1. An Evaluation of Auto-Scoping in OpenMP
Michael Voss, Eric Chiu, Patrick Chow, Catherine Wong and Kevin Yuen
ECE Department
University of Toronto

2. An Overview of Auto-scoping
Dieter an Mey proposed Auto-scoping as an extension to OpenMP (
Relieve users from burden of explicit scoping
error prone
tedious
compromise: explicit and automatic parallelization
analysis is similar to automatic parallelization
successful in 1 of 2 scientific programs
WOMPAT 2004

3. Using DEFAULT(AUTO) C$OMP PARALLEL DO SHARED(A,B) C$OMP&PRIVATE(I,J)
DO I = 1,100
DO J = 1,100
A(I,J) = A(J,I)
+ B(I,J)
ENDDO
C$OMP END PARALLEL DO
C$OMP PARALLEL DO
C$OMP&DEFAULT(AUTO)
DO I = 1,100
DO J = 1,100
A(I,J) = A(J,I)
+ B(I,J)
ENDDO
C$OMP END PARALLEL DO
WOMPAT 2004

4. Outline of Talk Introduction Implementing DEFAULT(AUTO) in Polaris
An evaluation of DEFAULT(AUTO) in Polaris
comparison with EA Sun Studio 9 F95 compiler
A Discussion of runtime support
Related Work
Conclusion
WOMPAT 2004

5. Implementing DEFAULT(AUTO) in Polaris
Polaris is auto-parallelizer for Fortran 77
Supports a range of advanced techniques
The Range Test
The Omega Test
Array and Scalar Privatization
Array and Scalar Reduction Recognition
Induction Variables Substitution
Interprocedural Constant Propagation
Most Interprocedural Optimization by Inlining
WOMPAT 2004

6. Polaris as an OMP to OMP Translator
Parser
DDtest pass
Reduction pass
Privatization pass …
OpenMP Backend
Fortran 77
Fortran 77 +
OpenMP
Polaris
Parser
Moerae Backend
Fortran 77 +
Moerae calls
Fortran 77 +
OpenMP
Original automatic parallelization path
OpenMP to explicitly threaded code path
New OpenMP to OpenMP path
WOMPAT 2004

7. Supporting DEFAULT(AUTO)
Parse DEFAULT(AUTO)
React appropriately to user directives
selective loop parallelization
no changes without AUTO directive
user scoping overrides Polaris scoping
can parallelize loops that cannot be fully auto-scoped
Limitations
only regions with PARALLEL DO semantics
bails out on general parallel regions
WOMPAT 2004

8. Example 1: No explicit scoping
!$OMP PARALLEL DEFAULT(AUTO)
DO N = 1,7
DO M = 1,7
!$OMP DO
DO L = LSS(itsub),LEE(itsub)
I = IG(L)
J = JG(L)
K = KG(L)
LIJK = L2IJK(L)
RHS(L,M) = RHS(L,M)
+ - FJAC(LIJK,LM00,M,N)*DQCO(i-1,j,k,n,NB)*FM00(L)
+ - FJAC(LIJK,LP00,M,N)*DQCO(i+1,j,k,n,NB)*FP00(L)
+ - FJAC(LIJK,L0M0,M,N)*DQCO(i,j-1,k,n,NB)*F0M0(L)
+ - FJAC(LIJK,L0P0,M,N)*DQCO(i,j+1,k,n,NB)*F0P0(L)
ENDDO
!$OMP END DO NOWAIT
!$OMP END PARALLEL
WOMPAT 2004

9. Example 1: No explicit scoping
!$OMP PARALLEL
!$OMP+DEFAULT(SHARED) !$OMP+PRIVATE(M,L,N)
DO n = 1, 7, 1
DO m = 1,7, 1
!$OMP DO
DO l = lss(itsub), lee(itsub), 1
rhs(l, m) = rhs(l, m)+(-dqco(ig(l), (-1)+jg(l), kg(l), n, nb))*
*f0m0(l)*fjac(l2ijk(l), l0m0, m, n)+(-dqco(ig(l), 1+jg(l), kg(l), n
*, nb))*f0p0(l)*fjac(l2ijk(l), l0p0, m, n)+(-dqco((-1)+ig(l), jg(l)
*, kg(l), n, nb))*fjac(l2ijk(l), lm00, m, n)*fm00(l)+(-dqco(1+ig(l)
*, jg(l), kg(l), n, nb))*fjac(l2ijk(l), lp00, m, n)*fp00(l)
ENDDO
!$OMP END DO NOWAIT
!$OMP END PARALLEL
WOMPAT 2004

10. Example 2: Explicit scoping
SUBROUTINE RECURSION(n,k,a,b,c,d,e,f,g,h,s)
REAL*8 A(*),B(*),C(*),D(*),E(*),F(*),G(*),H(*)
REAL*8 T,S
INTEGER N,K,I
S = 0.0D0
C$OMP PARALLEL SHARED(D)
C$OMP+DEFAULT(AUTO)
C$OMP DO
DO I = 1,N
T = F(I) + G(I)
A(I) = B(I) + C(I)
D(I+K) = D(I) + E(I)
H(I) = H(I) * T
S = S + H(I)
END DO
C$OMP END DO
C$OMP END PARALLEL
END
WOMPAT 2004

