[Bug c/55700] New: i386: incorrect divide-by-zero exception generated

[soren.sandmann at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55700

             Bug #: 55700
           Summary: i386: incorrect divide-by-zero exception generated
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: soren.sandmann@gmail.com


Created attachment 28972
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=28972
The reproducer program

The attached program generates a divide-by-zero on i686 when compiled with -O2.
The divide-by-zero comes from this fragment:

    if (t == 0.0f)
        b = sa * da;
    else
        b = sa * sa * d / (t);

which as far as I know should be a legitimate way to ensure division by zero
doesn't happen. In particular, the t value generated is a negative zero, which
should compare equal to 0.0f unless I'm misunderstanding something.

I have reproduced the bug with 4.7.0 and 4.7.2 running in 32 bit mode, and with
4.7.2 with -m32 on x86-64.

On x86-64:

localhost:~% gcc -m32 -W -Wextra -Wall -g -O2 gccfloat.c -lm
localhost:~% ./a.out 
Floating exception

localhost:~% gcc -v
Using built-in specs.
COLLECT_GCC=/usr/bin/gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/4.7.2/lto-wrapper
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla
--enable-bootstrap --enable-shared --enable-threads=posix
--enable-checking=release --disable-build-with-cxx
--disable-build-poststage1-with-cxx --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-gnu-unique-object
--enable-linker-build-id --with-linker-hash-style=gnu
--enable-languages=c,c++,objc,obj-c++,java,fortran,ada,go,lto --enable-plugin
--enable-initfini-array --enable-java-awt=gtk --disable-dssi
--with-java-home=/usr/lib/jvm/java-1.5.0-gcj-1.5.0.0/jre
--enable-libgcj-multifile --enable-java-maintainer-mode
--with-ecj-jar=/usr/share/java/eclipse-ecj.jar --disable-libjava-multilib
--with-ppl --with-cloog --with-tune=generic --with-arch_32=i686
--build=x86_64-redhat-linux
Thread model: posix
gcc version 4.7.2 20120921 (Red Hat 4.7.2-2) (GCC)