Hi all,
I think, I have run into a similar issue.
There are cases where a control instruction will take the execution to
another control instruction. If both of them are taken and the branch
predictor predicts correctly only for the first one it will:
* correctly set the predTaken flag and
* correctly set the pc field of the PCState structure however the npc
might be incorrect.
Now this is obviously a misprediction for the second control instruction
but not necessarily for the first one as well. The execution could
continue without squashing both.
Consider the execution of the following simple example in ALPHA.
...
12001847c: beq s2,1200184ac
...
1200184ac: beq s1,1200184d8
...
If both of them are taken then the target PCState of the first branch
should be set to
targetPC = { .pc = 1200184ac, .npc = 1200184d8 }.
But the predictor might set it to:
targetPC = { .pc = 1200184ac, npc = 1200184b0 }
thus it correctly predicts the next instruction after the first branch
but misses the right target for the second branch.
Are there any unwanted side-effects if we apply the fix that Nathanaël
proposed to change the way gem5 detects mispredictions? Or is there
something I am missing here?
On 27.06.2012 10:56, Nathanaël Prémillieu wrote:
>
> src/cpu/base_dyn_inst.hh:505
> - return !(tempPC == predPC);
> + return !(tempPC.instAddr() == predPC.instAddr());
>
Thanks,
Nikos
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