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Tomasulos Approach

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Tomasulo's Approach. Recall the scoreboard would allow us to bypass stalls from ... The reservation station stores 6 items: the operation to be performed (Op) ... – PowerPoint PPT presentation

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Title: Tomasulos Approach


1
Tomasulos Approach
  • Recall the scoreboard would allow us to bypass
    stalls from data hazards to continue executing
    code not impacted by the stalls
  • see the example code to the right where the ADD.D
    must stall, but there is no reason for the SUB.D
    to stall
  • The scoreboard had its own problems though
  • for instance, stalling because of WAR hazards
    which really should not cause stalls since they
    arise from name conflicts, not data conflicts
  • So we turn to another form of dynamic scheduling
    Tomasulos approach
  • this approach was designed for a more modern
    version of the IBM 360 mainframe but similar
    approaches have found their way into a variety of
    modern processors
  • the idea is to enhance the scoreboard with extra
    registers
  • these registers, stored in reservation stations,
    allow name conflicts to be avoided by renaming
    from a compiler-generated name to one of these
    reservation stations
  • this will allow the hardware to avoid WAW and WAR
    hazard stalls although the solution brings about
    additional cost and restrictions

2
Register Renaming
  • The concept is to rename registers
  • to avoid WAR and WAW hazards
  • this approach can also support overlapped
    execution of multiple iterations of a loop
  • Reservation stations are buffers for operands of
    instructions waiting to be issued
  • reservation stations for each operand of each
    functional unit
  • used as buffers for each operand, grabbing the
    operand as it becomes available from memory,
    register file or a functional unit
  • pending instruction designates the reservation
    station that will hold its operand(s)
  • as instructions are issued, register specifiers
    for pending operands are renamed to the names of
    the reservation stations holding them
  • when successive writes appear to a register, only
    the last one is actually accepted and written
  • eliminates WAW hazards

3
Differences from the Scoreboard
  • Issue logic combines reservation station
    selection and register renaming
  • in this way, all WAW and WAR hazards are
    eliminated without causing stalls unlike the
    scoreboard
  • WAW hazards are eliminated because out of order
    writes are not allowed to write
  • WAR hazards are eliminated because of register
    renaming
  • Hazard detection and execution control are
    distributed among the reservation stations
  • rather than centralized as in the scoreboard, or
    the ID stage of the MIPS pipeline
  • Results from one function unit are passed
    directly to other functional units to waiting
    reservation stations
  • so RAW hazards do not have to wait for register
    writes
  • this difference requires a mechanism to connect
    functional units and reservation stations
    together the Common Data Bus
  • This approach is more expensive because
  • there are more reservation stations than real
    registers to accommodate all WAW and WAR hazards
  • there are also multiple connections to the CBD

4
Reservation Station Architecture
  • Reservation stations hold
  • issued instructions that have not started
    executing
  • operand values for the instruction or the source
    of where they will be provided
  • control information
  • Load/store buffers hold
  • data or addresses coming from or going to memory
  • Register file
  • will be used as before, but forwarding via CDB is
    also used
  • CDB connects everything together

5
3 Stage Approach
  • Instructions go through the hardware like the
    Scoreboard or the 5-stage MIPS pipeline
  • but here we will separate memory operations
    leaving a 3 sequence execution
  • Issue
  • get instruction from instruction queue
  • if floating-point operation and there is an
    available functional unit then
  • issue instruction, send operands to reservation
    station if available (in registers)
  • if load/store instruction and if buffer available
    then
  • issue load/store to buffer
  • while issuing, detect WAR hazards and rename
    registers as needed
  • if reservation station for needed unit is busy,
    then stall until station is freed
  • Execute (for floating point instructions)
  • if one or more operands are not yet available,
    monitor CDB for them
  • this is RAW hazard detection
  • when operand becomes available, place operand in
    reservation station
  • when both operands are in reservation station,
    execute the operation
  • Write result
  • when functional unit has produced the result,
    place it on the CDB and from there, registers and
    reservations stations will grab it and store it

