1. Blusepc-5: Dead cycles, bubbles and Forwarding in Pipelines Arvind
Computer Science & Artificial Intelligence Lab
Massachusetts Institute of Technology
February 23, 2007

2. Topics Simultaneous enq & deq in a FIFO The RWire solution
Dead cycle elimination in the IP circular pipeline code
Two-stage processor pipeline
Value forwarding to reduce bubbles
February 23, 2007

3. Implicit guards (conditions)
Rule
rule <name> (<guard>); <action>; endrule
where
<action> ::= r <= <exp>
| m.g(<exp>)
| if (<exp>) <actions> endif
m.gB(<exp>) when m.gG
make implicit guards explicit
February 23, 2007

4. Guards vs If’s
A guard on one action of a parallel group of actions affects every action within the group
(a1 when p1); (a2 when p2)
==> (a1; a2) when (p1 && p2)
A condition of a Conditional action only affects the actions within the scope of the conditional action
(if (p1) a1); a2
p1 has no effect on a2 ...
Mixing ifs and whens
(if (p) (a1 when q)) ; a2
 ((if (p) a1); a2) when (p&q | !p)
February 23, 2007

5. Example: making guards explicit
rule recirculate (True);
if (p) fifo.enq(8);
r <= 7;
endrule
rule recirculate ((p && fifo.engG) || !p);
if (p) fifo.enqB(8);
r <= 7;
endrule
February 23, 2007

6. A problem ... (from the last lecture)
rule recirculate (True);
TableEntry p <- ram.resp();
match {.rip, .tok} = fifo.first();
if (isLeaf(p)) cbuf.put(tok, p);
else begin
fifo.enq(tuple2(rip << 8, tok));
ram.req(p+signExtend(rip[15:8]));
end
fifo.deq();
endrule
The fifo needs to be able to do enq and deq simultaneously for this rule to make sense
February 23, 2007

7. One Element FIFO module mkFIFO1 (FIFO#(t));
Reg#(t) data <- mkRegU();
Reg#(Bool) full <- mkReg(False);
method Action enq(t x) if (!full);
full <= True; data <= x;
endmethod
method Action deq() if (full);
full <= False;
method t first() if (full);
return (data);
method Action clear();
endmodule
enq and deq cannot even be enabled together much less fire concurrently!
The functionality we want is as if deq “happens” before enq; if deq does not happen then enq behaves normally n
not empty
not full
rdy
enab
enq
deq
module
FIFO
February 23, 2007

8. RWire to rescue interface RWire#(type t); method Action wset(t x);
method Maybe#(t) wget();
endinterface
Like a register in that you can read and write it but unlike a register
- read happens after write
- data disappears in the next cycle
February 23, 2007

9. One Element “Loopy” FIFO
module mkLFIFO1 (FIFO#(t));
Reg#(t) data <- mkRegU();
Reg#(Bool) full <- mkReg(False);
RWire#(void) deqEN <- mkRWire();
method Action enq(t x) if
(!full || isValid (deqEN.wget()));
full <= True; data <= x;
endmethod
method Action deq() if (full);
full <= False; deqEN.wset(?);
method t first() if (full);
return (data);
method Action clear();
full <= False;
endmodule
not empty
not full
rdy
enab
enq
deq
module
FIFO
!full or
February 23, 2007

10. Problem solved! What if fifo is empty?
LFIFO fifo <- mkLFIFO; // use a loopy fifo
rule recirculate (True);
TableEntry p <- ram.resp();
match {.rip, .tok} = fifo.first();
if (isLeaf(p)) cbuf.put(tok, p);
else begin
fifo.enq(tuple2(rip << 8, tok));
ram.req(p+signExtend(rip[15:8]));
end
fifo.deq();
endrule
What if fifo is empty?
February 23, 2007

11. The Dead Cycle Problem rule enter (True);
done?
RAM
cbuf
inQ
fifo
rule enter (True);
Token tok <- cbuf.getToken();
IP ip = inQ.first();
ram.req(ext(ip[31:16]));
fifo.enq(tuple2(ip[15:0], tok)); inQ.deq();
endrule
rule recirculate (True);
TableEntry p <- ram.resp();
match {.rip, .tok} = fifo.first();
if (isLeaf(p)) cbuf.put(tok, p);
else begin
fifo.enq(tuple2(rip << 8, tok));
ram.req(p+signExtend(rip[15:8]));
end
fifo.deq();
endrule
Can a new request enter the system simultaneously with an old one leaving?
February 23, 2007

