[Bug c++/62623] New: Behavior change in test case with -m32 and -O3

["theJ893 at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62623

            Bug ID: 62623
           Summary: Behavior change in test case with -m32 and -O3
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: theJ893 at gmail dot com

Created attachment 33424
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=33424&action=edit
Source file that I am using to reproduce the issue

When compiling unit tests for my code with g++, I discovered a particular unit
test was failing when I compiled it with the -m32 and -O3 options.

Background:
I have a class template for a Vector (the kind used in physics, not the
container). Vector contains a fixed with array of data, and an invert() method
that performs x = 1/x on each element of the array, modifying it in-place.

The unit test consists of initializing a vector "o" with an array of predefined
values, calling o.invert(), and then comparing the values in the vector to a
set of predefined, expected values for equality. The test returns "true" when
compiling with "-m32", "-m64", and "-m64 -O3", but returns "false" when
compiling with "-m32 -O3".

It should be noted, if you change the invert method to treat the Vector as
"volatile", or make use of the values in some way (e.g. by doing srand(
o.data[0] ); or std::cout << o.data[0] << " " o.data[1] << ... << std::endl;)
this circumvents the problem.

I've managed to isolate and reproduce the issue in its own file. I had a friend
compile and run the test program as well and verify that he is getting the same
result. I've attached the source file to this bug report. The .ii file was too
large to attach.

Here is the command line I am using to compile the program, using the requested
switches:
g++ -O3 -m32 -Wall -Wextra -fno-strict-aliasing -fwrapv
-fno-aggressive-loop-optimizations -save-temps -std=c++11 G++O3Bug.cpp -o
bin/G++O3Bug && ./bin/G++O3Bug

As requested, here is the output from gcc -v on my system:
owner@mint ~/Desktop/test $ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.8/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.8.1-10ubuntu9' --with-bugurl=file:///usr/share/doc/gcc-4.8/README.Bugs
--enable-languages=c,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.8 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.8 --libdir=/usr/lib --enable-nls
--with-sysroot=/ --enable-clocale=gnu --enable-libstdcxx-debug
--enable-libstdcxx-time=yes --enable-gnu-unique-object --enable-plugin
--with-system-zlib --disable-browser-plugin --enable-java-awt=gtk
--enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64/jre
--enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-amd64
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --enable-multiarch --disable-werror --with-arch-32=i686
--with-abi=m64 --with-multilib-list=m32,m64,mx32 --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 4.8.1 (Ubuntu/Linaro 4.8.1-10ubuntu9)