[Bug 1770676] Re: gcc optimizer bug

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** Changed in: gcc
       Status: Unknown => New

** Changed in: gcc
   Importance: Unknown => Medium

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https://bugs.launchpad.net/bugs/1770676

Title:
  gcc optimizer bug

Status in gcc:
  New
Status in gcc-5 package in Ubuntu:
  Confirmed
Status in gcc-7 package in Ubuntu:
  Confirmed

Bug description:
  While investigating why cronolog was no longer changing a log symlink
  in bionic, I discovered a bug in gcc (reproducable in 7.3.0-16ubuntu3
  in bionic and 5.4.0-6ubuntu1~16.04.9 in xenial) for code compiled with
  -O2. I have created a simple testcase to reproduce the problem, which
  is attached.

  You can reproduce the problem with the following:

  # ====
  set -x
  echo "First, we compile and run without optimization:"
  gcc -o testcase testcase.c && ./testcase

  echo "Then, we compile and run with optimization:"
  gcc -O2 -o testcase testcase.c && ./testcase
  # ====

  Which outputs:
  + echo 'First, we compile and run without optimization:'
  First, we compile and run without optimization:
  + gcc -o testcase testcase.c
  + ./testcase
  foo in func() is NULL (expected)
  foo in func() is NULL (expected)
  foo in main() is NULL (expected)
  foo in main() is NULL (expected)
  + echo 'Then, we compile and run with optimization:'
  Then, we compile and run with optimization:
  + gcc -O2 -o testcase testcase.c
  + ./testcase
  foo in func() is NULL (expected)
  foo in func() is not NULL (NOT EXPECTED!)
  foo in main() is NULL (expected)
  foo in main() is NULL (expected)

  The problem occurs after calling:

    stat(foo, &stat_buf);

  Where foo is a NULL pointer. After the return from this function, foo
  will no longer be NULL when the code is compiled with optimization -
  but only when the pointer is a function parameter of func(). The issue
  does not occur when the same codepath is called in main(). (You can
  argue calling stat with a NULL pointer is bad behavior, however this
  code has been in cronolog, working fine when compiled with -O2, for
  years).

  I can reproduce this easily on multiple systems, bionic with "gcc
  (Ubuntu 7.3.0-16ubuntu3) 7.3.0" and xenial with "gcc (Ubuntu
  5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609".

  I could not initially reproduce the cronolog testcase in xenial,
  leading me to some suspicion that it may be related to retpoline or
  other recent compiler changes - and in fact after recompiling cronolog
  on xenial I can reproduce the issue there as well.

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