[Bug math/16531] New: cosine very occasionally takes an exceptionally long time

["john.wilkinson at spirent dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=16531

            Bug ID: 16531
           Summary: cosine very occasionally takes an exceptionally long
                    time
           Product: glibc
           Version: 2.18
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
          Assignee: unassigned at sourceware dot org
          Reporter: john.wilkinson at spirent dot com

Created attachment 7394
  --> https://sourceware.org/bugzilla/attachment.cgi?id=7394&action=edit
demo code

The cosine function, cos, can be exceedingly slow for a few number of values.
>From my measurement on my PC using the attached C++ program I find that cos
almost always takes in the order of 2e-8 seconds, but occasionally it will take
around 1.5e-4 seconds, 0.15ms. It always takes around this time for particular
values, e.g. 1.27730245420149213 (radians). (This difference, 4 order of
magnitude, is quite a problem for us as we have to repeatedly perform a large
number of calculations within 1ms.)

GCC versions 4.6.3 and 4.7.2, GLIBC version 2.15, 2.18, i7 PCs, Ubuntu 12.04,
Yocto

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