------- Comment #11 from xinliangli at gmail dot com 2008-03-13 06:20 ------- (In reply to comment #10) > Subject: Re: New: Error with -fprofile-use > > xinliangli at gmail dot com wrote: > > In the following example, profile data generated by -O0 binary run can not > > be > > used for profile-use at -O2. This is either a bug or design flaw if not > > supported. > > Gcc constructs a minimal spanning tree for the CFG, and then only adds > instrumentation code to arcs that are not on the minimal spanning tree. > We can compute arc counts for the rest of the CFG from this set. We > can compute basic block execution counts from the arc counts. This > reduces the run-time cost of the instrumentation code, since only a > small fraction of the basic blocks require profiling code. However, it > also requires that we have exactly the same CFG with -fprofile-generate > as we have with -fprofile-use, and hence you must use the same > optimization options with both compiles. > > Jim >
Gcc's design minimizes runtime overhead for edge profiling -- this is a very good design. Same CFG is the requirement for other compilers as well, but some compilers do tolerate some differences even with source some source changes -- this give user the flexibility to use slightly stale profile data while still getting some performance benefit. Please note that people already hate FDO due to its complicated steps -- adding more constraints won't help to make them happy. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35544
