[Bug libfortran/78549] New: Very slow formatted internal file output

[mecej4 at operamail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78549

            Bug ID: 78549
           Summary: Very slow formatted internal file output
           Product: gcc
           Version: 6.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libfortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mecej4 at operamail dot com
  Target Milestone: ---

Created attachment 40163
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40163&action=edit
Program to demonstrate bug

Formatted internal writes to a string variable are very slow in many recent
versions of GFortran, especially the Windows ports. The attached F77 program,
and F95 versions of the same, demonstrate the problem. The heart of the problem
is the slowness of

     write (str,'(1P,10E12.3)') x

(where x is a real array of dimension 10), compared to the equivalent call to
sprintf in C. Unlike in some old Gfortran bug reports, there are no issues
related to Windows versus Linux file system and I/O system discrepancies. NO
external FILE I/O is involved.

Here are some timings, in seconds, on two systems.

SYSTEM: Linux, XEON E5-2630, GFortran/GCC 6.1.1/F2C

   gfortran -Ofast                     6.4
   F2C and gcc -Ofast                  4.8
   gcc -Ofast                          1.8

SYSTEM: Windows 10-64 laptop, Haswell i5-4200U CPU

   F2C, gcc 6.2 -O2 -march=haswell     7.8
   G77 -O2 -march=pentium             12.7
   gfortran 5.2 32-bit Cygwin         49.6
   gfortran 6.2 Mingw-64, Eq.com      65.6

   gcc 6.2                             5.2
   Ifort 17.0.1                        2.3

Here is the source code of the C program, which I am placing here since I am
allowed only one attachment.

   #include <stdio.h>
   int main(){
      char str[121];
      float x[10]={ 7.569, 5.556, -1.640,  9.362,  1.057,
                   -2.385,-9.541,  1.449, -7.885, -1.108};
      int i,j;
      for(i=0; i<1000000; i++){      
sprintf(str,"%12.3e%12.3e%12.3e%12.3e%12.3e%12.3e%12.3e%12.3e%12.3e%12.3e",
      x[0],x[1],x[2],x[3],x[4],x[5],x[6],x[7],x[8],x[9]);
      if((i+1)%200000 == 0)printf("%8d %s\n",i+1,str);
      }
}