------- Comment #6 from bonzini at gnu dot org 2009-02-04 12:02 -------
We have:
4.3 4.4
-O2 3.36 3.32
-O3 3.02 3.50
-O3 -funroll-all-loops 2.92 3.45
Interestingly enough, if I _really_ hand-unroll the loop as suggested by the
wrong summary, 4.4 is faster than 4.3:
4.3 4.4
-O2 3.53 3.23
-O3 3.53 3.23
-O3 -funroll-all-loops 3.45 3.18
This means:
1) that -funroll-all-loops is not really affecting the benchmark, but only the
test code
2) that the regression is actually at -O3, not at -O2, and there's really no
guarantee of what performance you get past -O2
3) ultimately, that this is not a regression, but just an enhancement request:
4.4 -O2 is better, but not as good as 4.3 -O3.
--- here is the hand-unrolled loop code:
module foo
implicit none
contains
subroutine unroll(a,b,c)
real, dimension(3,3) :: a,b,c
c(1,1) = a(1,1)*b(1,1) + a(1,2)*b(2,1) + a(1,3)*b(3,1)
c(1,2) = a(1,1)*b(1,2) + a(1,2)*b(2,2) + a(1,3)*b(3,2)
c(1,3) = a(1,1)*b(1,3) + a(1,2)*b(2,3) + a(1,3)*b(3,3)
c(2,1) = a(2,1)*b(1,1) + a(2,2)*b(2,1) + a(2,3)*b(3,1)
c(2,2) = a(2,1)*b(1,2) + a(2,2)*b(2,2) + a(2,3)*b(3,2)
c(2,3) = a(2,1)*b(1,3) + a(2,2)*b(2,3) + a(2,3)*b(3,3)
c(3,1) = a(3,1)*b(1,1) + a(3,2)*b(2,1) + a(3,3)*b(3,1)
c(3,2) = a(3,1)*b(1,2) + a(3,2)*b(2,2) + a(3,3)*b(3,2)
c(3,3) = a(3,1)*b(1,3) + a(3,2)*b(2,3) + a(3,3)*b(3,3)
end subroutine unroll
end module foo
program main
use foo
implicit none
integer :: k
real, dimension(3,3) :: a,b,c, a0, b0
real :: t1, t2
character(len=90) :: line
call random_number(a0)
call random_number(b0)
a = a0
b = b0
call cpu_time(t1)
do k=1,10**8
call unroll(a,b,c)
end do
call cpu_time(t2)
print *,"subroutine with explicit interface and unroll(1): ",t2-t1, " s"
write (unit=line,fmt='(9G10.3)') c
end program main
--
bonzini at gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Priority|P2 |P3
Last reconfirmed|2008-12-10 13:36:41 |2009-02-04 12:02:14
date| |
Summary|[4.4 Regression] speed |big performance boost from
|regression with hand- |4.3's -O3 vs. -O2 (in any
|unrolled matmul |version) on 3x3 matmul
Target Milestone|4.4.0 |---
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38434
