https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91936
Bug ID: 91936
Summary: Slowness in exp()
Product: gcc
Version: 4.4.7
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: waikng at hotmail dot com
Target Milestone: ---
Created attachment 46982
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=46982&action=edit
slow exp() example
I found two arrays of double numbers in our application such that the
calculation time of exp() for the first set is at least twice slower than the
other one.
Please see attached the example. It calculates the mean of the two arrays four
times. In the output, the first column is the calculation time and the second
column is the mean. It shows about 11 microseconds versus 4 microseconds
consistently on our PC with -O -m64. We have gcc 4.4.7 and /lib64/libm-2.12.so.
I can also replicate this with gcc 6.5.0 in an online compiler
(https://www.onlinegdb.com/H1CATk3DH).
In some other case, of which I haven't extracted the arrays out, our
application is even four times slower. When I implement an exp() algorithm
myself we don't see the performance degradation, so I believe there is some
issue about the stock exp(). Also I don't see this degradation on my personal
MacBook Pro running XCode 10.2.
