can’t this be simplified to:

return native_exp2(y * native_log2(x));

?

If x < 0, then native_log2 yields NaN, and the NaN propagates through the operations.

Jeroen

> On 13 Nov 2017, at 21:09, Jan Vesely <jan.vesely at rutgers.edu> wrote:
> 
> On Mon, 2017-11-13 at 20:50 +0100, Jeroen Ketema wrote:
>> This one is not immediately obvious to me. Maybe add some appropriate comment?
> 
> it exploits that x == 2^{log2 x} and log2 {x^y} == y * log2 x
> thus x^y == 2^{log2 x^y} == 2^{y * log2 x}.
> However, it only works for x > 0, which is OK since powr(<0, ...)
> should return NaN anyway.
> IMO powr was added to allow this kind of expansion instead of more
> expensive full pow (which needs to handle negative x).
> 
> Jan
> 
>> 
>> Jeroen
>> 
>>> On 6 Nov 2017, at 23:15, Jan Vesely via Libclc-dev <libclc-dev at lists.llvm.org> wrote:
>>> 
>>> Signed-off-by: Jan Vesely <jan.vesely at rutgers.edu>
>>> ---
>>> sent wrong patch the first time. This one passes the CTS.
>>> 
>>> Jan
>>> 
>>> generic/include/clc/math/native_powr.h | 8 +++++++-
>>> generic/lib/SOURCES                    | 1 +
>>> generic/lib/math/native_powr.cl        | 5 +++++
>>> generic/lib/math/native_powr.inc       | 6 ++++++
>>> 4 files changed, 19 insertions(+), 1 deletion(-)
>>> create mode 100644 generic/lib/math/native_powr.cl
>>> create mode 100644 generic/lib/math/native_powr.inc
>>> 
>>> diff --git a/generic/include/clc/math/native_powr.h b/generic/include/clc/math/native_powr.h
>>> index e8a37d9..c31161a 100644
>>> --- a/generic/include/clc/math/native_powr.h
>>> +++ b/generic/include/clc/math/native_powr.h
>>> @@ -1 +1,7 @@
>>> -#define native_powr pow
>>> +#define __CLC_BODY <clc/math/binary_decl_tt.inc>
>>> +#define __CLC_FUNCTION native_powr
>>> +
>>> +#include <clc/math/gentype.inc>
>>> +
>>> +#undef __CLC_BODY
>>> +#undef __CLC_FUNCTION
>>> diff --git a/generic/lib/SOURCES b/generic/lib/SOURCES
>>> index 355741c..ad5a743 100644
>>> --- a/generic/lib/SOURCES
>>> +++ b/generic/lib/SOURCES
>>> @@ -127,6 +127,7 @@ math/native_exp2.cl
>>> math/native_log.cl
>>> math/native_log10.cl
>>> math/native_log2.cl
>>> +math/native_powr.cl
>>> math/native_recip.cl
>>> math/native_rsqrt.cl
>>> math/native_sin.cl
>>> diff --git a/generic/lib/math/native_powr.cl b/generic/lib/math/native_powr.cl
>>> new file mode 100644
>>> index 0000000..452bc6f
>>> --- /dev/null
>>> +++ b/generic/lib/math/native_powr.cl
>>> @@ -0,0 +1,5 @@
>>> +#include <clc/clc.h>
>>> +
>>> +#define __CLC_BODY <native_powr.inc>
>>> +#define __FLOAT_ONLY
>>> +#include <clc/math/gentype.inc>
>>> diff --git a/generic/lib/math/native_powr.inc b/generic/lib/math/native_powr.inc
>>> new file mode 100644
>>> index 0000000..841b1ff
>>> --- /dev/null
>>> +++ b/generic/lib/math/native_powr.inc
>>> @@ -0,0 +1,6 @@
>>> +_CLC_OVERLOAD _CLC_DEF __CLC_GENTYPE native_powr(__CLC_GENTYPE x, __CLC_GENTYPE y) {
>>> +  const __CLC_GENTYPE zero = (__CLC_GENTYPE)0.0f;
>>> +  const __CLC_GENTYPE res = native_exp2(y * native_log2(x));
>>> +  const __CLC_INTN condnan = isless(x, zero);
>>> +  return select(res, (__CLC_GENTYPE)NAN, condnan);
>>> +}
>>> -- 
>>> 2.13.6
>>> 
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>> 
>>