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Static Outcome Prediction
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From the course by Princeton University
Computer Architecture
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From the lesson
Branch Prediction
This lecture covers the motivation and implementation of branch predictors.
Meet the Instructors
David WentzlaffAssociate Professor
Electrical Engineering
So let's look at some techniques to deal with outcome prediction for branches, and
we're gonna look at this mostly, first of all, in the static domain.
So things that are not trying to watch the genetic instruction sequence, but instead
are, can, basically statically analyze the instruction sequence and with some high
probability, we'll say, try to predict the location.
The first technique is actually not a prediction at all.
The first technique, we had talked about this a few lectures ago, or we, actually
we talked about this in lecture, two adding delay slots.
So, instead of, I don't know, having that time just be dead cycles, when we're
trying to resolve the branch. We, let's say we put a, if we don't
resolve the branch 'til here, we have what say one delay slot, two delay slots, three
delay slots, and we have three instructions, we'll say.
Or maybe you can redirect at this point out of X and you have two branched delay
slots, depending on how you sort of wire that in if you can make the cycle time.
Let's say if two branch delay slots, what ends up happening is you have instruction
EEQZ here. Let's say that is at address 100.
Then you have two instructions which execute no matter whether the branch is
taken or not taken, that follow it. So these instructions are in the delay
slots of the branch, and you know, if you have architectures have which have things
like load delay slots and other things like that.
But for right now we'll talk about branches.
And we can force these instructions always to execute.
Unfortunately, if you go look at the sort of percentages of probability that you can
actually fill these delay slots, it's pretty low.
It's, always not easy to find work to, to shove down here, 'cuz you're basically
taking work that was above the branch, and in the compiler, reordering the code to
put the, instructions that were before the branch, after the branch.
Hm. Okay.
Well, this is good. I mean, if you can actually make this work
out, this is, this is great. One of the problems with this, though, is
the probability of filling one branch delay slot, let's say is 70%.
The probability to filling two branch delay slots is even lower than that.
It's maybe, 50ish to 40ish%. As we said before, if you have a monkey
pulling out of a, a hat, or just some random process pulling out of a hat,
you're going to do 50%, correct. So.
And as we'll see today, if you use some more fancy branch prediction, or the
outcome prediction techniques, you can get the prediction accuracy of a branch all
the way up into the sort of 95, 96, 97 percent probability.
And all the way people have built, if you look at your sort of out of order
super-scaler on your desktop, your core, or I-7 these days from Intel, that's
probably somewhere in the neighborhood of 98 to 99 percent correct.
So, being able to fill these delay slots, with some probability which is less than
98%, is not a good, good trade off. You would've better been served possibly
by allowing some fancier prediction mechanism trying to fill it, if you care
about performance. Now if you care about complexity, or
reducing complexity in area, a static prediction might be a good, good way to
go. So let's, let's look at some basic rules
of thumb here about static prediction. So the, the overall probability of branch
is taken with say out of something like spec int is 60 to 70%.
But it's not equally distributed. Backwards branches have a much higher
probability of being taken than forward branches.
Okay. So, we got a question coming up here.
Why is this? Yes, so, so loops with high probability or
by definition, to be a loop, you don't have to jump backwards.
You jump forwards, it's, it's pretty hard to loop.
So you jump backwards, it's a loop. And in fact, people like to execute loops,
loops, and stay in loops for a while, 'cause that's where a lot of work is done.
So if you're seeing a loop, and you're just spinning in this, this is increasing
the probability that the backwards branch is taken, and that says a high
probability. Such that this half forward branches 50%.
Hm. Can we, what's, what's going on there?
Forward branch, 50 percent going forward. These are usually some swarm of data
dependent branches, like an if then else clause.
So that's why the probability of this is much, much lower.
You sit in loops for long periods of time, forward branches typically sort of, are if
then else's, and there's you're checking some data value and then you're either
executing or not. Now this gets a little trickier with some
more advanced compiler techniques. Cuz sometimes with advanced compiler
techniques, you'll actually, effectively convert a loop with an with an if then
else into it, and a condition check at the end into a unconditional backwards branch
and then a little sort of branch that goes around the, the, piece of code which
checks the loop sensible or checks the, the whether the loop is completing or not.
But even that's actually not a horrible thing, cause at least that backwards
branch will always be predicted to take in a hundred percent, cause its just turned
into a jump. So that's not bad for a performance.
It just, you know, might change these percentages a little bit.
But things do like to sit in loops, is what you should take away from this slide.
Okay, so let's think about a technique to try to take advantage of this.
So one technique they didn't take advantage of this with is actually to add
extra bits to your instruction set, and allow the compiler to hint to the
architecture whether the branch is taken or not taken.
Now just a little hint, if it get's it wrong, we still won't get correct
execution, so if it takes a branch, we still want to go correct, we still want to
execute the right piece of code, but the performance might just be worse.
So let's take a look at two branches here. We have branch .T, and branch .NT.
Branch .T is a branch which is taken or predicted taken, and branch .NT is a
branch that's predicted not taken. So what do I want to say about this?
Well, architectures do have this. I was gonna say the itanium architecture
actually has static branch prediction completely.
Some things have sort of intermediary things with this, for instance the
Motorola 68K kind of has something like this but not, you can't do with all
branches. So this, this exist in real architectures
and one of the, the things is that this could actually be very accurate,
especially if you profile your code. So if you run the code once and you log
the compiler to C, which way the branch was taken, it can do quite good.
