No, that would not be enough. Doing that change would just make the front end assume no branch was ever taken (and the sim would actually crash because the commit logic would later would try to inform the front end about branches it never predicted).
To do this (with the saving branch outcomes to a file & later just replaying them as if you had predicted them right the first time), you'd have to hook the proper spaces in the squash/update functions in bpred_unit_impl.hh to figure out what the correct predictions should have been. Then save these correct decisions off to a gzipped file (I haven't thought how big this is going to be, but this file will be pretty massive). Then re-run the simulation with the "predict" from bpred_unit_impl.hh just feeding the correct path information from this file. On Mon, Oct 22, 2012 at 9:14 PM, mihai pricopi <[email protected]>wrote: > Thanks. > Would be enough to change a bit the lookupAndUpdateNextPC() function such > that the return is always false ? > I mean changing the: > > if (!inst->isControl()) { > TheISA::advancePC(nextPC, inst->staticInst); > inst->setPredTarg(nextPC); > inst->setPredTaken(false); > return false; > } > > > to > > // if (!inst->isControl()) { > TheISA::advancePC(nextPC, inst->staticInst); > inst->setPredTarg(nextPC); > inst->setPredTaken(false); > return false; > // } > > ? > > > On Tue, Oct 23, 2012 at 10:02 AM, Mitch Hayenga < > [email protected]> wrote: > >> Since gem5 the O3 cpu model actually executes instructions @ execute (not >> fetch/decode) a perfect branch predictor is a bit tricky. Assuming you are >> running a single-threaded app in SE mode (so you don't have >> OS/multi-threaded time variance issues), you could simply run the >> application twice. Save the branch outcomes from the first run. Then >> re-run the app, with a "predictor" that just uses the branch results from >> the first time you ran the application. >> >> On Mon, Oct 22, 2012 at 8:43 PM, mihai pricopi >> <[email protected]>wrote: >> >>> Hi, >>> >>> I am using arm_detailed SE mode. >>> Is there a simple way to force the branch predictor to do perfect >>> prediction ? >>> >>> Thanks >>> >>> _______________________________________________ >>> gem5-users mailing list >>> [email protected] >>> http://m5sim.org/cgi-bin/mailman/listinfo/gem5-users >>> >> >> >> _______________________________________________ >> gem5-users mailing list >> [email protected] >> http://m5sim.org/cgi-bin/mailman/listinfo/gem5-users >> > > > _______________________________________________ > gem5-users mailing list > [email protected] > http://m5sim.org/cgi-bin/mailman/listinfo/gem5-users >
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