On Fri, Nov 22, 2013 at 4:51 AM, Rong Xu <x...@google.com> wrote: > Hi, > > This patch injects a condition into the instrumented code for edge > counter update. The counter value will not be updated after reaching > value 1. > > The feature is under a new parameter --param=coverage-exec_once. > Default is disabled and setting to 1 to enable. > > This extra check usually slows the program down. For SPEC 2006 > benchmarks (all single thread programs), we usually see around 20%-35% > slow down in -O2 coverage build. This feature, however, is expected to > improve the coverage run speed for multi-threaded programs, because > there virtually no data race and false sharing in updating counters. > The improvement can be significant for highly threaded programs -- we > are seeing 7x speedup in coverage test run for some non-trivial google > applications. > > Tested with bootstrap.
Err - why not simply emit counter = 1 for the counter update itself with that --param (I don't like a --param for this either). I assume that CPUs can avoid data-races and false sharing for non-changing accesses? Richard. > Thanks, > > -Rong
