[Bug c/79920] New: Incorrect floating point results when compiling with -O3

[wence at gmx dot li]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79920

            Bug ID: 79920
           Summary: Incorrect floating point results when compiling with
                    -O3
           Product: gcc
           Version: 6.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wence at gmx dot li
  Target Milestone: ---

Created attachment 40895
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40895&action=edit
Preprocessed source

The attached (preprocessed) code, which should produce the answer "12",
produces "0" when compiled with -O3 using gcc 6.3.0, and also a 7.0.0 snapshot
I had access to.

Details:

$ gcc-6 -v
Using built-in specs.
COLLECT_GCC=gcc-6
COLLECT_LTO_WRAPPER=/usr/local/Cellar/gcc/6.3.0_1/libexec/gcc/x86_64-apple-darwin16.3.0/6.3.0/lto-wrapper
Target: x86_64-apple-darwin16.3.0
Configured with: ../configure --build=x86_64-apple-darwin16.3.0
--prefix=/usr/local/Cellar/gcc/6.3.0_1
--libdir=/usr/local/Cellar/gcc/6.3.0_1/lib/gcc/6
--enable-languages=c,c++,objc,obj-c++,fortran --program-suffix=-6
--with-gmp=/usr/local/opt/gmp --with-mpfr=/usr/local/opt/mpfr
--with-mpc=/usr/local/opt/libmpc --with-isl=/usr/local/opt/isl
--with-system-zlib --enable-libstdcxx-time=yes --enable-stage1-checking
--enable-checking=release --enable-lto --with-build-config=bootstrap-debug
--disable-werror --with-pkgversion='Homebrew GCC 6.3.0_1'
--with-bugurl=https://github.com/Homebrew/homebrew-core/issues --enable-plugin
--disable-nls --enable-multilib
Thread model: posix
gcc version 6.3.0 (Homebrew GCC 6.3.0_1)

Also occurs with:

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/6/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 6.2.0-5ubuntu12'
--with-bugurl=file:///usr/share/doc/gcc-6/README.Bugs
--enable-languages=c,ada,c++,java,go,d,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-6 --program-prefix=x86_64-linux-gnu- --enable-shared
--enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --libdir=/usr/lib --enable-nls --with-sysroot=/
--enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie
--with-system-zlib --disable-browser-plugin --enable-java-awt=gtk
--enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-6-amd64/jre
--enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-6-amd64
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-6-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 --enable-multilib
--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 6.2.0 20161005 (Ubuntu 6.2.0-5ubuntu12) 

and
$ gcc-snapshot -v
Using built-in specs.
COLLECT_GCC=/usr/lib/gcc-snapshot/bin/gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc-snapshot/libexec/gcc/x86_64-linux-gnu/7.0.0/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
20161006-1ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-snapshot/README.Bugs
--enable-languages=c,ada,c++,go,fortran,objc,obj-c++
--prefix=/usr/lib/gcc-snapshot --program-prefix= --enable-shared
--enable-linker-build-id --disable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie
--with-system-zlib --enable-objc-gc --enable-multiarch --disable-werror
--with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --with-tune=generic --enable-checking=yes
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 7.0.0 20161006 (experimental) [trunk revision 240826] (Ubuntu
20161006-1ubuntu1) 

Steps to reproduce:
$ gcc-6 -O3 -save-temps bug.c -o bug; ./bug
0 # expected 12

Debugging the execution, it looks like the initialisation of t33 is wrong. 
After the  first nested loop (on line 20), every entry of t33 ought to be 1,
however, when compiling with -O3, every entry is 0.

Bug goes away if I constant-fold t43 (either into the initialisation list or
the increment into A).