Bug#768574: gcc-4.9: Miscompilation of boolean negation on SH4 using -O2

[Michael Karcher]

Package: gcc-4.9
Version: 4.9.1-16
Severity: important

The currently available version of gcc 4.9 for mips miscompiles boolean 
negation under certain circumstances.
The assembly contains the "not" instruction, which represents bitwise negation, 
which is not appropriate, as
both 0 and 1 get mapped to a non-zero bit-pattern, but the following check of 
the boolean tests it against 0.

See the attached example file. The assertion fails if compiled with -O2 but 
succeeds without optimization.
This is actually a heavily stripped-down example of a miscompilation of 
binutils, making linking of most C++
programs on SH4 fail. The misbehaviour of binutils is reported in
https://sourceware.org/bugzilla/show_bug.cgi?id=17553 , but this bug has been 
correctly rejected as it is
not caused by the binutils source code.

-- System Information:
Debian Release: jessie/sid
  APT prefers unreleased
  APT policy: (500, 'unreleased'), (500, 'unstable')
Architecture: sh4 (sh4a)

Kernel: Linux 3.2.44-00829-g14e6110 (PREEMPT)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash

Versions of packages gcc-4.9 depends on:
ii  binutils        2.24.90.20141104-1
ii  cpp-4.9         4.9.1-16
ii  gcc-4.9-base    4.9.1-16
ii  libc6           2.19-11
ii  libcloog-isl4   0.18.2-1
ii  libgcc-4.9-dev  4.9.1-16
ii  libgmp10        2:6.0.0+dfsg-6
ii  libisl10        0.12.2-2
ii  libmpc3         1.0.2-1
ii  libmpfr4        3.1.2-1
ii  zlib1g          1:1.2.8.dfsg-2

Versions of packages gcc-4.9 recommends:
ii  libc6-dev  2.19-11

Versions of packages gcc-4.9 suggests:
pn  gcc-4.9-doc      <none>
pn  gcc-4.9-locales  <none>
pn  libasan1-dbg     <none>
pn  libatomic1-dbg   <none>
pn  libcilkrts5-dbg  <none>
pn  libgcc1-dbg      <none>
pn  libgomp1-dbg     <none>
pn  libitm1-dbg      <none>
pn  liblsan0-dbg     <none>
pn  libquadmath-dbg  <none>
pn  libtsan0-dbg     <none>
pn  libubsan0-dbg    <none>

-- no debconf information

*** /home/glaubitz/gcctest/gccfail.c
#include <assert.h>

int decision_result;
int truecount = 0;
int val;

void buggy(int flag)
{
  int condition;
  if(flag == 0)
    condition = val != 0;
  else
    condition = !decision_result;
  if (condition)
  {
     truecount++;
  }
}

int main(void)
{
  decision_result = 1;
  buggy(1);
  assert(truecount == 0);
}


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