On 29 August 2007 18:31, Brian Dessent wrote:
> Dave Korn wrote:
>
>> Since when was pi less than 2^-27?
>
> This is a kernel sin, a function over the limited range [-pi/4, pi/4],
> where the general case sin is reduced to kernel sin by the remainder of
> modulus pi/2.
Ah, I see.
>> I think it may be an artefact of FP precision and/or rounding mode, but
>> I'd need to do more experiments to make sure. OTOH it could be an
>> artifact of the printf %g specfier. Like I said, more experimentation is
>> needed!
>
> A problem with the testcase is the .10. It helps to expand that so that
> you see the entire value. Or use %a. newlib returns 3CED1A62633145C0
> whereas both MSVCRT and x86 linux seem to return 3CED1A6000000000. It
> sort of looks like a truncation problem but since glibc agrees with
> msvcrt (shock!) I tend to think the latter is right. The actual
> mantissas are:
>
> newlib 1.1101000110100110001001100011001100010100010111000000
> other 1.1101000110100110000000000000000000000000000000000000
This looks more and more like an extended precision issue. Good ol' PR323!
I'll dig out my fpctl macro and have a try...
cheers,
DaveK
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