[Bug tree-optimization/32309] New: Unnecessary conversion from short to unsigend short breaks vectorization

["gangren at google dot com" <gcc-bugzilla at gcc dot gnu dot org>]

void Sub(short * __restrict src1row, short * __restrict src2row, int
num_in_row) {
  for(int i=num_in_row; i--;) {
    *src1row -= *src2row;
    ++src1row;
    ++src2row;
  }
}

In the test case above, GCC inserts several explicit conversions soon after the
gimple transformation stage and gets,

        D.2097 = *src1row;
        D.2098 = (short unsigned int) D.2097;
        D.2099 = *src2row;
        D.2100 = (short unsigned int) D.2099;
        D.2101 = D.2098 - D.2100;
        D.2102 = (short int) D.2101;

These conversions breaks the vectorization and GCC reports,

/* i686-unknown-linux-gnu-gcc -O3 -ftree-vectorize -ftree-vectorizer-verbose=5
-march=nocona -fno-strict-aliasing -c test.cc */

......
test.cc:2: note: not vectorized: relevant stmt not supported: D.2430_11 =
(short unsigned int) D.2429_10
test.cc:1: note: vectorized 0 loops in function.


-- 
           Summary: Unnecessary conversion from short to unsigend short
                    breaks vectorization
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: gangren at google dot com
 GCC build triplet: i686-unknown-linux-gnu-gcc
  GCC host triplet: i686-unknown-linux-gnu-gcc
GCC target triplet: i686-unknown-linux-gnu-gcc


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32309