[Bug rtl-optimization/62011] New: False Data Dependency in popcnt instruction

["debiandev at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62011

            Bug ID: 62011
           Summary: False Data Dependency in popcnt instruction
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: debiandev at gmail dot com

On Sandy/Ivy Bridge and Haswell processors, the instruction:

popcnt src, dest

appears to have a false dependency on the destination register dest. Even
though the instruction only writes to it, the instruction will wait until dest
is ready before executing.

This causes a loss in performance as explained here:
http://stackoverflow.com/questions/25078285/replacing-a-32-bit-loop-count-variable-with-64-bit-introduces-crazy-performance