[see http://www.polyhedron.co.uk/pb05/linux/f90bench_AMD.html for the original
polyhedron benchmark results, an explanation of what the benchmark is and the
source code]
Typical timings for the gas_dyn.f90 benchmark on my AMD64/linux system are:
* ifort -O3 -xW -ipo -static -V gas_dyn.f90 -o gas_dyn.intel
=> ./gas_dyn.intel 10.53s user 0.43s system 99% cpu 10.976 total
* gfortran -static -ftree-vectorize -march=opteron -ffast-math -funroll-loops
-O3 gas_dyn.f90 -o gas_dyn.gfortran
./gas_dyn.gfortran 15.92s user 0.05s system 99% cpu 15.969 total
Experimenting a bit with Intel options to understand why it is so fast, I found
that:
* disabling inlining doesn't change the execution time
* disabling vectorization drops it to the same execution time as gfortran
(roughly speaking)
Following an analysis by Tobias Burnus, and noting that 22.16% of the total
time is spent in the MINLOC library routine, I modified the source by replacing
a call to MINLOC by inline code:
--- gas_dyn.f90 2007-03-07 09:36:23.000000000 +0100
+++ gas_dyn.modified.f90 2007-03-07 10:44:14.000000000 +0100
@@ -234,12 +234,23 @@ end module ints
!-----------------------------------------------
! L o c a l V a r i a b l e s
!-----------------------------------------------
- INTEGER :: ISET(1)
- REAL :: VSET, SSET
+ INTEGER :: ISET(1), I
+ REAL :: VSET, SSET, T
REAL, DIMENSION (NODES) :: DTEMP
!-----------------------------------------------
DTEMP = DX/(ABS(VEL) + SOUND)
- ISET = MINLOC (DTEMP)
+! FXC replace this:
+! ISET = MINLOC (DTEMP)
+! by this:
+ ISET(1) = 0
+ T = HUGE(T)
+ DO I = 1, NODES
+ IF (DTEMP(I) < T) THEN
+ T = DTEMP(I)
+ ISET(1) = I
+ END IF
+ END DO
+! end of modification
DT = DTEMP(ISET(1))
VSET = VEL(ISET(1))
SSET = SOUND(ISET(1))
this makes the code faster by 14%:
./gas_dyn.modified.gfortran 13.56s user 0.05s system 99% cpu 13.614 total
Maybe we should have MINLOC inlined when there's no mask, stride 1 and
one-dimensional?
PS: Other hot spots are:
% cumulative self self total
time seconds seconds calls Ts/call Ts/call name
29.13 4.18 4.18 eos_ (gas_dyn.f90:410 @
413386)
14.22 6.22 2.04 chozdt_ (gas_dyn.f90:241
@ 4152b3)
Both lines are whole-array operations, corresponding to:
CS(:NODES) = SQRT(CGAMMA*PRES(:NODES)/DENS(:NODES))
and
DTEMP = DX/(ABS(VEL) + SOUND)
I filed PR31066, which is I think a small reproducer for the two lines above.
--
Summary: MINLOC should sometimes be inlined (gas_dyn is sooooo
sloooow)
Product: gcc
Version: 4.3.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: fortran
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: fxcoudert at gcc dot gnu dot org
BugsThisDependsOn: 31066
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31067
