[PR42542] g++ -O3 produces incorrect results for less-than operator

[Nathan Bell]

Binary package hint: g++-4.3

g++ appears to be (incorrectly) using a signed int comparison when -O3 optimization is enabled. Compiling with -O2 or lower produces the correct output.

This problem was observed on Ubuntu 9.04 x86_64. The source code (bug.cpp) is attached.

$ lsb_release -rd
Description: Ubuntu 9.04
Release: 9.04

$ uname -a
Linux rabota 2.6.28-13-generic #45-Ubuntu SMP Tue Jun 30 22:12:12 UTC 2009 x86_64 GNU/Linux

$ apt-cache policy g++-4.3
g++-4.3:
  Installed: 4.3.3-5ubuntu4
  Candidate: 4.3.3-5ubuntu4
  Version table:
 *** 4.3.3-5ubuntu4 0
        500 http://us.archive.ubuntu.com jaunty/main Packages
        100 /var/lib/dpkg/status

$ g++ --version
g++ (Ubuntu 4.3.3-5ubuntu4) 4.3.3

$ g++ -O2 bug.cpp && ./a.out
4264598888

$ g++ -O3 bug.cpp && ./a.out
1898580612

$ g++ -v -O3 bug.cpp
Using built-in specs.
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.3.3-5ubuntu4' --with-bugurl=file:///usr/share/doc/gcc-4.3/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/include/c++/4.3 --program-suffix=-4.3 --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --enable-mpfr --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.3.3 (Ubuntu 4.3.3-5ubuntu4)
COLLECT_GCC_OPTIONS='-v' '-O3' '-shared-libgcc' '-mtune=generic'
 /usr/lib/gcc/x86_64-linux-gnu/4.3.3/cc1plus -quiet -v -D_GNU_SOURCE bug.cpp -D_FORTIFY_SOURCE=2 -quiet -dumpbase bug.cpp -mtune=generic -auxbase bug -O3 -version -fstack-protector -o /tmp/cc1tnMId.s
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/4.3.3/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/include/c++/4.3
 /usr/include/c++/4.3/x86_64-linux-gnu
 /usr/include/c++/4.3/backward
 /usr/local/include
 /usr/lib/gcc/x86_64-linux-gnu/4.3.3/include
 /usr/lib/gcc/x86_64-linux-gnu/4.3.3/include-fixed
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
GNU C++ (Ubuntu 4.3.3-5ubuntu4) version 4.3.3 (x86_64-linux-gnu)
 compiled by GNU C version 4.3.3, GMP version 4.2.4, MPFR version 2.4.0.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: d9dd67dcc191271ac69fd2c05266cf9c
COLLECT_GCC_OPTIONS='-v' '-O3' '-shared-libgcc' '-mtune=generic'
 as -V -Qy -o /tmp/ccZsdHhv.o /tmp/cc1tnMId.s
GNU assembler version 2.19.1 (x86_64-linux-gnu) using BFD version (GNU Binutils for Ubuntu) 2.19.1
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.3.3/:/usr/lib/gcc/x86_64-linux-gnu/4.3.3/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.3.3/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.3.3/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.3.3/:/usr/lib/gcc/x86_64-linux-gnu/4.3.3/:/usr/lib/gcc/x86_64-linux-gnu/4.3.3/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/4.3.3/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-O3' '-shared-libgcc' '-mtune=generic'
 /usr/lib/gcc/x86_64-linux-gnu/4.3.3/collect2 --eh-frame-hdr -m elf_x86_64 --hash-style=both -dynamic-linker /lib64/ld-linux-x86-64.so.2 -z relro /usr/lib/gcc/x86_64-linux-gnu/4.3.3/../../../../lib/crt1.o /usr/lib/gcc/x86_64-linux-gnu/4.3.3/../../../../lib/crti.o /usr/lib/gcc/x86_64-linux-gnu/4.3.3/crtbegin.o -L/usr/lib/gcc/x86_64-linux-gnu/4.3.3 -L/usr/lib/gcc/x86_64-linux-gnu/4.3.3 -L/usr/lib/gcc/x86_64-linux-gnu/4.3.3/../../../../lib -L/lib/../lib -L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/4.3.3/../../.. /tmp/ccZsdHhv.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/lib/gcc/x86_64-linux-gnu/4.3.3/crtend.o /usr/lib/gcc/x86_64-linux-gnu/4.3.3/../../../../lib/crtn.o

[Matthias Klose (doko)]

it's a bug in your test case, result is overflowing. use `long' instead

[Nathan Bell (wnbell)]

All of those numbers are less than 4294967296 (i.e. 2^32), so they can be stored in 32-bit *unsigned* integers.

Nevermind that "overflow" refers to the result of an arithmetic operation, and no arithmetic operations are being used here.

Please don't dismiss this issue with nothing more than a cursory look. I'm quite confident this is a bug.

[Launchpad Janitor (janitor)]

This bug was fixed in the package gcc-4.4 - 4.4.2-9ubuntu1

---------------
gcc-4.4 (4.4.2-9ubuntu1) lucid; urgency=low

  * Merge with Debian.

gcc-4.4 (4.4.2-9) unstable; urgency=low

  [ Matthias Klose ]
  * Update to SVN 20100108 from the gcc-4_4-branch (r155727).
    - Fixes PR tree-optimization/42337, PR target/42448, PR middle-end/42099,
      PR target/42549 (closes: #560812), PR tree-optimization/41956,
      PR target/42503, PR other/42611 (LP: #501921), PR target/42511,
      PR target/42564, PR tree-optimization/42614,
      PR target/42542 (LP: #403744).
  * libgcj10: Move .so symlinks into gcj-4.4-jdk. Addresses: #563280.
  * Don't use expect-tcl8.3 on hppa anymore.
  * Merge gnat-4.4 changes back from 4.4.2-5.
  * Default to v9a (ultrasparc) on sparc*-linux.
  * gcj-4.4-jdk: Include /usr/lib/jvm-exports.
  * Use 4.4.3 as the upstream version (we'll end up with this version
    for the next release anyway).
  * Always build libgcc4 on hppa, even if not building libgcc for other
    archs (gcc-4.5 bumps the soversion on hppa to 6).
  * Base the source build-dependency on the package version instead of the
    gcc version.
  * Build-depend on autogen again.

  [ Nobuhiro Iwamatsu ]
  * Update gcc-multiarch patch for sh4.
 -- Matthias Klose <email address hidden> Fri, 08 Jan 2010 08:47:20 +0100

[Nathan Bell (wnbell)]

Thanks Matthias!

[Matthias Klose (doko)]

closing the 4.3 task as won't fix. fixed in lucid.