11. Example 2: Explicit scoping
SUBROUTINE recursion(n, k, a, b, c, d, e, f, g, h, s)
DOUBLE PRECISION a, b, c, d, e, f, g, h, s, t
INTEGER*4 i, k, n
DIMENSION a(*), b(*), c(*), d(*), e(*), f(*), g(*), h(*)
s = 0.0D0
!$OMP PARALLEL
!$OMP+DEFAULT(SHARED)
!$OMP+PRIVATE(T,I)
!$OMP DO
!$OMP+REDUCTION(+:s)
DO i = 1, n, 1
t = f(i)+g(i)
a(i) = b(i)+c(i)
d(i+k) = d(i)+e(i)
h(i) = h(i)*t
s = h(i)+s
ENDDO
!$OMP END DO
!$OMP END PARALLEL
RETURN
END
WOMPAT 2004

12. Evaluation of DEFAULT(AUTO)
Fortran 77 Benchmarks from SPEC OpenMP
removed all explicit scoping
added DEFAULT(AUTO) to all regions
used Omni OpenMP compiler as backend (-O2)
Explicit speedup –vs- auto-scope speedup
four processor Xeon server
1.8 GHz processors, 16 GBytes main memory
Hyperthreaded, but only used 1 thread per CPU
Also used EA Sun Studio 9 Fortran 95 compiler
supports DEFAULT(__AUTO)
report number of regions auto-scoped
WOMPAT 2004

13. Performance of Auto-scoping
Benchmark
Regions
% Scoped
Polaris
Sun
Explicit
Speedup
Auto-Scoped
Applu 33
94%
72%
5.8
1.1
Apsi 32
47%
25%
2.8 1
Mgrid 12
100%
83%
3.6
Swim 8
63%
1.9
Wupwise 10
40%
20%
3.0
1.25
Sun results are for the Early Access Version of the Sun Microsystems Studio 9 Fortran 95 compiler.
WOMPAT 2004

14. Discussion Many regions were not fully analyzable
Polaris could not fully inline the regions
several regions were general parallel regions
Early Access Sun Studio 9 compiler
auto-scoped fewer regions in general
missed important regions in Swim and Mgrid
regions could be parallelized but not auto-scoped
Sun compiler could auto-scope some regions that Polaris could not
can analyze general parallel regions
WOMPAT 2004

15. A general parallel region from Wupwise Polaris fails but the Sun compiler succeeds
C$OMP PARALLEL DEFAULT(AUTO)
LSCALE = ZERO
LSSQ = ONE
C$OMP DO
DO IX = 1, 1 + (N - 1) *INCX, INCX
IF (DBLE (X(IX)) .NE. ZERO) THEN
...
LSSQ = ONE + LSSQ* (LSCALE / TEMP) ** 2
LSCALE = TEMP
END IF
END DO
C$OMP END DO
C$OMP CRITICAL
IF (SCALE .LT. LSCALE) THEN
SSQ = ((SCALE / LSCALE) ** 2) * SSQ + LSSQ
SCALE = LSCALE
ELSE
SSQ = SSQ + ((LSCALE / SCALE) ** 2) * LSSQ
C$OMP END CRITICAL
C$OMP END PARALLEL
WOMPAT 2004

16. Runtime Support for Auto-scoping
add speculate directive for regions that cannot be auto-scoped
applies to very few regions in SPEC OpenMP
requires interprocedural marking of reads/writes
only 2 regions not auto-scoped can be fully analyzed
!$OMP PARALLEL
!$OMP+DEFAULT(SHARED)
!$OMP+PRIVATE(U51K,U41K,U31K,Q,U21K,M,K,I,U41,U31KM1,U51KM1,U21KM1)
!$OMP+PRIVATE(U41KM1,TMP,J)
!$OMP+SPECULATE(UTMP,RTMP)
!$OMP DO
!$OMP+LASTPRIVATE(FLUX2)
DO j = jst, jend, 1
...
ENDDO
!$OMP END DO
!$OMP END PARALLEL
(a region from the RHS subroutine of Applu)
WOMPAT 2004

17. Related Work DEFAULT(AUTO) proposed by Dieter an Mey
Many commercial and research auto-parallelizers
Polaris, SUIF, CAPO, …
Perform parallelization and scoping
The EA Sun Studio 9 Fortran 95 Compiler
paper also here at WOMPAT
thanks to Yuan Lin for pointing me to it
Runtime dependence testing
Saltz, Rauchwerger, …
WOMPAT 2004

18. Conclusion Implemented DEFAULT(AUTO) in Polaris
created full OpenMP to OpenMP translator
added facilities for auto-scoping
Evaluated implementation
2 of 5 benchmarks fully auto-scoped
remainder showed significant loss of speedup
results different from EA Sun compiler
performance not portable across compilers
Discussed speculative parallelization support
WOMPAT 2004

19. Conclusion cont… Combination of loop and region analyzer
Polaris auto-scoped more regions
Sun compiler can handle general regions
Performance not be portable across compilers
never is but…
sacrifice performance for convenience
perhaps a useful tool during manual parallelization
Future work
general region support in Polaris
WOMPAT 2004