[Bug tree-optimization/64365] New: Predictive commoning after loop vectorization produces incorrect code.

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64365

            Bug ID: 64365
           Summary: Predictive commoning after loop vectorization produces
                    incorrect code.
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: congh at google dot com

Compiling the following loop with -O3 on x86-64 produces incorrect code:


void foo(int *in) {
  for (int i = 14; i >= 10; i--) {
    in[i - 8] -= in[i];
    in[i - 5] += in[i] * 2;
    in[i - 4] += in[i];
  }
}


The incorrect code appears starting from pcom pass. Note that after this loop
is vectorized there exists read-after-write data dependence between the second
and third statements in the loop. The correct way to get the vector from in[i -
4] in the third statement is reading the memory after the write from the second
statement. However, in pcom pass, that vector is actually preloaded before the
loop. I think pcom ignores the aliasing between the memory addresses of vector
types (in this case MEM[&{in[i-3] : in[i-0]}] and MEM[&{in[i-5] : in[i-1]}].