6
Reservation Station Structure
  • The reservation station stores 6 items
  • the operation to be performed (Op)
  • the reservation stations that will produce the
    operands, or 0 if the operand is already
    available (Qj, Qk)
  • this will include the register if it is coming
    from the register file, or the load/store buffer
    if it is coming from memory
  • the values of the operands (Vj, Vk)
  • whether the reservation station and functional
    unit is available (Busy)
  • When an instruction is issued, the reservation
    station information is supplied and the register
    file and load/store buffer are modified to store
    Qi (reservation station that will produce the
    result to be moved to the register or buffer
  • if this field is blank, then no current
    instruction will produce the value for this
    register or buffer

7
How It Works
  • Instruction fetch unit fetches next instruction
  • unless instruction queue becomes full
  • Instruction at front of queue is issued if
  • the needed reservation station is available
  • Once at a reservation station
  • Qj/Qk ? reservation station that will produce the
    value (as determined by comparing the register
    number to the register status information) OR
  • Vj/Vk ? value from register for source operand
    (if the register value is correct)
  • this step takes care of RAW hazards
  • Once both source operands are in Vj/Vk (forwarded
    from CDB), the instruction can execute
  • update register status to include that this
    reservation station will produce a result to the
    destination register
  • a later instruction which may write to the same
    register will override this reservation station,
    and so WAW hazards are avoided
  • Once computed, if the value is needed at another
    reservation station, forward it via CDB, and
    forward it to the register file if this
    reservation station is still listed in the
    register status list to avoid WAR hazards
  • the control logic is shown in figure 2.12 on page
    101

8
Summary
  • Differences from the Scoreboard
  • WAW and WAR hazards eliminated entirely rather
    than stalling
  • CDB broadcasts results rather than waiting for a
    register to be available
  • load and store treated as basic functional unit
  • reservation stations contain logic to detect and
    eliminate hazards
  • data structure used in reservation stations are
    tags (virtual names for registers generated
    through register renaming)
  • Advantages
  • distribution of hazard detection allows for
    instructions to move passed Issue stage and
    improve ILP
  • elimination of stalls for WAW and WAR hazards due
    to register renaming
  • can provide high performance as long as branch
    penalties can be kept small
  • Disadvantages
  • complexity of the hardware
  • restrictions because of the single CDB
  • expense of the associative memory used as tags in
    each reservation station
  • see http//www.nku.edu/foxr/CSC462/NOTES/tomasulo
    -example1.xls http//www.nku.edu/foxr/CSC462/NOTE
    S/tomasulo-example2.xls for examples

9
Hardware Based Speculation
  • Using dynamic scheduling, we might have multiple
    instructions being executed in the same cycle
  • To obtain better control, we need to improve on
    our branch mechanism
  • for instance, as it is now, a mispredicted branch
    may not be able to prevent an imprecise exception
  • So we turn to hardware-based speculation
  • predict the next instruction and issue it before
    determining the branch result
  • if prediction is wrong, instruction must be
    killed off before it can affect a change to the
    machines state (it cannot update registers or
    memory or cause an exception)
  • We add a new buffer called the reorder buffer
  • buffer stores the results of completed
    instructions that were speculated, until the
    speculation is proven true or false
  • if true, allow the instructions results to be
    written to registers/memory
  • if false, remove instruction and all instructions
    that followed it
  • We add a new state to instruction execution
    called commit to our Tomasulo-based superscalar
    architecture
  • should the result be stored in the destination
    register?
  • this becomes the final step for all instructions

10
New Architecture
  • Add the Reorder buffer to
  • store the output from each functional unit and
    load buffer
  • Enhance control hardware
  • instruction cannot issue if the reorder buffer is
    full
  • upon issue, update register status to include
    reorder buffer entry number, and enter reorder
    buffer entry number into destination field of
    reservation station
  • execution remains the same
  • write result remains the same except that values
    are not written to registers here, but they are
    forwarded via CDB
  • in each cycle, commit the instruction at the
    front of the reorder buffer if it has reached the
    write result stage and the speculation for the
    instruction was correct
  • otherwise, if the speculation for the instruction
    was wrong, flush the instruction and all others
    in the reorder buffer until you reach the first
    instruction fetched after the branch condition
    was determined

11
Examples
  • Note this architectures new control logic
    shown in figure 2.17 on page 113
  • Two examples are provided in the text
  • assume FP add takes 2 cycles, FP multiply takes
    10 cycles, FP divide takes 40 cycles
  • example 1 code below to the left
  • solution at http//www.nku.edu/foxr/CSC462/NOTES/
    /reorder1.xls
  • example 2 code below to the right
  • this example uses a loop to clearly illustrate
    the use of speculation and the reorder buffer
  • solution at http//www.nku.edu/foxr/CSC462/NOTES/
    /reorder2.xls

