[Bug tree-optimization/53265] New: Warn when undefined behavior implies smaller iteration count

["amonakov at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53265

             Bug #: 53265
           Summary: Warn when undefined behavior implies smaller iteration
                    count
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: amonakov@gcc.gnu.org


As requested in PR 53128, this is an enhancement request that asks to produce a
warning when loop bound inferred from undefined behavior may be used to
transform the loop in a surprising way.

Example code:

enum {N=4};
int a[N], pfx[N];
void foo()
{
  int i, accum;
  for (i=0, accum=a[0]; i < N; i++, accum+=a[i])
    pfx[i] = accum;
}

VRP in trunk produces an infinite loop from the above. By the way, with
-fno-tree-vrp one would expect that cunroll unrolls just 3 iterations of the
loop, but in fact it unrolls 4, because nb_iterations is not updated when
nb_iterations_upper_bound is reduced from UB analysis.

Can we simply warn when UB analysis reduces nb_iterations_upper_bound and makes
it statically smaller than symbolic nb_iterations?