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
