Yes, because not all the squashes are caused by control instructions.
Another source of squash is memory violations
(IEW::SquanshDueToMemOrder).
There, the squash bit toCommit->squash is set, but
toCommit->mispredictInst is set to NULL.
Thus, when the reason for the squash is later checked in Commit::commit() and
then in
Fetch::checkSignalsAndUpdate, the if(fromIEW->mispredictInst[tid]) branch is
not executed.
The branchPred->squash() function in the else-part is to delete all the branch
predictor states that has been made after the squash instruction
(BPredUnit::Squash).
Hope this helps.
Eliot Moss via gem5-users @ 2024-10-21 14:54 :
> 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
> _______________________________________________
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--
Best regards,
Yuan Yao
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