[Bug c/61108] New: gcc 4.8.2 generates incorrect integer arithmetic at O3 (ok at O2)

["corydoras at ridiculousfish dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=61108

            Bug ID: 61108
           Summary: gcc 4.8.2 generates incorrect integer arithmetic at O3
                    (ok at O2)
           Product: gcc
           Version: 4.8.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: corydoras at ridiculousfish dot com

(My first time filing a gcc bug, apologies if I screw it up)

I have a loop with a high multiply and some arithmetic. gcc generates code that
computes the wrong value as the result, under -O3 on x86-64. It computes the
correct value under -O2. The key difference appears to be that the loop is
vectorized under -O3.

To reproduce: compile the following as 'gcc -O3 test.c' When run, it will
abort. Now compile as 'gcc -O2 test.c' and it will not abort.

#include <stdlib.h>
#include <stdint.h>

#define ITERS 8

__attribute__((noinline))
uint32_t sum(const uint32_t *vals)
{
    uint32_t result = 0;
    int i;
    for (i=0; i < ITERS; i++)
    {
        uint32_t val = vals[i];
        uint32_t q = (val * 613566757LLU) >> 32;
        uint32_t t = ((val + q) >> 1);
        result += t;
    }
    return result;
}

int main(void)
{
    uint32_t vals[ITERS] = {};
    vals[1] = 7;
    if (sum(vals) != 4)
        abort();
    return 0;
}





> 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 4.8.2-19ubuntu1'
--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 --disable-libmudflap
--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 4.8.2 (Ubuntu 4.8.2-19ubuntu1)