在Fortran中快速交换数组中的二维

Question

假设我在Fortran A(:,:,:,:)中有一个多维数组，其中每个维数的大小都是相同的，即n。

我想换一下第二和第三个维度。我可以重塑A2 = reshape(A, (/ n, n, n, n/), order = (/1,3,2,4/) )，有没有更快的方法？因为我只需要部分交换。

更新的示例代码

Program reshape_test

  Use, Intrinsic :: iso_fortran_env, Only :  wp => real64, li => int64

  real :: A(32,32,32,32), A2(32,32,32,32)
  integer :: i,j,k,l,n,m, m_iter
  Integer( li ) :: start, finish, rate
  
  
  m_iter = 3200
  n = 32
  do l = 1, n
    do k = 1, n
       do j = 1, n
         do i = 1, n
           A(i,j,k,l) = i + j * k**2 - l
         end do 
       end do   
    end do      
  end do        
 
  Call System_clock( start, rate )
  do m=1,m_iter
    A2 = reshape(A, (/n, n, n, n/), order = (/1,3,2,4/) ) 
  end do
  
 ! write (*,*) A(1,2,3,4), A(1,3,2,4), A2(1,2,3,4), A2(1,3,2,4)
  Call System_clock( finish, rate )  
  
  Write(*,*) 'Time for reshape-1', Real( finish - start, wp ) / rate  
  
  
  Call System_clock( start, rate )
  do m=1,m_iter

  do j = 1, n
    do i = 1, n  
      A2(:,i,j,:) =  A(:,j,i,:)
    end do      
  end do      
  end do
  
  !write (*,*) A(1,2,3,4), A(1,3,2,4), A2(1,2,3,4), A2(1,3,2,4)
  Call System_clock( finish, rate )  
  
  Write(*,*) 'Time for reshape-2', Real( finish - start, wp ) / rate  
    
    
  Call System_clock( start, rate )
  do m=1,m_iter 
  do l = 1, n
    do k = 1, n
       do j = 1, n
         do i = 1, n
           A2(i,k,j,l) = A(i,j,k,l)
         end do 
       end do   
    end do      
  end do   
  end do
  
 ! write (*,*) A(1,2,3,4), A(1,3,2,4), A2(1,2,3,4), A2(1,3,2,4)
  Call System_clock( finish, rate )      
  
  Write(*,*) 'Time for reshape-3', Real( finish - start, wp ) / rate  
    
        

end program reshape_test

在这个例子中，通过gfortran -o3，我得到了(我运行了三次)

 Time for reshape-1   13.500307800000000
 Time for reshape-2   10.146418400000000
 Time for reshape-3   20.489294800000000

 Time for reshape-1   11.421597100000000
 Time for reshape-2   9.3823936999999997
 Time for reshape-3   19.856221900000001

 Time for reshape-1   14.376207500000000
 Time for reshape-2   10.756465400000000
 Time for reshape-3   21.301044500000000

将-march=native引线添加到(我运行了三次)

 Time for reshape-1   12.517529200000000
 Time for reshape-2   10.240939200000000
 Time for reshape-3   26.637998799999998

 Time for reshape-1   12.436479700000000
 Time for reshape-2   9.9803838000000002
 Time for reshape-3   21.760061700000001

 Time for reshape-1   11.419214300000000
 Time for reshape-2   10.908747200000001
 Time for reshape-3   21.445951399999998

总的来说，方法2是最快的。-march=native似乎不是很有效。

(在How to speed up reshape in higher rank tensor contraction by BLAS in Fortran?中，报告定时的代码样式跟随在答案之后)

Answer 1

为什么不试着将reshape与显式循环进行比较，比如

do i = 1, n
  do j = 1, n
     do k = 1, n
       do l = 1, n
         A2(i,k,j,l) = A(i,j,k,l)
       end do 
     end do   
  end do      
end do 

然后你告诉我们答案。它也可以依赖于优化。我建议使用-O3 -march=native，试着换成-O2，-O1和-O0。