L.D F6, 34(R2) L.D F2, 45(R3) MUL.D F0, F2,
F4 SUB.D F8, F6, F2 DIV.D F10, F0, F6 ADD.D F6,
F8, F2
Loop L.D F0, 0(R1) MUL.D F4, F0, F2 S.D F4,
0(R1) DSUBI R1, R1, 8 BNE R1, R2, Loop
12
Multiple Instruction Issue
  • We have attempted to limit stalls from hazards to
    lower the average CPI to the ideal CPI of 1
  • can we decrease CPI to under 1? How?
  • issue and execute more than 1 instruction at a
    time
  • Multiple-issue processors come in two kinds
  • superscalars
  • use static and/or dynamic scheduling mechanisms
    and multiple functional units to issue more than
    1 instruction at a time
  • static scheduling uses the compiler to piece
    together instructions that might be able to
    execute simultaneously (for instance, the
    compiler might couple up an integer operation
    with a FP operation since they should not
    interfere with each other)
  • dynamic scheduling will use a Tomasulo-like
    architecture
  • VLIW (very long instruction word)
  • use instructions which are themselves multiple
    instructions, scheduled by a compiler
  • all instructions in the long word are executed in
    parallel

13
Superscalar
  • Hardware issues from 1 to 8 instructions per
    clock cycle
  • these instructions must be independent and
    satisfy other constraints
  • avoid structural hazards - use different
    functional units, make up to 1 memory reference
    combined
  • Scheduling of instructions can be done statically
    by a compiler or dynamically by hardware
  • while a superscalar can issue any combination of
    instructions, for simplicity, we will concentrate
    on a 2 instruction superscalar for MIPS where
  • one instruction will be an integer operation
  • and the other, if available will be a floating
    point operation
  • this simplification reduces the complexity of the
    hardware, but also reduces the usefulness of the
    superscalar

14
How it works
  • For the 1 int/1 FP superscalar
  • instruction Fetch gets two simultaneous
    instructions (consecutive) (64 bits worth of
    instruction)
  • if first instruction is an int instruction and
    the int unit is available (it should be since the
    int EX takes 1 cycle to execute) then issue the
    instruction
  • if second instruction is an FP instruction and
    the proper FP unit is available, issue it
  • with 2 instructions fetched and issued in 1
    cycle, we could ultimately have a CPI of 0.5
  • to make this worthwhile
  • we will want to either increase the number of FP
    functional units, or pipeline these units as
    every clock cycle might involve issuing an FP
    instruction
  • compiler can schedule instructions to be paired
    as int/FP, or we can use hardware to detect if
    two instructions are an int/FP pair and issue both

15
Superscalar Example
  • Now we combine speculation, dynamic scheduling
    and multiple issue with a superscalar processor
  • the code is given below
  • assume there are separate integer units for
    effective address calculation, ALU operations,
    and branch condition evaluation
  • notice that there are no FP operations here, so
    all instructions should execute in 1 cycle
  • we will look at the cycles at which each
    instruction issues, executes, and writes to the
    CDB without speculation, and issues, executes,
    writes and commits with speculation

Loop LD R2, 0(R1) DADDIU R2,
R2, 1 SD R2, 0(R1)
DADDIU R1, R1, 4 BNE R2, R3, Loop
16
Without Speculation
17
With Speculation
18
Compiler Scheduling a Superscalar
  • Here, the compilers goal is to look beyond just
    two instructions (as we saw with the
    hardware-based approach)
  • to select compatible instructions
  • if we assume, like our previous example, that we
    want to execute 1 integer and 1 FP operation,
    then the compiler will try to couple up such
    operations into the executable code
  • we might group a L.D with an ADD.D that uses the
    loaded value, thus causing the ADD.D to stall and
    gaining no benefit from the superscalar
  • we can then combine superscalar scheduling with
    loop unrolling and scheduling to optimize the
    performance of the superscalar and the available
    functional units
  • assume floating point functional units are
    pipelined so that we can schedule multiple FP
    operations in a row

