Embedded Systems in Silicon TD5102 MIPS Pipelining

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Embedded Systems in SiliconTD5102MIPS
Pipelining
Henk Corporaal http//www.ics.ele.tue.nl/heco/cou
rses/EmbSystems Technical University
Eindhoven DTI / NUS Singapore 2005/2006
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Topics

• Pipelining
• Pipelined datapath
• Pipelined control
• Hazards
• Structural
• Data
• Control
• Exceptions
• Scheduling

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Pipelining

• Improve performance by increasing instruction
  throughput

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Pipelining

• Ideal speedup number of stages
• Do we achieve this?

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Pipelining

• What makes it easy
• all instructions are the same length
• just a few instruction formats
• memory operands appear only in loads and stores
• What makes it hard?
• structural hazards suppose we had only one
  memory
• control hazards need to worry about branch
  instructions
• data hazards an instruction depends on a
  previous instruction
• Well build a simple pipeline and look at these
  issues
• Well talk about modern processors and what
  really makes it hard
• exception handling
• trying to improve performance with out-of-order
  execution, etc.

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Basic idea start from single cycle impl.

• What do we need to add to actually split the
  datapath into stages?

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Pipelined Datapath

• Can you find a problem even if there are no
  dependencies? What instructions can we execute
  to manifest the problem?

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Corrected Datapath
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Graphically Representing Pipelines

• Can help with answering questions like
• how many cycles does it take to execute this
  code?
• what is the ALU doing during cycle 4?
• use this representation to help understand
  datapaths

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Pipeline Control
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Pipeline control

• We have 5 stages. What needs to be controlled in
  each stage?
• Instruction Fetch and PC Increment
• Instruction Decode / Register Fetch
• Execution
• Memory Stage
• Write Back
• How would control be handled in an automobile
  plant?
• a fancy control center telling everyone what to
  do?
• should we use a finite state machine?

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Pipeline Control

• Pass control signals along just like the data

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Datapath with Control
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Hazards
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Hazards

• Hazards problems due to pipelining
• Hazard types
• Structural
• same resource is needed multiple times in the
  same cycle
• Data
• data dependencies limit pipelining
• Control
• next executed instruction may not be the next
  specified instruction

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Structural hazards

• Examples
• Two accesses to a single ported memory
• Two operations need the same function unitat the
  same time
• Two operations need the same function unitin
  successive cycles, but the unit is not pipelined
• Solutions
• stalling
• add more hardware

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Structural hazards

• Simple pipelining diagram (but not MIPS!)
• IF instruction fetch
• ID instruction decode
• OF operand fetch
• EX execute stage(s)
• WB write back

time
Instruction stream
Pipeline stalls due to lack of resources
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Structural hazards

• Non-pipelined units

Same non-pipelined FU
time
IF
ID
OF
EX
WB
IF
ID
OF
EX
WB
EX
IF
ID
OF
EX
WB
EX
Instruction stream
IF
ID
OF
EX
WB
IF
ID
OF
EX
WB
IF
ID
OF
EX
WB
IF
ID
OF
EX
WB
Stall cycle
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Structural hazards on MIPS

• Q Do we have structural hazards on our simple
  MIPS pipeline?

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Data hazards

• Data dependencies
• RaW (read-after-write)
• WaW (write-after-write)
• WaR (write-after-read)
• Hardware solution
• Forwarding / Bypassing
• Detection logic
• Stalling
• Software solution Scheduling

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Data dependences

• Three types RaW, WaR and WaW
• add r1, r2, 5 r1 r25
• sub r4, r1, r3 RaW of r1
• add r1, r2, 5
• sub r2, r4, 1 WaR of r2
• add r1, r2, 5
• sub r1, r1, 1 WaW of r1
• st r1, 5(r2) Mr25 r1
• ld r5, 0(r4) RaW if 5r2 0r4

WaW and WaR do not occur in simple pipelines, but
they limit scheduling freedom! Problems for
your compiler and Pentium! ? use register
renaming to solve this!
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RaW dependence
add r1, r2, 5 r1 r25 sub r4, r1, r3 RaW of
r1
Without bypass circuitry
time
add r1, r2, 5
sub r4, r1, r3
OF
EX
WB
IF
ID
With bypass circuitry
time
add r1, r2, 5
Saves two cycles
sub r4, r1, r3
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RaW on MIPS pipeline
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Forwarding

• Use temporary results, dont wait for them to be
  written
• register file forwarding to handle read/write to
  same register
• ALU forwarding

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Forwarding hardware
ALU forwarding circuitry principle

• Note there are two options
• buf - ALU bypass mux - buf
• buf - bypass mux ALU - buf

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Forwarding
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Forwarding check

• Check for matching register-ids
• For each source-id of operation in the EX-stage
  check if there is a matching pending dest-id

Q. How many comparators do we need?
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Can't always forward

• Load word can still cause a hazard
• an instruction tries to read register r following
  a load to the same r
• Need a hazard detection unit to stall the load
  instruction

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Stalling

• We can stall the pipeline by keeping an
  instruction in the same stage

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Hazard Detection Unit
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Software only solution?

