[Bug fortran/82938] New: Speed regression in internal read

["manfred99 at gmx dot ch" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82938

            Bug ID: 82938
           Summary: Speed regression in internal read
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: manfred99 at gmx dot ch
  Target Milestone: ---

#!/bin/sh

seq --format="%.1f" 1 1000000 > read.txt
cat > read.f <<EOF
      real f
      character*12 buffer
      open(10,file="read.txt")
  1   read(10,'(a)',end=2) buffer
      read(buffer,'(f12.0)') f
      !!read(buffer,*) f
      print*,f
      goto 1
  2   close(10)
      end
EOF
cmd="gfc-bin"
$cmd --version
$cmd read.f -g
time ./read >/dev/null


On my box, this short program takes
  2.4s for GNU Fortran (GCC) 8.0.0 20170828 (experimental) [trunk revision
251373]
  4.4s for GNU Fortran (GCC) 8.0.0 20170829 (experimental) [trunk revision
251395]

This is most likely due to
  http://gcc.gnu.org/viewcvs/gcc?view=revision&revision=251374

Of these 4.4s, about 4s are solely taken by the
line "read(buffer,'(f12.0)') f".

It is really "amazing" how slow internal read is,
compare e.g. with a C program ...
This issue clearly shows as a regression in a operational program of mine,
so no artificial, constructed issue.