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    SubjectRe: [PATCH 1/5] [PATCH 1/5] function-graph/x86: replace unbalanced ret with jmp
    On Tue, Oct 13, 2009 at 05:12:46PM -0400, Steven Rostedt wrote:
    > On Tue, 2009-10-13 at 23:02 +0200, Frederic Weisbecker wrote:
    > > On Tue, Oct 13, 2009 at 04:33:50PM -0400, Steven Rostedt wrote:
    > > > From: Steven Rostedt <srostedt@redhat.com>
    > > >
    > > > The function graph tracer replaces the return address with a hook to
    > > > trace the exit of the function call. This hook will finish by returning
    > > > to the real location the function should return to.
    > > >
    > > > But the current implementation uses a ret to jump to the real return
    > > > location. This causes a imbalance between calls and ret. That is
    > > > the original function does a call, the ret goes to the handler
    > > > and then the handler does a ret without a matching call.
    > > >
    > > > Although the function graph tracer itself still breaks the branch
    > > > predictor by replacing the original ret, by using a second ret and
    > > > causing an imbalance, it breaks the predictor even more.
    > >
    > >
    > >
    > > I have troubles to understand by it breaks the predictor, especially
    > > since there is not conditional branch in return_to_handler.
    > > But still I don't understand why a ret would break more the branch
    > > prediction than a jmp.
    >
    > Calls are branch prediction jumps. Which associates the "ret" with the
    > call. As it approaches the ret, it starts to receive the code after the
    > call.
    >
    > But this is stack order. Every call should hit one ret. But with the
    > original code, we break this stack. We have one call and two rets. Which
    > means that the branch prediction will also get messed up with the
    > previous stored call.


    Oh, ok I got it.
    Thanks.



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