12. Scheduling conflicting rules
When two rules conflict on a shared resource, they cannot both execute in the same clock
The compiler produces logic that ensures that, when both rules are applicable, only one will fire
Which one?
source annotations
(* descending_urgency = “recirculateh, enter” *)
February 23, 2007

13. A slightly simpler example
enter?
done?
RAM
inQ
fifo
rule enter (True);
IP ip = inQ.first();
ram.req(ip[31:16]);
fifo.enq(ip[15:0]); inQ.deq();
endrule
rule recirculate (True);
TableEntry p = ram.peek(); ram.deq();
IP rip = fifo.first();
if (isLeaf(p)) outQ.enq(p);
else begin
fifo.enq(rip << 8);
ram.req(p + rip[15:8]);
end
fifo.deq();
endrule
In general these two rules conflict but when isLeaf(p) is true there is no apparent conflict!
February 23, 2007

14. Rule Spliting
rule foo (True);
if (p) r1 <= 5;
else r2 <= 7;
endrule
rule fooT (p);
r1 <= 5;
endrule
rule fooF (!p);
r2 <= 7; 
rule fooT and fooF can be scheduled independently with some other rule
February 23, 2007

15. Spliting the recirculate rule
rule recirculate (!isLeaf(ram.peek()));
IP rip = fifo.first(); fifo.enq(rip << 8);
ram.req(ram.peek() + rip[15:8]);
fifo.deq(); ram.deq();
endrule
rule exit (isLeaf(ram.peek()));
outQ.enq(ram.peek()); fifo.deq(); ram.deq();
endrule
rule enter (True);
IP ip = inQ.first(); ram.req(ip[31:16]);
fifo.enq(ip[15:0]); inQ.deq();
endrule
Now rules enter and exit can be scheduled simultaneously
February 23, 2007

16. Sometimes rule splitting is not possible
makes life difficult
rule recirculate (True);
TableEntry p <- ram.resp();
match {.rip, .tok} = fifo.first();
if (isLeaf(p)) cbuf.put(tok, p);
else begin
fifo.enq(tuple2(rip << 8, tok));
ram.req(p+signExtend(rip[15:8]));
end
fifo.deq();
endrule
You will have to resort to interface changes and/or the use of RWires
February 23, 2007

17. Packaging a module: Turning a rule into a method
enter?
done?
RAM
cbuf
fifo
inQ
rule enter (True);
Token t <- cbuf.getToken();
IP ip = inQ.first();
ram.req(ip[31:16]);
fifo.enq(tuple2(ip[15:0], t)); inQ.deq();
endrule
method Action enter (IP ip);
Token t <- cbuf.getToken();
ram.req(ip[31:16]);
fifo.enq(tuple2(ip[15:0], t));
endmethod
February 23, 2007

18. Processor with a two-stage pipeline
5-minute break to stretch you legs
February 23, 2007

19. Processor Pipelines and FIFOspc rf
decode
execute
memory
write-
back
fetch
iMem
dMem
CPU
February 23, 2007

20. SFIFO (glue between stages)
interface SFIFO#(type t, type tr);
method Action enq(t); // enqueue an item
method Action deq(); // remove oldest entry
method t first(); // inspect oldest item
method Action clear(); // make FIFO empty
method Bool find(tr); // search FIFO
endinterface n
enab
enq
rdy
not full
n = # of bits needed
to represent the
values of type “t“
m = # of bits needed
values of type “tr"
enab
rdy
deq
SFIFO
module
not empty n
first
not empty
rdy
enab
more on searchable FIFOs later
clear
bool m
find
February 23, 2007

21. Two-Stage Pipeline module mkCPU#(Mem iMem, Mem dMem)(Empty);
fetch & decode
execute pc rf
CPU bu
module mkCPU#(Mem iMem, Mem dMem)(Empty);
Reg#(Iaddress) pc <- mkReg(0);
RegFile#(RName, Bit#(32)) rf <- mkRegFileFull();
SFIFO#(InstTemplate, RName) bu
<- mkSFifo(findf);
Instr instr = iMem.read(pc);
Iaddress predIa = pc + 1;
InstTemplate it = bu.first();
rule fetch_decode ...
endmodule
This first method describe what happens in the fetch of this pipeline.
This methods says that given any processor term, we can always rewrite it to a new term wehre, and we enqueue the current instruction into bf.
And at the same time the pc field is incremende by one
It may look like this method can fire forever, but because bf is bounded, when a method enqueue into bf, there is an implied predicate that say bf must not be full.
February 23, 2007 8 8 6 8

