[patch] Cleanup and improvement of if-conversion for vectorization

[Sebastian Pop]

Hi,

The attached patch-set cleans the code of the current if-conversion
pass, separates the analysis of the basic block predicates from the
transformation, and improves the if-conversion by transforming loops
containing conditions with memory references.  Conditional writes to
memory are handled as non conditional writes by reading and writing
back the same value, like this:

+/* Predicate each write to memory in LOOP.
+
+   Replace a statement "A[i] = foo" with "A[i] = cond ? foo : A[i]"
+   with the condition COND determined from the ->aux field of the
+   basic block containing the statement.  */

With this patch set, I am XFAIL-ing vect-ifcvt-18.c that I am
considering unsafe for the if-conversion as currently implemented in
trunk.  I am XFAIL-ing it until I get a patch that checks that a
statically allocated data reference cannot trap in a loop accessed
niter times.  vect-ifcvt-18.c looks like the testcase of
http://gcc.gnu.org/PR43423

__attribute__ ((noinline)) void
foo (int mid, int n)
{
  int i;

  for (i = 0; i < n; i++)
    if (i < mid)
      A[i] = A[i] + B[i];
    else
      A[i] = A[i] + C[i];
}

After if-conversion the accesses to B and C would be executed for
every iteration, and without proving that B and C have "n" elements,
the if-converted code could trap.

The patch-set passes test and bootstrap with BOOT_CFLAGS="-g -O3".
Ok for trunk?

Thanks,
Sebastian
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