[Bug tree-optimization/66974] New: -Warray-bounds false positive with -O3

[gajjanagadde at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66974

            Bug ID: 66974
           Summary: -Warray-bounds false positive with -O3
           Product: gcc
           Version: 5.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gajjanagadde at gmail dot com
                CC: gajjanagadde at gmail dot com
  Target Milestone: ---

Created attachment 36039
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36039&action=edit
gcc_warraybounds_false.i

Compiling the following code on GCC 5.2.0 produces a bogus out of bounds
warning at -O3 level:

-------
int foo(unsigned order) {
    int c[3] = {1, 2, 3};
    unsigned i, j;
    for (i = 1; i < order; i++) {
        for (j = 0; j < i / 2; j++) {
            c[j] += c[i] * c[i-j-1];
            c[i-j-1] += c[i] * c[j];
        }
    }
    return c[0];
}

----------------------
%gcc -Warray-bounds -O3 -c gcc_warraybounds_false.c

gcc_warraybounds_false.c: In function ‘foo’:
gcc_warraybounds_false.c:6:22: warning: array subscript is above array bounds
[-Warray-bounds]
             c[j] += c[i] * c[i-j-1];
                      ^
gcc_warraybounds_false.c:7:26: warning: array subscript is above array bounds
[-Warray-bounds]
             c[i-j-1] += c[i] * c[j];
                          ^
gcc_warraybounds_false.c:6:22: warning: array subscript is above array bounds
[-Warray-bounds]
             c[j] += c[i] * c[i-j-1];
                      ^
gcc_warraybounds_false.c:6:29: warning: array subscript is above array bounds
[-Warray-bounds]
             c[j] += c[i] * c[i-j-1];
                             ^
gcc_warraybounds_false.c:7:22: warning: array subscript is above array bounds
[-Warray-bounds]
             c[i-j-1] += c[i] * c[j];
                      ^
gcc_warraybounds_false.c:7:26: warning: array subscript is above array bounds
[-Warray-bounds]
             c[i-j-1] += c[i] * c[j];
                          ^
gcc_warraybounds_false.c:7:22: warning: array subscript is above array bounds
[-Warray-bounds]
             c[i-j-1] += c[i] * c[j];


-----------------------

config:
Using built-in specs.
COLLECT_GCC=/usr/bin/gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-unknown-linux-gnu/5.2.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: /build/gcc/src/gcc-5.2.0/configure --prefix=/usr
--libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man
--infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared
--enable-threads=posix --enable-libmpx --with-system-zlib --with-isl
--enable-__cxa_atexit --disable-libunwind-exceptions --enable-clocale=gnu
--disable-libstdcxx-pch --disable-libssp --enable-gnu-unique-object
--enable-linker-build-id --enable-lto --enable-plugin
--enable-install-libiberty --with-linker-hash-style=gnu
--enable-gnu-indirect-function --disable-multilib --disable-werror
--enable-checking=release --with-default-libstdcxx-abi=gcc4-compatible
Thread model: posix
gcc version 5.2.0 (GCC)