[Bug middle-end/41969] New: [LTO] 23% slow-down with -flto -fwhole-program

[burnus at gcc dot gnu dot org]

The following test case takes

  9.100s with gfortran -O3
 11.113s with gfortran -flto -fwhole-program -O3
  9.582s with ifort -O3

  5.205s with gfortran -ffast-math -march=native -funroll-loops -O3
  7.414s with gfortran -ffast-math -march=native -funroll-loops -flto
-fwhole-program -O3
  9.624s with ifort -xHost -O3

on an AMD Athlon(tm) 64 X2 Dual Core Processor 4800+
running openSUSE 11.2 RC 1 (x86_64).


Thus the LTO version takes 23% more - with unrolled loops/fast math and
-march=native, the LTO version is even 42% slower.


Without the "print" statement, the "ifort" version only takes 0.002s which
means that it optimizes the complete loop away - GCC does not (why?).
Otherwise, gfortran wins the race by 5% to 46%.


module vector_ops
contains
  FUNCTION vector_function(v1,v2) RESULT(v3)
    IMPLICIT NONE
    INTEGER, PARAMETER :: ndim = 100
    REAL*8, DIMENSION(ndim), INTENT(IN) :: v1, v2
    REAL*8, DIMENSION(ndim) :: v3
    INTEGER :: i

    DO i = 1, ndim
      v3(i) = 0.5D0 * v1(i) + 0.5D0 * v2(i)
    END DO
  END FUNCTION vector_function
end module vector_ops

PROGRAM vectorsyntax
  USE vector_ops
  IMPLICIT NONE
  INTEGER, PARAMETER :: ndim = 100
  REAL*8, DIMENSION(ndim) :: u, v, w
  INTEGER :: i, iter

  DO i = 1, ndim
    CALL random_number( u(i) )
    CALL random_number( v(i) )
  END DO

  iter = 50000000
  DO i = 1, iter
    w = vector_function( u, v )
  END DO
  print *, w(1),w(50),w(100)
END PROGRAM


-- 
           Summary: [LTO] 23% slow-down with -flto -fwhole-program
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Keywords: lto
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: burnus at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41969