On 9/27/07, Hariharan Sandanagobalane <[EMAIL PROTECTED]> wrote: > Hello, > I am implementing support for PBO on picochip port of GCC (not yet > submitted to mainline). > > I see that GCC generates 2 files, xx.gcno and xx.gcda, containing the > profile information, the former containing the flow graph > information(compile-time) and later containing the edge profile > information(run-time). The CFG information seems to be getting emitted > quite early in the compilation process(pass_tree_profile). Is the > instrumentation also done at this time? If it is, as later phases change
Yes. > CFG, how is the instrumentation code sanity maintained? If it isnt, How Instrumentation code sanity is naturally maintained since those are global load/stores. The compiler transformations naturally preserve the original semantic of the input and since profile counters are global variables, update to those are preserved to provide what unoptimized code would do. > would you correlate the CFG in gcno file to the actual CFG at > execution(that produces the gcda file)? > As for our port's case, we are already able to generate profile > information using our simulator/hardware, and it is not-too-difficult > for me to format that information into .gcno and .gcda files. But, i > guess the CFG that i would have at runtime would be quite different from > the CFG at initial phases of compilation (even at same optimization > level). Any suggestions on this? Would i be better off keeping the gcno > file that GCC generates, try to match the runtime-CFG to the one on the > gcno file and then write gcda file accordingly? Not only better off, you *need* to provide information that matches what's in gcno, otherwise gcc can't read that gcda nor use it. How you match gcno is a different problem - there's no guarantee that you'll be able to recover enough information from the output assembly of gcc, because without instrumentation, gcc can optimize away the control flow. pass_tree_profile is when both the instrumentation (with -fprofile-generate) and reading of the profile data (with -fprofile-use) are done. The CFG has to remain the same between generate and use - otherwise the compiler isn't able to use the profile data. Seongbae
