Bug#859116: GCC miscompiles following code, unlike clang

[Roman Lebedev <lebedev.ri@gmail.com>]

Package: g++-6
Version: 6.3.0-11
Severity: normal
Tags: upstream

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Hi.

Yes, this issue should be sent upstream. But i'm unable to do that
because their bugzilla does not want to reset my password.

The attached testcase was minimized.
With gcc (6), the exit code is 10.
With clang (tried with current svn version), the exit code is 0.
As it should be pretty clear from the code, 0 is the correct answer.

$ ./test.sh
0 compilation passed, GOOD
1 expected output, GOOD
2 compilation passed, GOOD
3 expected output, GOOD
ALL GOOD, valid testcase

- -- System Information:
Debian Release: 9.0
  APT prefers unstable
  APT policy: (990, 'unstable'), (500, 'unstable-debug'), (1, 'experimental-debug'), (1, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 4.9.0-2-amd64 (SMP w/8 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages g++-6 depends on:
ii  gcc-6            6.3.0-11
ii  gcc-6-base       6.3.0-11
ii  libc6            2.24-9
ii  libgmp10         2:6.1.2+dfsg-1
ii  libisl15         0.18-1
ii  libmpc3          1.0.3-1+b2
ii  libmpfr4         3.1.5-1
ii  libstdc++-6-dev  6.3.0-11
ii  zlib1g           1:1.2.8.dfsg-5

g++-6 recommends no packages.

Versions of packages g++-6 suggests:
pn  g++-6-multilib    <none>
ii  gcc-6-doc         6.1.0-1
ii  libstdc++6-6-dbg  6.3.0-11

- -- no debconf information

*** buffer.cpp
struct a {
  int b;
  int c = 0;
  a(int d) : b(d) {}
  int e() { return b - c; }
  void *f(int d) {
    c = d;
    return 0;
  }
};
int g(void *, int h) { return h; }
a i(10);
int j = i.e();
int main() { return g(i.f(j), i.e()); }

*** test.sh
#!/bin/bash

# gcc -E, no -P
# $ creduce --n 16 test.sh ${FILE}

GCC=/usr/bin/gcc-6
CLANG=/usr/bin/clang++-5.0

FILE=buffer.cpp

FLAGS="-std=c++11 -Wall -Wextra -Werror -o buffer"

$GCC $FLAGS $FILE #>/dev/null 2>&1

if [ $? -ne 0 ]
then
  echo "0 compilation failed, bad"
  exit 1;
else
  echo "0 compilation passed, GOOD"
fi

./buffer >/dev/null 2>&1

if [ $? -ne 10 ]
then
  echo "1 unexpected output, bad"
  exit 1;
else
  echo "1 expected output, GOOD"
fi


$CLANG $FLAGS $FILE >/dev/null 2>&1

if [ $? -ne 0 ]
then
  echo "2 compilation failed, bad"
  exit 1;
else
  echo "2 compilation passed, GOOD"
fi

./buffer >/dev/null 2>&1

if [ $? -ne 0 ]
then
  echo "3 unexpected output, bad"
  exit 1;
else
  echo "3 expected output, GOOD"
fi

echo "ALL GOOD, valid testcase"

exit 0;

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