http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52576
Bug #: 52576
Summary: fib.c (attached) is slower on current (4.8.0) than
4.6.x
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: c...@f00f.org
The following is found to be slower using gcc 4.8.0 (svn head) compared to gcc
4.6.2 (Debian provided gcc).
gcc version 4.6.2 (Debian 4.6.2-12) 0m24.395s
gcc version 4.8.0 20120309 (experimental) (GCC) 0m29.159s
The results are quite repeatable. The generated code is different (gcc -O3 -c,
objdump -d, diff)
Built using:
gcc -Wall -c -O3 fib.c
---------------
#include <stdio.h>
unsigned int fib(unsigned int n)
{
if (n < 2)
return n;
return fib(n-2) + fib(n-1);
}
int main()
{
printf("%d\n", fib(48));
return 0;
}
---------------
(48 was chosen to show the different in performance but also run in a tolerable
about of time)
Testing various options didn't close the gap, with only a slight improvement
from -mtune=opteron with is unexpected as these are Intel Westmere systems.