19
Example
  • The code on the left can be scheduled for the
    MIPS superscalar as shown on the right
  • Both sets of code have RAW hazard stalls not
    shown in the code above, but the superscalar will
    achieve a speedup of approximately 17 / 14
    1.214 or 21

L.D F0, 0(R1) L.D F1, 0(R2) ADD.D F2, F0,
F1 L.D F3, 0(R3) L.D F4, 0(R4) ADD.D F5, F3,
F4 SUB.D F6, F3, F2 DADDI R1, R1, 8 DADDI R2,
R2, 8 DADDI R3, R3, 8 DADDI R4, R4,
8 DADDI R5, R5, 8 S.D F6, 0(R5)
Integer FP L.D F0, 0(R1) L.D F1, 0(R2)
L.D F3, 0(R3) ADD.D F2, F0, F1 L.D F4, 0(R4)
DADDI R1, R1, 8 ADD.D F5, F3, F4 DADDI R2, R2,
8 SUB.D F6, F3, F2 DADDI R3, R3, 8 DADDI R4,
R4, 8 DADDI R5, R5, 8 S.D F6, -8(R5)
20
Another Example
  • Now we improve by also performing loop unrolling
    with superscalar scheduling
  • this example uses the same loop that we had
    previously used in loop unrolling and scheduling

Our loop executes 5 iterations in 12 cycles for
200 total iterations 200 12 2400 cycles,
for a speedup of 3500 / 2400 1.46 or 46 We
could also compute speedup as (14 / 4) / (12 /
5) 1.46
21
VLIW
  • Very Long Instruction Word approach
  • statically scheduled and attempting to issue more
    than 2 instructions per cycle
  • the compiler must select and group instructions
    that can be executed together
  • to fill out an entire VLIW instruction, other
    compiler techniques are needed like loop
    unrolling, scheduling across blocks and global
    techniques
  • VLIW might be between 5 and 7 instructions long
    (160 224 bits long)
  • instruction fetch gets one VLIW
  • fetch 1 VLIW (up to 7 instructions!)
  • issue it all at once on up to 7 functional units
  • the compiler must know how many functional units
    are available
  • As an example, the hardware might have
  • 2 integer functional units, 2 floating point
    function units, 2 load/store units, 1 branch unit

22
VLIW Example
  • Consider a VLIW compiler for MIPS
  • assume two load/store units, 1 integer unit
    (which also performs branch calculations), and
    two floating point functional units
  • here is an example of our previous loop,
    unrolled, scheduled and grouped together for our
    MIPS VLIW
  • 7 loop iterations in 9 cycles takes 143 total
    iterations or 143 9 1287 cycles giving us a
    speedup of 2400 / 1287 1.86 over our
    superscalar and 3500 / 1287 2.72 over loop
    unrolling/scheduling
  • we could also compute this as (12 cycles / 5
    iterations) / (9 cycles / 7 iterations) 1.87
    and (14 / 4) / (9 / 7) 2.72

23
Design Issues
  • Reorder buffer vs. more registers
  • we could forego the reorder buffer by providing
    additional temporary storage in essence, the
    two are the same solution, just a slightly
    different implementation
  • both require a good deal more memory than we
    needed with an ordinary pipeline, but both
    improve performance greatly
  • How much should we speculate?
  • other factors cause our multiple-issue
    superscalar to slow cache issues or exceptions
    for instance, so a large amount of speculation is
    defeated by other hardware failings, we might try
    to speculate over a couple of branches, but not
    more
  • Speculating over multiple branches
  • imagine our loop has a selection statement, now
    we speculate over two branches speculation over
    more than one branch greatly complicates matters
    and may not be worthwhile

24
Renaming Table
  • One implementation for register renaming is to
    use a queue of available registers and a renaming
    table
  • This allows for on-the-fly renaming or
    substituting in a superscalar

25
Superscalar Loop Example
  • We now examine dynamic scheduling on our
    superscalar
  • In this case, we will schedule instructions in a
    loop, allowing multiple iterations to be
    executing at the same time, similar to what we
    did with Tomasulos approach
  • For this example, assume that
  • both a floating-point and an integer operation
    can be issued at each cycle even if they are
    dependent
  • one integer functional unit is available, and
    used for both ALU operations as well as
    loads/stores/branch calculations
  • a separate floating point pipeline is available
    for the execution of floating point operations
  • issue/write stages each take 1 cycle
  • latencies loads 2 cycles, FP add 3 cycles,
    ALU ops 1 cycle
  • 2 CDBs available, one for integer operations, one
    for floating point
  • assume no branch delay (perfect prediction)

