[Bug c++/57206] New: Auto-vectorization fails when array index is an unsigned int expression

["snagavallis at outlook dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57206

             Bug #: 57206
           Summary: Auto-vectorization fails when array index is an
                    unsigned int expression
    Classification: Unclassified
           Product: gcc
           Version: 4.7.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: snagavallis@outlook.com


Basic examples given below. Please see attachment for more examples. 

Auto-vectorize fails:
void bad0(float * d, unsigned int n)
{
  unsigned int i;
  for (i=n; i>0; --i) 
    d[n-i] = 0.0;
}


Auto-vectorize succeeds:
void good0(float * d, int n)
{
  int i;
  for (i=0; i<n; i++) 
    d[i] = 0.0;
}

void good1(float * d, int n)
{
  unsigned int i;
  for (i=0; i<n; i++) 
    d[i] = 0.0;
}

void good2(float * d, int n)
{
  int i;
  for (i=n; i>0; --i) 
    d[n-i] = 0.0;
}

$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.7/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu/Linaro
4.7.3-1ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-4.7/README.Bugs
--enable-languages=c,c++,go,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.7 --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.7 --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 --enable-objc-gc --with-cloog --enable-cloog-backend=ppl
--disable-cloog-version-check --disable-ppl-version-check --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 4.7.3 (Ubuntu/Linaro 4.7.3-1ubuntu1) 

$ gcc -march=native -Q --help=target
The following options are target specific:
  -m128bit-long-double                [disabled]
  -m32                                [disabled]
  -m3dnow                             [disabled]
  -m3dnowa                            [disabled]
  -m64                                [enabled]
  -m80387                             [enabled]
  -m8bit-idiv                         [disabled]
  -m96bit-long-double                 [enabled]
  -mabi=                              sysv
  -mabm                               [disabled]
  -maccumulate-outgoing-args          [disabled]
  -maddress-mode=                     short
  -maes                               [disabled]
  -malign-double                      [disabled]
  -malign-functions=                  0
  -malign-jumps=                      0
  -malign-loops=                      0
  -malign-stringops                   [enabled]
  -mandroid                           [disabled]
  -march=                             corei7
  -masm=                              att
  -mavx                               [disabled]
  -mavx2                              [disabled]
  -mavx256-split-unaligned-load     [disabled]
  -mavx256-split-unaligned-store     [disabled]
  -mbionic                            [disabled]
  -mbmi                               [disabled]
  -mbmi2                              [disabled]
  -mbranch-cost=                      0
  -mcld                               [disabled]
  -mcmodel=                           32
  -mcpu=                              
  -mcrc32                             [disabled]
  -mcx16                              [enabled]
  -mdispatch-scheduler                [disabled]
  -mf16c                              [disabled]
  -mfancy-math-387                    [enabled]
  -mfentry                            [enabled]
  -mfma                               [disabled]
  -mfma4                              [disabled]
  -mforce-drap                        [disabled]
  -mfp-ret-in-387                     [enabled]
  -mfpmath=                           387
  -mfsgsbase                          [disabled]
  -mfused-madd                        
  -mglibc                             [enabled]
  -mhard-float                        [enabled]
  -mieee-fp                           [enabled]
  -mincoming-stack-boundary=          0
  -minline-all-stringops              [disabled]
  -minline-stringops-dynamically     [disabled]
  -mintel-syntax                      
  -mlarge-data-threshold=             0x10000
  -mlwp                               [disabled]
  -mlzcnt                             [disabled]
  -mmmx                               [disabled]
  -mmovbe                             [disabled]
  -mms-bitfields                      [disabled]
  -mno-align-stringops                [disabled]
  -mno-fancy-math-387                 [disabled]
  -mno-push-args                      [disabled]
  -mno-red-zone                       [disabled]
  -mno-sse4                           [disabled]
  -momit-leaf-frame-pointer           [disabled]
  -mpc32                              [disabled]
  -mpc64                              [disabled]
  -mpc80                              [disabled]
  -mpclmul                            [disabled]
  -mpopcnt                            [enabled]
  -mprefer-avx128                     [disabled]
  -mpreferred-stack-boundary=         0
  -mpush-args                         [enabled]
  -mrdrnd                             [disabled]
  -mrecip                             [disabled]
  -mrecip=                            
  -mred-zone                          [enabled]
  -mregparm=                          0
  -mrtd                               [disabled]
  -msahf                              [enabled]
  -msoft-float                        [disabled]
  -msse                               [enabled]
  -msse2                              [enabled]
  -msse2avx                           [disabled]
  -msse3                              [enabled]
  -msse4                              [enabled]
  -msse4.1                            [enabled]
  -msse4.2                            [enabled]
  -msse4a                             [disabled]
  -msse5                              
  -msseregparm                        [disabled]
  -mssse3                             [enabled]
  -mstack-arg-probe                   [disabled]
  -mstackrealign                      [enabled]
  -mstringop-strategy=                [default]
  -mtbm                               [disabled]
  -mtls-dialect=                      gnu
  -mtls-direct-seg-refs               [enabled]
  -mtune=                             corei7
  -muclibc                            [disabled]
  -mveclibabi=                        [default]
  -mvect8-ret-in-mem                  [disabled]
  -mvzeroupper                        [disabled]
  -mx32                               [disabled]
  -mxop                               [disabled]

  Known assembler dialects (for use with the -masm-dialect= option):
    att intel

  Known ABIs (for use with the -mabi= option):
    ms sysv

  Known code models (for use with the -mcmodel= option):
    32 kernel large medium small

  Valid arguments to -mfpmath=:
    387 387+sse 387,sse both sse sse+387 sse,387

  Known vectorization library ABIs (for use with the -mveclibabi= option):
    acml svml

  Known address mode (for use with the -maddress-mode= option):
    long short

  Valid arguments to -mstringop-strategy=:
    byte_loop libcall loop rep_4byte rep_8byte rep_byte unrolled_loop

  Known TLS dialects (for use with the -mtls-dialect= option):
    gnu gnu2