• Have compiler guarantee that no hazards occur
• Example where do we insert the NOPs
  ? sub 2, 1, 3 and 12, 2, 5 or 13,
  6, 2 add 14, 2, 2 sw 15, 100(2)
• Problem this really slows us down!

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Control hazards

• Control operations may change the sequential flow
  of instructions
• branch
• jump
• call (jump and link)
• return
• (exception)

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Control hazard Branch

• Branch actions
• Compute new address
• Determine condition
• Perform the actual branch (if taken) PC new
  address

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Branch example
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Branching

• Squash pipeline
• When we decide to branch, other instructions are
  in the pipeline!
• We are predicting branch not taken
• need to add hardware for flushing instructions if
  we are wrong

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Branch with predict not taken
Clock cycles
Branch L
IF
ID
EX
MEM
WB
Predict not taken
IF
ID
EX
MEM
WB
IF
ID
EX
MEM
WB
L
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Branch speedup

• Earlier address computation
• Earlier condition calculation
• Put both in the ID pipeline stage
• adder
• comparator

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Improved branching / flushing IF/ID

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Exception support

• Types of exceptions
• Overflow
• I/O device request
• Operating system call
• Undefined instruction
• Hardware malfunction
• Page fault
• Precise exception
• finish previous instructions (which are still in
  the pipeline)
• flush excepting and following instructions, redo
  them after handling the exception(s)

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Exceptions

• Changes needed for handling overflow exception of
  an operation in EX stage (see book for details)
• Extend PC input mux with extra entry with fixed
  address
• Add EPC register recording the ID/EX stage PC
• this is the address of the next instruction !
• Cause register recording exception type
• E.g., in case of overflow exception insert 3
  bubblesflush the following stages
• IF/ID stage
• ID/EX stage
• EX/MEM stage

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Scheduling, why?

• Lets look at the execution time
• Texecution Ncycles x Tcycle
• Ninstructions x CPI x Tcycle
• Scheduling may reduce Texecution
• Reduce CPI (cycles per instruction)
• early scheduling of long latency operations
• avoid pipeline stalls due to structural, data and
  control hazards
• allow Nissue gt 1 and therefore CPI lt 1
• Reduce Ninstructions
• compact many operations into each instruction
  (VLIW)

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Scheduling data hazardsexample 1

• Try and avoid RaW stalls (in this case load
  interlocks)!
• E.g., reorder these instructions

lw t0, 0(t1) lw t2, 4(t1) sw t0, 4(t1) sw
t2, 0(t1)
lw t0, 0(t1) lw t2, 4(t1) sw t2, 0(t1) sw
t0, 4(t1)
?
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Scheduling data hazardsexample 2
Avoiding RaW stalls
Reordering instructions for following program (by
you or the compiler)
Code a b c d e - f
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Scheduling control hazards

• Texecution Ninstructions x CPI x Tcycle
• CPI CPIideal fbranch x Pbranch
• Pbranch Ndelayslots x miss_rate
• Modern processors tend to have large branch
  penalty, Pbranch,due to many pipeline stages
• Note that penalties have larger effect when
  CPIideal is low

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Scheduling control hazards

• What can we do about control hazards and CPI
  penalty?
• Keep penalty Pbranch low
• Early computation of new PC
• Early determination of condition
• Visible branch delay slots filled by compiler
  (MIPS)
• Branch prediction
• Reduce control dependencies (control height
  reduction) Schlansker and Kathail, Micro95
• Remove branches if-conversion
• Conditional instructions CMOVE, cond skip next
• Guarding all instructions TriMedia

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Branch delay slot

• Add a branch delay slot
• the next instruction after a branch is always
  executed
• rely on compiler to fill the slot with
  something useful
• Is this a good idea?
• let's look how it works

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Branch delay slot scheduling
Q. What to put in the delay slot?
op 1
beq r1,r2, L
.............
op 2
.............
'fall-through'
L op 3
branch target
.............
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Summary

• Modern processors are (deeply) pipelined, to
  reduce Tcycle and aim at CPI 1
• Hazards increase CPI
• Several software and hardware measure to avoid or
  reduce hazards are taken
• Not discussed, but important developments
• Multi-issue further reduces CPI
• Branch prediction to avoid high branch penalties
• Dynamic scheduling
• In all cases a scheduling compiler needed