Loop L.D F0, 0(R1) ADD.D F4, F0, F2 S.D F4,
0(R1) DSUBI R1, R1, 8 BNE R1, R2, Loop
26
Execution of the Loop
27
Resource Usage for Loop
28
Example 2
Same loop, but now 2 integer ALUs, one for
load/store address calculations, one for int ALU
operations
29
Execution of the Loop
30
Resource Usage for Loop
31
Superscalar Problems
  • We must now expand the potential problems that
    arise with a superscalar pipeline over an
    ordinary pipeline
  • RAW hazards could exist between the two
    instructions issued at the same time
  • there are new potential WAW and WAR hazards
  • we need to have twice as many register reads and
    writes as before, our register file must be
    expanded to accommodate this
  • loads and stores are integer operations even if
    they are dealing with floating point registers
  • we might be reading floating point registers for
    a FP operation and also reading/writing floating
    point registers for an FP load or store
  • maintaining precise exceptions is difficult
    because an integer operation may have already
    completed
  • hardware must detect these problems (and quickly)

32
Cost of a Superscalar
  • We already had the multiple functional units, so
    there is no added cost in terms of having an int
    and a FP instruction issue and execute in
    parallel
  • there are added costs though for
  • hazard detection
  • the complexity here is increased because now
    instructions must be compared not only to
    instructions further down the pipeline, but to
    the instruction at the same stage, plus there is
    a potential for twice as many instructions being
    active at one time!
  • maintaining precise exceptions
  • two sets of buses
  • integer operations from integer registers to
    integer ALU data cache
  • FP operations from FP registers to FP functional
    unit data cache
  • ability to access floating point register file by
    up to 3 instructions during the same cycle (a
    load or store FP in the ID or WB stage, an FP
    instruction in ID and an FP instruction in WB)

33
Sample Problem 1
  • We are going to implement Tomasulos approach
  • assume floating point /- take 4 cycles to
    execute, take 7 cycles to execute and / takes
    10 cycles to execute
  • assume loads and stores take 3 cycles to execute
  • assume all int operations take 2 cycles to
    execute
  • Using the average FP SPEC benchmark statistics in
    figure 2.33
  • how many of each type of functional unit do you
    suppose we should provide assuming the
    distribution of operations across a program match
    their frequency
  • e.g., 25 means that we expect this operation to
    arise 1 in every 4 instructions

34
Solution
  • The FP averages are 39 (load/store), 38 (ALU),
    4 (branch), 10 FP /-, 8 FP , 0 FP /
  • with 39 load/store operations a load or store
    occurs roughly every 2.5 instructions, since
    loads and stores take 3 cycles, we should have 2
    load/store units
  • with 38 ALU 4 branch operations an ALU/branch
    operation occurs roughly every 2.5 instructions,
    since ALU operations take 2 cycles, we only need
    1 ALU
  • with 10 FP /- an FP /- occurs roughly every 10
    instructions, so we only need 1 FP adder
  • with 8 FP an FP occurs roughly every 12
    instructions, so we only need 1 FP
  • with about 0 FP / we only need 1 FP /

35
Sample Problem 2
  • Given the following latencies
  • find a sequence of no more than 10 instructions
    that causes contention on the CDB using
    Tomasulos approach
  • FP ? FP ALU 6 cycles
  • FP ? FP ALU 4 cycles
  • FP ? FP Store 5 cycles
  • FP ? FP Store 3 cycles
  • Int (including LD) ? any 0 cycles
  • if we have an FP followed 2 cycles later by an
    FP that uses different source operands, we will
    have bus contention in the situation below, the
    MUL.S is postponed 1 cycle because of waiting for
    the L.S to write the result, so the ADD.S causes
    contention which follows 3 cycles later

Operation Issues at Executes at Writes at L.S
F2, 0(R1) 1 2 3 MUL.S F3, F2, F1
2 4 11 int op 3 4 5 int op 4 5 6 ADD.S
F4, F2, F5 5 6 11
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