http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47298
Summary: -O3 destroys beautifully vectorized code obtained at
-O2
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: joost.vandevond...@pci.uzh.ch
current trunk generates really fast vectorized code for the following testcase
(a 12x12x12 matrix multiply, c=c+a*b, benchmarked with a,b,c in cache) as can
be seen from the assembly:
> cat compare.f90
SUBROUTINE HARD_NN_12_12_12(C,A,B)
REAL(KIND=8), INTENT(INOUT) :: C(12,*)
REAL(KIND=8), INTENT(IN) :: B(12,*), A(12,*)
INTEGER ::i,j,l
DO j=1,12 ; DO i=1,12; DO l=1,12
C(i,j)=C(i,j)+A(i,l)*B(l,j)
ENDDO ; ENDDO ; ENDDO
END SUBROUTINE HARD_NN_12_12_12
however, this only happens with:
gfortran-trunk -O2 -funroll-loops -ftree-vectorize -ffast-math -march=corei7
-msse4.2 compare.f90
while switch -O2 to -O3 causes 'bad' code.
gfortran-trunk -O3 -funroll-loops -ftree-vectorize -ffast-math -march=corei7
-msse4.2 compare.f90
with the following tester below
-O2 runs in about 4.4s
-O3 runs in about 7.0s
> cat test_compare.f90
REAL(KIND=8), DIMENSION(12,12) :: A,B,C
A=0 ; B=0 ; C=0
DO I=1,10000000
CALL HARD_NN_12_12_12(C,12,A,12,B,12)
ENDDO
END
