Re: [PATCH 2/6] Remove slow paths from sin/cos

[Zack Weinberg <zackw@panix.com>]

On Fri, Mar 9, 2018 at 1:19 PM, Ondřej Bílka <neleai@seznam.cz> wrote:
> On Fri, Mar 09, 2018 at 09:47:09PM +0530, Siddhesh Poyarekar wrote:
>> On Friday 09 March 2018 09:16 PM, Wilco Dijkstra wrote:
>> > This patch removes 2nd of the 3 range reduction cases and defer to the final one.
>> > Input values above 2^27 are extremely rare, so this case doesn't need to as be
>> > optimized as smaller inputs.
>>
>> Please elaborate on two more points:
>>
>> - The basis for your claim that values above 2^27 are extremely rare,
>>   i.e. papers that you may have referred that conclude this and/or
>>   applications you may have profiled
>>
> Main reason is that for these inputs accuracy doesn't make a sense.
> There is already millions bigger error caused by limited precision when
> rounding input to float.

More concretely, for IEEE single, the gap between representable values
is bigger than 2π for values whose exponent is 2^26 or above.  Since
the sine function is periodic over 2π, that means the result of sinf()
is effectively meaningless for any input at least that big - _any
value_ within the input period could have been rounded to the
representable, so the "true" answer could be anything.  (I am tempted
to suggest that we return NaN for inputs this large, without even
bothering to do argument reduction.)

For double, this happens at 2^55, and for x86-64 long double it
happens at 2^66.  I _think_ x86-64 long double is 80-bit IEEE
extended, not quad, but I could be wrong.

zw