[Bug tree-optimization/54132] New: Incorrect loop transformation with -ftree-loop-distribute-patterns

[mans at mansr dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54132

             Bug #: 54132
           Summary: Incorrect loop transformation with
                    -ftree-loop-distribute-patterns
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: mans@mansr.com


The following function is incorrectly optimised when
-ftree-loop-distribute-patterns is used (enabled by -O3):

void foo(char *p, int n)
{
    int i;
    for (i = 0; i < n; i++)
        p[i] = p[i - 1];
}

The loop is replaced by a call to memmove(), whereas the correct operation is
equivalent to memset(p, p[-1], n).