weird optimization in sin+cos, x86 backend

[Robert Dewar]

On 2/3/2012 10:28 AM, Vincent Lefevre wrote:
> On 2012-02-03 10:13:58 -0500, Robert Dewar wrote:
>> On 2/3/2012 10:01 AM, Michael Matz wrote:
>>> No normal math library supports such an extreme range, even basic
>>> identities (like cos^2+sin^2=1) aren't retained with such inputs.
>>
>> I agree: the program is complete nonsense.
>
> I disagree: there may be cases where large inputs can be meaningful.
> And it may be important that some identities (like cos^2+sin^2=1) be
> preserved.
>
>> It would be useful to know what the intent was.
>
> If the user requested such a computation, there should at least be
> some intent. Unless an option like -ffast-math is given, the result
> should be accurate.

What is the basis for that claim? to me it seems useless to expect
anything from such absurd arguments. Can you site a requirement to
the contrary (other than your (to me) unrealistic expectations).
In particular, such large numbers are of course represented
imprecisely. Given that, i do not see a possible useful intent,
obviously a 1 bit rounding error makes a gigantic difference to
the final result here.
>
> For the glibc, I've finally reported a bug here:
>
>    http://sourceware.org/bugzilla/show_bug.cgi?id=13658
>
> Note that there have been other (somewhat different) complaints in
> the past, e.g.
>
>    http://sourceware.org/bugzilla/show_bug.cgi?id=13381
>