[Bug 837303] [NEW] gcov produce incorrect numbers for while loops and for loops with no increment part

[Kasper Dupont]

Public bug reported:

gcov produce correct execution count for a typical for loop such as:
for (i=0;i<10;++i)

However if the increment part is removed from the for statement and put
at the end of the loop instead, the program is equivalent, but gcov in
that case will double count the line with the for statement once per
time the for loop is reached. That is if the loop was not entered at
all, the line where the for loop starts is counted twice.

The same incorrect behaviour is seen with any while loop.

Here is the gcov output from four variations of a program demonstrating
incorrect execution counts in three of the four cases.

        -:    1:#include <stdio.h>
        -:    2:
        1:    3:int main()
        -:    4:{
        -:    5:  int i;
       11:    6:  for (i=0;i<10;++i) {
       10:    7:    printf("%d\n",i);
        -:    8:  }
        1:    9:  return 0;
        -:   10:}

        -:    1:#include <stdio.h>
        -:    2:
        1:    3:int main()
        -:    4:{
        -:    5:  int i;
       12:    6:  for (i=0;i<10;) {
       10:    7:    printf("%d\n",i);
       10:    8:    ++i;
        -:    9:  }
        1:   10:  return 0;
        -:   11:}

        -:    1:#include <stdio.h>
        -:    2:
        1:    3:int main()
        -:    4:{
        1:    5:  int i=0;
       12:    6:  while (i<10) {
       10:    7:    printf("%d\n",i);
       10:    8:    ++i;
        -:    9:  }
        1:   10:  return 0;
        -:   11:}

        -:    1:#include <stdio.h>
        -:    2:
        1:    3:int main()
        -:    4:{
        1:    5:  int i=20;
        2:    6:  while (i<10) {
    #####:    7:    printf("%d\n",i);
    #####:    8:    ++i;
        -:    9:  }
        1:   10:  return 0;
        -:   11:}

ProblemType: Bug
DistroRelease: Ubuntu 10.04
Package: gcc 4:4.4.3-1ubuntu1
ProcVersionSignature: Ubuntu 2.6.32-33.72-generic 2.6.32.41+drm33.18
Uname: Linux 2.6.32-33-generic i686
Architecture: i386
Date: Tue Aug 30 12:47:01 2011
EcryptfsInUse: Yes
InstallationMedia: Ubuntu 10.04.3 LTS "Lucid Lynx" - Release i386 (20110720.1)
ProcEnviron:
 PATH=(custom, user)
 LANG=en_DK.utf8
 SHELL=/bin/bash
SourcePackage: gcc-defaults

** Affects: gcc-defaults (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: apport-bug i386 lucid

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https://bugs.launchpad.net/bugs/837303

Title:
  gcov produce incorrect numbers for while loops and for loops with no
  increment part

Status in “gcc-defaults” package in Ubuntu:
  New

Bug description:
  gcov produce correct execution count for a typical for loop such as:
  for (i=0;i<10;++i)

  However if the increment part is removed from the for statement and
  put at the end of the loop instead, the program is equivalent, but
  gcov in that case will double count the line with the for statement
  once per time the for loop is reached. That is if the loop was not
  entered at all, the line where the for loop starts is counted twice.

  The same incorrect behaviour is seen with any while loop.

  Here is the gcov output from four variations of a program
  demonstrating incorrect execution counts in three of the four cases.

          -:    1:#include <stdio.h>
          -:    2:
          1:    3:int main()
          -:    4:{
          -:    5:  int i;
         11:    6:  for (i=0;i<10;++i) {
         10:    7:    printf("%d\n",i);
          -:    8:  }
          1:    9:  return 0;
          -:   10:}

          -:    1:#include <stdio.h>
          -:    2:
          1:    3:int main()
          -:    4:{
          -:    5:  int i;
         12:    6:  for (i=0;i<10;) {
         10:    7:    printf("%d\n",i);
         10:    8:    ++i;
          -:    9:  }
          1:   10:  return 0;
          -:   11:}

          -:    1:#include <stdio.h>
          -:    2:
          1:    3:int main()
          -:    4:{
          1:    5:  int i=0;
         12:    6:  while (i<10) {
         10:    7:    printf("%d\n",i);
         10:    8:    ++i;
          -:    9:  }
          1:   10:  return 0;
          -:   11:}

          -:    1:#include <stdio.h>
          -:    2:
          1:    3:int main()
          -:    4:{
          1:    5:  int i=20;
          2:    6:  while (i<10) {
      #####:    7:    printf("%d\n",i);
      #####:    8:    ++i;
          -:    9:  }
          1:   10:  return 0;
          -:   11:}

  ProblemType: Bug
  DistroRelease: Ubuntu 10.04
  Package: gcc 4:4.4.3-1ubuntu1
  ProcVersionSignature: Ubuntu 2.6.32-33.72-generic 2.6.32.41+drm33.18
  Uname: Linux 2.6.32-33-generic i686
  Architecture: i386
  Date: Tue Aug 30 12:47:01 2011
  EcryptfsInUse: Yes
  InstallationMedia: Ubuntu 10.04.3 LTS "Lucid Lynx" - Release i386 (20110720.1)
  ProcEnviron:
   PATH=(custom, user)
   LANG=en_DK.utf8
   SHELL=/bin/bash
  SourcePackage: gcc-defaults

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