Re: [Bug libm] pow() function execution time increase by factor 4000

["Carlos O'Donell" <carlos at redhat dot com>]

On 05/02/2014 04:44 PM, Stefani Seibold wrote:
> A pow(10.0, x) where x has the hex equivalent of 0xc022247c7a9c48f9l
> needs 4000 times longer as x equal 0xc022247c7a9c4800l, which is nearly
> the same value.
> 
> This happens on x86_64 Linux. I have tested it with the current git tree
> and GLIB 2.19, but the behavior can also reproduced on an old EGLIBC
> 2.12 32 bit PowerPC Linux system.
> 
> The origin the fdlibm pow() function does not show this performance
> impact, but is nearly three times slower as the best case libm execution
> time. The result of the function call are the same.
> 
> As i figured out the value 0xc022247c7a9c48f9l run into the __slowpow()
> branch, where 0xc022247c7a9c4800l does bypass this branch.
> 
> A "perf stat -e instructions" show that the 0xc022247c7a9c48f9 value
> needs round about 1505962 instructions, the 0xc022247c7a9c4800l needs
> only 385 instructions.
> 
> On the old PowerPC device the execution time is 1 us vs. 5000 us.
> 
> This makes pow() not very useable in a realtime environment.
> 
> Maybe the exp() has the same problem.
> 
> - Stefani

This is not the right place to report a bug.

Please see:
http://www.gnu.org/software/libc/bugs.html

In addition what you're reporting is not a bug, but we can
discuss this in the bug you open.

Cheers,
Carlos.