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From: "dominiq at lps dot ens.fr" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug libfortran/38199] [4.7/4.8/4.9 Regression] missed optimization: I/O performance
Date: Wed, 12 Mar 2014 15:16:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38199

--- Comment #41 from Dominique d'Humieres <dominiq at lps dot ens.fr> ---
> Created attachment 32335 [details]
> Updated patch taking care of NIST failures and cleaned up
>
> This patch regression tests and passes NIST on x86-64.  I have restructured it 
> and took care of leaping on uninitialized values.
>
> Dominiq. Can you give it a spin? Thanks

Confirmed. Now the two test in comment 35 execute in ~0.1s. Note that the test

      character buffer(1)*100000
      integer i,j

      buffer = ''
      buffer(1)(6:100000) = repeat('a',99995)

      DO j=1,9999
        write(buffer(1)(1:4),'(i4)') j
        read(buffer,*) i
      ENDDO
      print *, "'",buffer(1)(1:10), '...', buffer(1)(99990:100000), "'"
      end

obviously does not benefit of the speed-up due to the trimming:

[Book15] f90/bug% time a.out
 '9999 aaaaa...aaaaaaaaaaa'
11.380u 0.007s 0:11.38 100.0%    0+0k 0+1io 0pf+0w

Without the patch, the run time is slightly shorter:

[Book15] f90/bug% time a.out
 '9999 aaaaa...aaaaaaaaaaa'
10.617u 0.001s 0:10.61 100.0%    0+0k 0+0io 0pf+0w

One possibility would be to "go to end" as soon as the value(s) is(are) read.
This would also fix pr58324 and pr59727.