// Check squash signals from commit.
    if (fromCommit->commitInfo[tid].squash) {

        DPRINTF(Fetch, "[tid:%i] Squashing instructions due to squash "
                "from commit.\n",tid);
        // In any case, squash.
        squash(*fromCommit->commitInfo[tid].pc,
               fromCommit->commitInfo[tid].doneSeqNum,
               fromCommit->commitInfo[tid].squashInst, tid);

        // If it was a branch mispredict on a control instruction, update the
        // branch predictor with that instruction, otherwise just kill the
        // invalid state we generated in after sequence number
        if (fromCommit->commitInfo[tid].mispredictInst &&
            fromCommit->commitInfo[tid].mispredictInst->isControl()) {
            branchPred->squash(fromCommit->commitInfo[tid].doneSeqNum,
                    *fromCommit->commitInfo[tid].pc,
                    fromCommit->commitInfo[tid].branchTaken, tid);
        } **else {
            branchPred->squash(fromCommit->commitInfo[tid].doneSeqNum,
                              tid);
        }**

        return true;
    }

Hello, In function checkSignalsAndUpdate(ThreadID tid) in src/cpu/o3/fetch.cc file, it seems miss prediction can still happen from commit and decode even if mispredictInst->isControl() is false. ``` // Check squash signals from commit. if (fromCommit->commitInfo[tid].squash) { DPRINTF(Fetch, "[tid:%i] Squashing instructions due to squash " "from commit.\n",tid); // In any case, squash. squash(*fromCommit->commitInfo[tid].pc, fromCommit->commitInfo[tid].doneSeqNum, fromCommit->commitInfo[tid].squashInst, tid); // If it was a branch mispredict on a control instruction, update the // branch predictor with that instruction, otherwise just kill the // invalid state we generated in after sequence number if (fromCommit->commitInfo[tid].mispredictInst && fromCommit->commitInfo[tid].mispredictInst->isControl()) { branchPred->squash(fromCommit->commitInfo[tid].doneSeqNum, *fromCommit->commitInfo[tid].pc, fromCommit->commitInfo[tid].branchTaken, tid); } **else { branchPred->squash(fromCommit->commitInfo[tid].doneSeqNum, tid); }** return true; } ``` Can someone please explain when would this else condition be true? How can there be miss prediction of non-control instructions?

// Check squash signals from commit.
     if (fromCommit->commitInfo[tid].squash) {

         DPRINTF(Fetch, "[tid:%i] Squashing instructions due to squash "
                 "from commit.\n",tid);
         // In any case, squash.
         squash(*fromCommit->commitInfo[tid].pc,
                fromCommit->commitInfo[tid].doneSeqNum,
                fromCommit->commitInfo[tid].squashInst, tid);

         // If it was a branch mispredict on a control instruction, update the
         // branch predictor with that instruction, otherwise just kill the
         // invalid state we generated in after sequence number
         if (fromCommit->commitInfo[tid].mispredictInst &&
             fromCommit->commitInfo[tid].mispredictInst->isControl()) {
             branchPred->squash(fromCommit->commitInfo[tid].doneSeqNum,
                     *fromCommit->commitInfo[tid].pc,
                     fromCommit->commitInfo[tid].branchTaken, tid);
         } **else {
             branchPred->squash(fromCommit->commitInfo[tid].doneSeqNum,
                               tid);
         }**

         return true;
     }

On 10/21/2024 8:41 AM, Atul Rahman via gem5-users wrote: > Hello, > In function checkSignalsAndUpdate(ThreadID tid) in src/cpu/o3/fetch.cc file, it seems miss prediction can still happen > from commit and decode even if mispredictInst->isControl() is false. > ``` > // Check squash signals from commit. >     if (fromCommit->commitInfo[tid].squash) { > >         DPRINTF(Fetch, "[tid:%i] Squashing instructions due to squash " >                 "from commit.\n",tid); >         // In any case, squash. >         squash(*fromCommit->commitInfo[tid].pc, >                fromCommit->commitInfo[tid].doneSeqNum, >                fromCommit->commitInfo[tid].squashInst, tid); > >         // If it was a branch mispredict on a control instruction, update the >         // branch predictor with that instruction, otherwise just kill the >         // invalid state we generated in after sequence number >         if (fromCommit->commitInfo[tid].mispredictInst && >             fromCommit->commitInfo[tid].mispredictInst->isControl()) { >             branchPred->squash(fromCommit->commitInfo[tid].doneSeqNum, >                     *fromCommit->commitInfo[tid].pc, >                     fromCommit->commitInfo[tid].branchTaken, tid); >         } **else { >             branchPred->squash(fromCommit->commitInfo[tid].doneSeqNum, >                               tid); >         }** > >         return true; >     } > ``` > Can someone please explain when would this else condition be true? How can there be miss prediction of non-control > instructions? I am thinking that these are non-control instructions that are on the mispredicted path. They also need to be squashed. If that's not it, then possibly instructions like CMOV (conditional move) that might be (mis)predicted even though they do not affect control flow. Not sure if there might be other cases ... Eliot Moss