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Predication Implementation
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From the course by Princeton University
Computer Architecture
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From the lesson
VLIW2
This lecture covers the common methods used to improve VLIW performance.
Meet the Instructors
David WentzlaffAssociate Professor
Electrical Engineering
Okay. So, let's take a look at our data path
here and see where predicates thin the datapath.
And we're, we're going to focus, actually, just on the conditional move predicate or,
or predication instruction here. We're not going to look at full
predication just yet on the datapath. But, it follows a similar idea.
Okay. So, what's, what do we need to, to do?
What do we, what do we add to our, sort of, boring nipstyle five stage pipeline to
add this instruction. Hm.
Okay. Instruction comes in, moves down the pipe.
Oh, this is interesting. I know.
This is a really cool trick. Let's just, if, if this condition is not
true, let's just kill the right back to the register file.
It's brilliant. We just have, we just suppress the right
back. We don't have to actually change datapath
at all, we just put an end gate in here. And this end gate depends on, you know,
this condition. Simple, it's easy.
Maybe this is what we should do. Looks simple, just add an end gate where
the big X is, and life, life is done. Okay.
Well, that looks, that looks good. Can we bypass this value?
So, can we have an instruction that directly follows this move zero, or this
conditional move zero, and reads rd? Well, where are we changing rd, or not
changing rd? Where are we making that decision?
Well, in this pipeline, because we did it in the write back stage, it doesn't happen
'till down here. Down here, or right back stage, or this
wire, the right back wire runs all the way back into the right enable on a registry
file. Huh, okay.
Well, that doesn't really help us a whole lot.
Especially, if we're trying to bypass and we're trying to bypass out of here, our
ALU, back around. Because at this point, we haven't figured
out any way to suppress this. So, we don't actually, we're not able to
actually suppress that. Hm.
So, what do we, what do we think about this?
So, how do we, how do we go about doing this?
So, let's, let's think about how to actually bypass out for this conditional
move instruction. Cuz condition move, it's a, just a simple
comparison of zero which we could do that in one cycle.
We don't really have to wait until the end of the pipe to do that.
And we will [inaudible] to go bypass it into a back to back instruction.
Okay. So, how do we, how do we do that?
Well, bypassing doesn't work. What if, we somehow pipe forward the
original value and the new value. Okay.
So, what do I mean by this? So, this, this instruction is very
interesting. It is much more interesting than your
standard like add instruction. So, why is it interesting?
Well, let's look at the semantics very closely here.
Move zero is going to write rs to rd, or It's going to write rd to rd.
I could say, why do I need to write rd to rd?
Well, in the bypass path, when we provide this value around back to our bypass
registers or forwarding logic here, or bypass luxes or for, forwarding logic, we
need the old value of rd. So, the traditional, sort of, something
sort of risk pipelined here. We're only going to fetch our two sources,
and we can only write one location. So, we're going to fetch rs or rt, and
then we're going to write to rd. Now, all of a sudden, in this instruction,
we need to read rs, okay? We need to read that cuz we need to
overwrite rd with rs if we need, if, if the condition is true, and we need to read
the condition rt. But, aha.
We may also need to read rd here. This is because when we get to this stage
here and we're going to use this bypass path to forward the value of what rd is
going to be in the future, we need the original rd.
So, sort of to draw this a little bit more succinctly because this is pretty
important. We have if, the registered value of rt
equals zero. We have R of rd gets rs.
That's the easy one. We can count the registers here.
Once one source, two sources, one destination.
That's simple. And what no one ever forgets, everyone
always forgets is the else case here. And what does this else case say?
Well, the else case is going to say, register rd gets register rd.
And you might say, well r, rd already had rd.
That's true, but our bypassing, or our forwarding logic didn't have that.
So, we need to actually read this rd. So, that means we have to read one, two,
three, and we need to write in one location.
Okay. So, that's going to cause us some problems
over here because all of a sudden, we had a register file which had two read ports,
and we need to now have three read ports. So, we need to add an extra read port on
our register file, and this can be expensive.
So, if we actually want to build predication, it's going to have some
costs. We might, if we want to build predication
actually bypass something like a predicated conditional move, we're going
to have to add another report to our register file.
And, that, that actually has some cost. And this is especially costly if you look
at something like a VLIW. So, let's, let's take, for example, a
three way VLIW something like the, the Tilera processor.
So, it's a three way, three wide VLIW. Each of those is going, each of those ways
or each of those pipelines is going to read two, if you don't have conditional
move, we'll say. And, it's going to write one value.
So, it's going to have six read ports, and three write ports.
So, it's a ten port register file. No, excuse me, it's a nine port register
file to begin with. And of all the sudden, we add something
like conditional move here, and we need to add these extra read ports.
We're going to go from a nine port register file to a twelve port register
file. We're going to have let's see, we're going
to have three write ports and nine read ports.
That's a, that's a hard to do. It's, you know, it's hard to build this
really heavily ported register files. Okay.
So, to, to sum up here, a problem, problems with full predication is that you
need to add another cork to the administer file, you need to bypass the predicates.
So, what I mean by that is you're computing predicates, and you want to use
it in the next instruction. So, if we go back to this instruction
sequence here. We compute these predicates and what we
use it very carefully, a very, very quickly after it.
We don't have to wait at the end of the pipeline for this predicates to be
computed. So, the effectively its going to make a,
make it, so that we are going to have a predicate register files, sitting
somewhere here, and have a bypassing around the predicate register file
forwarding of the predicates to, to get the, the, the predicates there are faster.
Or, get the, the predicates to be used in the next instruction.
And, you're going to have to add extra pipeline registers to pipe forward the old
value cuz you might need to keep the old value in the bypass.
And, in fact, actually a lot of times when people do these things, they actually
always write the register file and just pipe forward both and at the end make the
decision. And, or, or along the way they make the
decision to go into the, the bypass or not but then sort of when, when the
instruction finishes, that's going to make the decision.
So, we're going to actually going to have to add more pipeline registers to pipe
forward the old value that was in, in this case, rd.
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