llvm dev - Aug 2013 - [LLVMdev] Convert fdiv - X/Y -> X*1/Y

On Thu, Aug 08, 2013 at 10:13:07AM -0700, Shuxin Yang
wrote:
> You should be very careful, reciprocal is very very very imprecise
> transformation. Sometimes you will see big different
> with and without this xform.
What about constant %a, %b in this case? At least for power-of-two
arguments, wouldn't it still be precise?

Joerg

Yes it is precise, if Y is constant and 1/Y is exact.  In this case, 
this transformation is conducted without "-fast-math".
What I meant is if 1/Y is not exact.  I measured the results before. 
Among all fast-math transformation,
reciprocal is most annoying one because it is very imprecise. I was 
arguing at that time that reciprocal should be
ranked as the most relax xfrom.  It seems the numerical guru Steve kinda 
agree with me.

On 8/8/13 10:32 AM, Joerg Sonnenberger wrote:
> On Thu, Aug 08, 2013 at 10:13:07AM -0700, Shuxin Yang wrote:
>> You should be very careful, reciprocal is very very very imprecise
>> transformation. Sometimes you will see big different
>> with and without this xform.
> What about constant %a, %b in this case? At least for power-of-two
> arguments, wouldn't it still be precise?
>
> Joerg
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On Thu, Aug 08, 2013 at 10:41:12AM -0700, Shuxin Yang
wrote:
> Yes it is precise, if Y is constant and 1/Y is exact.  In this case,
> this transformation is conducted without "-fast-math".
I meant the other case, if the X/Y with power of two X. Some other forms
may also apply depending on the precise value of Y.

Joerg