[patch, libgfortran] Bug 78549 - [7/8 Regression] Very slow formatted internal file output

[Jerry DeLisle]

Hi all,

After the usual considerable digging around I determined that for the test case
doing 1000000 writes to a string that we were allocating 1000000 newunit numbers
and the associated unit structures on the unit treap. Consequently, at the end
of the program, when all left open units are cleaned up and closed, a lot of
time was being burned.  We did not benefit from the previously allocated unit
structures saved on the treap that could be reused.

IO operations on strings complete with st_read_done or st_write_done of the
parent IO procedure.  To fix the problem, I simply declared newunit_alloc
accessible to transfer.c and invoked it.

Now unit numbers and their associated unit structures are reused as they should
be and internal unit I/O is just a little bit better than on gfortran version 6.

My test results with the test case in the PR are as follows. (I commented out
the intermediate write to stdout for clarity)

Patched trunk:

$ gfc -static pr78549.f
$ time ./a.out

real	0m22.476s
user	0m22.183s
sys	0m0.180s

And gfortran 6:

$ gfc6 -static pr78549.f
$ time ./a.out

real	0m22.790s
user	0m22.633s
sys	0m0.011s

Unpatched gfortran 7:

$ gfc7 -static pr78549.f
$ time ./a.out

real	0m29.915s
user	0m28.750s
sys	0m0.971s


Regression tested on x86-64-linux.

I will commit to trunk in a day and back port to 7 in a few more days if no
objections.  The patch is simple.

Regards,

Jerry

2017-11-19  Jerry DeLisle  <jvdelisle@gcc.gnu.org>

	PR libgfortran/78549
	* io/io.h (newunit_free): Add declaration. Clean some whitespace.
	* io/transfer.c (st_read_done, st_write_done): Call newunit_free.
	* io/unit.c (newunit_free): Change type from static void to void.
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