22. Instructions & Templates
typedef union tagged {
struct {RName dst; RName src1; RName src2} Add;
struct {RName cond; RName addr} Bz;
struct {RName dst; RName addr} Load;
struct {RName value; RName addr} Store;
} Instr deriving(Bits, Eq);
typedef union tagged
{ struct {RName dst; Value op1; Value op2} EAdd;
struct {Value cond; Iaddress tAddr} EBz;
struct {RName dst; Daddress addr} ELoad;
struct {Value data; Daddress addr} EStore;
} InstTemplate deriving(Eq, Bits);
typedef Bit#(32) Iaddress;
typedef Bit#(32) Daddress;
typedef Bit#(32) Value;
February 23, 2007

23. Rules for Add CPU implicit check: pc rf fetch & decode execute bu
rule decodeAdd(instr matches Add{dst:.rd,src1:.ra,src2:.rb})
bu.enq (EAdd{dst:rd,op1:rf[ra],op2:rf[rb]});
pc <= predIa;
endrule
implicit check:
This first method describe what happens in the fetch of this pipeline.
This methods says that given any processor term, we can always rewrite it to a new term wehre, and we enqueue the current instruction into bf.
And at the same time the pc field is incremende by one
It may look like this method can fire forever, but because bf is bounded, when a method enqueue into bf, there is an implied predicate that say bf must not be full.
bu notfull
rule executeAdd(it matches EAdd{dst:.rd,op1:.va,op2:.vb})
rf.upd(rd, va + vb);
bu.deq();
endrule
bu notempty
February 23, 2007 8 6 8 8

24. Fetch & Decode Rule: Reexamined
execute pc rf
CPU bu
rule decodeAdd (instr matches Add{dst:.rd,src1:.ra,src2:.rb})
bu.enq (EAdd{dst:rd, op1:rf[ra], op2:rf[rb]});
pc <= predIa;
endrule
This first method describe what happens in the fetch of this pipeline.
This methods says that given any processor term, we can always rewrite it to a new term wehre, and we enqueue the current instruction into bf.
And at the same time the pc field is incremende by one
It may look like this method can fire forever, but because bf is bounded, when a method enqueue into bf, there is an implied predicate that say bf must not be full.
Wrong! Because instructions in bu may be modifying ra or rb
stall !
February 23, 2007 8 6 8 8

25. Fetch & Decode Rule: corrected
execute pc rf
CPU bu
rule decodeAdd (instr matches Add{dst:.rd,src1:.ra,src2:.rb}
bu.enq (EAdd{dst:rd, op1:rf[ra], op2:rf[rb]});
pc <= predIa;
endrule
&&& !bu.find(ra) &&& !bu.find(rb))
This first method describe what happens in the fetch of this pipeline.
This methods says that given any processor term, we can always rewrite it to a new term wehre, and we enqueue the current instruction into bf.
And at the same time the pc field is incremende by one
It may look like this method can fire forever, but because bf is bounded, when a method enqueue into bf, there is an implied predicate that say bf must not be full.
February 23, 2007 8 8 6 8

26. Rules for Branch CPU rule-atomicity ensures that pc update, and
fetch & decode
execute pc rf
CPU bu
rule-atomicity ensures that
pc update, and
discard of pre-
fetched instrs in bu, are done
consistently
rule decodeBz(instr matches Bz{cond:.rc,addr:.addr}) &&&
!bu.find(rc) &&& !bu.find(addr));
bu.enq (EBz{cond:rf[rc],addr:rf[addr]});
pc <= predIa;
endrule
This first method describe what happens in the fetch of this pipeline.
This methods says that given any processor term, we can always rewrite it to a new term wehre, and we enqueue the current instruction into bf.
And at the same time the pc field is incremende by one
It may look like this method can fire forever, but because bf is bounded, when a method enqueue into bf, there is an implied predicate that say bf must not be full.
rule bzTaken(it matches EBz{cond:.vc,addr:.va}) &&&
(vc==0));
pc <= va; bu.clear(); endrule
rule bzNotTaken (it matches EBz{cond:.vc,addr:.va}) &&&
(vc != 0));
bu.deq; endrule
February 23, 2007 8 6 8 8

27. The Stall Signal Bool stall = case (instr) matches
tagged Add {dst:.rd,src1:.ra,src2:.rb}:
return (bu.find(ra) || bu.find(rb)); tagged Bz {cond:.rc,addr:.addr}:
return (bu.find(rc) || bu.find(addr)); tagged Load {dst:.rd,addr:.addr}:
return (bu.find(addr)); tagged Store {value:.v,addr:.addr}:
return (bu.find(v)) || bu.find(addr));
endcase;
This first rule describe what happens in the fetch of this pipeline.
This rules says that given any processor term, we can always rewrite it to a new term wehre, and we enqueue the current instruction into bf.
And at the same time the pc field is incremende by one
It may look like this rule can fire forever, but because bf is bounded, when a rule enqueue into bf, there is an implied predicate that say bf must not be full.
Need to extend the fifo interface with the “find” method where “find” searches the fifo using the findf function
February 23, 2007 8 8 6 8

