Re: [patch, libfortran] [4.7/4.8/4.9 Regression] PR38199 missed optimization: I/O performance

[Manfred Schwarb]

Am 08.03.2014 23:37, schrieb Manfred Schwarb:

Am 08.03.2014 07:38, schrieb Jerry DeLisle:

The attached patch addresses the problem identified in comment #22 of the PR.
For character array internal unit reads, eat_spaces must call next_char to
advance every single character until the end of the string is reached.  In the
case sited which is very contrived, this amounts to about 100000 calls to 
next_char.

For clarity, this test case:

       character buffer(1)*100000
       integer i,j

       j = 1234
       write(buffer(1),'(i4)') j

       DO j=1,9999
!        write(*,*) buffer(1)(1:4)
         read(buffer,*) i
!        write(*,*) i
       ENDDO
       end

Without the patch takes about 25 seconds to run.

With the patch this takes about 2.8 seconds.

This is great.

However, this is still 10 times slower than the LEN_TRIM variant:
         character buffer(1)*100000
         integer i,j

         j = 1234
         write(buffer(1),'(i4)') j

         DO j=1,9999
  !        write(*,*) buffer(1)(1:4)
           read(buffer(1)(1:LEN_TRIM(buffer(1))),*) i
  !        write(*,*) i
         ENDDO
         end

and also 10 times slower than the scalar variant (which was fixed by Thomas):
      character buffer*100000
      integer i,j

      j = 1234
      write(buffer,'(i4)') j

      DO j=1,9999
!        write(*,*) buffer(1:4)
        read(buffer,*) i
!        write(*,*) i
      ENDDO
      end

which takes 0.23s on my box. So on the on hand the improvement is great,
on the other hand it is really sad, because the user will still
need to do manual LEN_TRIM's when reading larger strings to get
optimal performance...

Thanks,
Manfred

The speedup is accomplished by simply skipping over spaces without calling
next_read, then backing up one character and letting the existing execution path
proceed, preserving all the end of record code needed in next_char.

I also remove some unneeded error checks.

Regression tested on X86_64 gnu.  No need for a new test case since no new
functionality is added.

OK for trunk? The PR is marked as a regression, so I think this could be the
last piece and call it done.

Regards,

Jerry

2014-03-08  Jerry DeLisle  <jvdeli...@gcc.gnu>

    PR libfortran/38199
    * io/list_read.c (next_char): Delete unuseful error checks.
    (eat_spaces): For character array reading, skip ahead over
    spaces rather than call next_char multiple times.