And some of the insight here is there's a lot of branches in a program which are
there just to check error cases. And the probability that they actually are
taken or not taken, one way or the other is, can basically almost be statically
determined at compile time. And definitely if you have feedback
information, of execution of the program once, that's, that's a, a very good
indicator. So for instance if you had a loop you
would, as a compiler would denote that the backwards branch is, is taken.
So we put a br.t and if it's let's say some piece of code which jumps over an
error case and checks an error condition and 99 percent of the time that never
falls into that. It would predict that taken.
So we jump over that little piece of code. So you can actually have very high
prediction accuracy. Now we're not, we're not up into the, 99
percent here yet, or something like that. This isn't, this isn't great.
We're still doing static things here. And, you know, at this point, it may
actually make sense still to have, a delay slot, 'cuz we're still not over our, 70th
percen-, or, or, or, or sort of a close trade-off here.
You have to get the stack creation correct and your delay slot might actually still
be a better approach at this, depending on sort of how, how this accuracy compares up
to how many times you can fill the delay slot.
But let's take a look at what happens in the pipeline here.
So here we have branch taken and it's been taken to this target.
We still end up with a dead cycle here because we do not know where that target
is until we've effectively decoded the instruction.
So that in the sort of intermediate time here we just have fetched let's say the
fall through case or something like that. But, because we predicted a taken, we can,
we can do better here. We don't have to have two delay slots, or
two dead cycles. We can fill it.
We can actually get the, the correct next instruction, in here.
For the target. This branch not taken, let's say we
speculatively execute the fall through. We, we don't actually have any penalty for
a correctly predicted branch here. So if we predict this branch not taken,
that's the fall through case, we can just fetch p c plus four, p c plus eight and
just keep executing. And we have no, no penalty there.
Okay, so if we get the hint wrong. What are we gonna do?
Well, we have effectively taken a mis-predict penalty here.
And if you look at the branch take-in case and you take a mis-predict penalty.
What's gonna happen here is you are actually going to end up having two, two
dead cycles, cuz we don't actually determine whether we took the mis-predict
or not until this execute stage. And that's the, the soonest we can go
redirect the front of the pipe. Now, if you look at this, and squint
really hard, you can see that we actually fetched op A twice here.
This was the fall through instruction. It is hypothetically possible that you
might be able to not hold off killing this instruction if you will, and in this case
you sort of have the pipe do something special and actually end up getting the
instruction after op A or op B here, if you will.
But that's pretty hard to do. So, for the base case we'll just say that
you end up with an extra cycle, a mispredict penalty, when you mispredict.
Yeah, so just to reiterate that, what I was trying to say here is that, well, if a
branch taken, we fetch the subsequent instruction in the follow through case.
We fetch the target of the branch. We try to kill this, but, lo and behold,
we end up fetching the exact same instruction again.
So, you could, if you really wanted, to try to optimize run that and not fetch
this twice, but then you are kind of having things out of order in the pipe
here. Because, you'd have this instruction.
This instruction is dead and you have to figure out how to kill, sort of,
sub-portions of your pipe or things out of order in your pipe.
That gets pretty, pretty hard to do. But this does really quite well if we do
static software based branch production Okay, so let's look at hardware branch
prediction. Now when we say hardware branch
prediction, that does not necessarily mean dynamic hardware branch prediction.
This could just mean that you have the hardware do either prediction without any
hints from the software. And we have a couple different cases we
can try to implement. Heuristics, if you will, in hardware.
The first one here always predicts not taken.
This is what we've done so far in all of our pipelines that we've designed.
We've predict, we predict fallthrough effectively, and we just fetch PC plus
four. We didn't put a name on it, this, but this
is actually what we were doing. We were doing speculative execution with a
static harbor branch prediction of PC plus four.
It's pretty simple to implement. It's what you guys have done in your labs
so far. You know the fall-through early.
Accuracy is not very good. You get all your loops wrong.
All your backward branches and your loops just are always predicted wrong.
Okay, let's, let's do the inverse here. Predict taken, let's have that be our
static strategy in hardware. Well, kinda hard to do here because we
don't really know the target of this branch in D code.
So it's like going back to here. If we predict everything taken, what do
we, what do we fetch in this cycle? Well, I don't know.
It's a big question mark there. It doesn't do super well on,
if-then-elses, cuz a lot of times those are forward branches over things, or, or
some, it depends on how you structure it. Some architectures which have things like
these static prediction techniques will actually restructure their code, such that
the compiler will figure out the probability of a branch being taken or not
taken, and then work the code around it, to make it work out.
That's actually pretty common the, the Motorola 68K had something similar to that
and they, the compilers there actually try to work around it.
So, hmm, well, this is definitely a bummer here.
Maybe we can fix that. Like, like we said, this is the second
part of today's lecture is trying to fix this problem.
Okay, how about we use a heuristic, that all backwards branches are taken and the
forward branches are not taken. Okay so this does a lot better.
This is her, our heuristic of what we were saying before, that loops and things like
that will get caught up in this. So forward branch is not part of the loop.
It'll, we'll predict that not taken, we'll predict the fall through.
Backward branch we'll predict taken. This does pretty good.
It's better accuracy. It's still nowhere near the sort of 80
percent accuracy that we had if the compiler were to get involved, cuz that's
effectively, the compiler case that we were talking about before could actually
implement this entire algorithm in software.
Or something much more sophisticated and that's usually what ends up happening.
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