28. Parameterization: The Stall Function
function Bool stallfunc (Instr instr,
SFIFO#(InstTemplate, RName) bu);
case (instr) matches
tagged Add {dst:.rd,src1:.ra,src2:.rb}:
return (bu.find(ra) || bu.find(rb)); tagged Bz {cond:.rc,addr:.addr}:
return (bu.find(rc) || bu.find(addr)); tagged Load {dst:.rd,addr:.addr}:
return (bu.find(addr)); tagged Store {value:.v,addr:.addr}:
return (bu.find(v)) || bu.find(addr));
endcase
endfunction
This first rule describe what happens in the fetch of this pipeline.
This rules says that given any processor term, we can always rewrite it to a new term wehre, and we enqueue the current instruction into bf.
And at the same time the pc field is incremende by one
It may look like this rule can fire forever, but because bf is bounded, when a rule enqueue into bf, there is an implied predicate that say bf must not be full.
We need to include the following call in the mkCPU module
Bool stall = stallfunc(instr, bu);
no extra gates!
February 23, 2007 8 8 6 8

29. The findf function function Bool findf (RName r, InstrTemplate it);
case (it) matches tagged EAdd{dst:.rd,op1:.ra,op2:.rb}:
return (r == rd); tagged EBz {cond:.c,addr:.a}:
return (False); tagged ELoad{dst:.rd,addr:.a}:
return (r == rd);
tagged EStore{value:.v,addr:.a}:
return (False);
endcase
endfunction
This first rule describe what happens in the fetch of this pipeline.
This rules says that given any processor term, we can always rewrite it to a new term wehre, and we enqueue the current instruction into bf.
And at the same time the pc field is incremende by one
It may look like this rule can fire forever, but because bf is bounded, when a rule enqueue into bf, there is an implied predicate that say bf must not be full.
SFIFO#(InstrTemplate, RName) bu <- mkSFifo(findf);
mkSFifo can be parameterized by the search function!
no extra gates!
February 23, 2007 8 8 6 8

30. Fetch & Decode Rule
rule fetch_and_decode(!stall); case (instr) matches
tagged Add {dst:.rd,src1:.ra,src2:.rb}:
bu.enq(EAdd{dst:rd,op1:rf[ra],op2:rf[rb]});
tagged Bz {cond:.rc,addr:.addr}:
bu.enq(EBz{cond:rf[rc],addr:rf[addr]});
tagged Load {dst:.rd,addr:.addr}:
bu.enq(ELoad{dst:rd,addr:rf[addr]}); tagged Store{value:.v,addr:.addr}:
bu.enq(EStore{value:rf[v],addr:rf[addr]});
endcase pc<= predIa;
endrule
February 23, 2007

31. Fetch & Decode Rule another style
InstrTemplate newIt =
case (instr) matches
tagged Add {dst:.rd,src1:.ra,src2:.rb}:
return EAdd{dst:rd,op1:rf[ra],op2:rf[rb]};
tagged Bz {cond:.rc,addr:.addr}:
return EBz{cond:rf[rc],addr:rf[addr]};
tagged Load {dst:.rd,addr:.addr}:
return ELoad{dst:rd,addr:rf[addr]}; tagged Store{value:.v,addr:.addr}:
return EStore{value:rf[v],addr:rf[addr]};
endcase;
rule fetch_and_decode (!stall);
bu.enq(newIt);
pc <= predIa;
endrule
Conceptually cleaner; hides unnecessary details
February 23, 2007

32. Execute Rule
rule execute (True); case (it) matches tagged EAdd{dst:.rd,src1:.va,src2:.vb}: begin
rf.upd(rd, va+vb); bu.deq();
end
tagged EBz {cond:.cv,addr:.av}:
if (cv == 0) then begin
pc <= av; bu.clear(); end
else bu.deq(); tagged ELoad{dst:.rd,addr:.av}: begin
rf.upd(rd, dMem.read(av)); bu.deq();
end tagged EStore{value:.vv,addr:.av}: begin
dMem.write(av, vv); bu.deq();
endcase
endrule
Next time simultaneous execution of fetch and execute rules
February 23, 2007