CS 152 Computer Architecture and
Engineering
Lecture 15 - Out-of-Order Memory,
Complex Superscalars Review
Krste Asanovic
Electrical Engineering and Computer Sciences
University of California at Berkeley
http://www.eecs.berkeley.edu/~krste
http://inst.eecs.berkeley.edu/~cs152
Review of Last 3 Lectures
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Phases of Instruction Execution
PC
I-cache
Fetch
Buffer
Issue
Buffer
Func.
Units
Result
Buffer
Arch.
State
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Fetch: Instruction bits retrieved
from cache.
Decode: Instructions decoded, registers
renamed, placed in appropriate issue buffer.
Execute: Instructions and operands sent to
execution units.
When execution completes, all results and
exception flags are available.
Commit: Instruction irrevocably updates
architectural state.
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In-Order Pipeline
ALU
IF
ID
Issue
Mem
WB
Fadd
Fmul
Instructions pass through
issue stage and enter
execution in-order.
May complete out-oforder, but must commit
in-order.
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Fdiv
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Exception Handling
Commit
Point
(In-Order Five-Stage Pipeline)
Inst.
Mem
PC
Select
Handler
PC
PC Address
Exceptions
Kill F
Stage
•
•
•
•
D
Decode
E
Illegal
Opcode
+
M
Overflow
Data
Mem
Data Addr
Except
W
Kill
Writeback
Exc
D
Exc
E
Exc
M
Cause
PC
D
PC
E
PC
M
EPC
Kill D
Stage
Kill E
Stage
Asynchronous
Interrupts
Hold exception flags in pipeline until commit point (M stage)
Exceptions in earlier pipe stages override later exceptions
Inject external interrupts at commit point (override others)
If exception at commit: update Cause and EPC registers, kill
all stages, inject handler PC into fetch stage
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Commit
Point
In-Order Superscalar Pipeline
PC
Inst. 2
D
Mem
Dual
Decode
GPRs
• Fetch two instructions per cycle;
issue both simultaneously if one is
integer/memory and other is floating
point
• Inexpensive way of increasing
throughput, examples include Alpha
21064 (1992) & MIPS R5000 series
(1996)
• Same idea can be extended to wider
issue by duplicating functional units
(e.g. 4-issue UltraSPARC) but regfile
ports and bypassing costs grow
quickly
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FPRs
X1
X1
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X2
Data
Mem
X3
W
X2
FAdd
X3
W
X2
FMul
X3
Unpipelined
divider X3
FDiv X2
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Out-of-Order Issue
ALU
IF
ID
Issue
Mem
WB
Fadd
Fmul
• Issue stage buffer holds multiple instructions waiting to issue.
• Decode adds next instruction to buffer if there is space and the instruction does
not cause a WAR or WAW hazard.
–
Note: WAR possible again because issue is out-of-order (WAR not possible with in-order issue and
latching of input operands at functional unit)
• Any instruction in buffer whose RAW hazards are satisfied can be issued (for now
at most one dispatch per cycle). On a write back (WB), new instructions may get
enabled.
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Register Renaming
ALU
IF
ID
Mem
Issue
WB
Fadd
Fmul
• Decode does register renaming and adds instructions to
the issue stage reorder buffer (ROB)
 renaming makes WAR or WAW hazards impossible
• Any instruction in ROB whose RAW hazards have been
satisfied can be dispatched.
 Out-of-order or dataflow execution
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Out-of-Order Execution Pipeline
In-order
Fetch
Out-of-order
Reorder Buffer
Decode
Kill
In-order
Commit
Kill
Kill
Execute
Inject handler PC
Exception?
• Instructions fetched and decoded into instruction
reorder buffer in-order
• Execution is out-of-order (  out-of-order completion)
• Commit (write-back to architectural state, i.e., regfile &
memory), is in-order
Temporary storage needed in ROB to hold results before commit
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“Data in ROB” Design
(HP PA8000, Pentium Pro, Core2Duo)
Register File
holds only
committed state
Ins# use exec
op
p1
src1
p2
src2
pd dest
t1
t2
.
.
tn
data
Reorder
buffer
Load
Unit
FU
FU
FU
Store
Unit
Commit
< t, result >
• On dispatch into ROB, ready sources can be in regfile or in ROB
dest (copied into src1/src2 if ready before dispatch)
• On completion, write to dest field and broadcast to src fields.
• On issue, read from ROB src fields
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Unified Physical Register File
(MIPS R10K, Alpha 21264, Pentium 4)
r1
r2
ti
tj
Rename
Table
t1
t2
.
tn
Snapshots for
mispredict recovery
Load
Unit
FU
FU
FU
(ROB not shown)
Reg
File
FU
Store
Unit
< t, result >
• One regfile for both committed
and speculative values (no data in ROB)
• During decode, instruction result allocated new physical register, source
regs translated to physical regs through rename table
• Instruction reads data from regfile at start of execute (not in decode)
• Write-back updates reg. busy bits on instructions in ROB (assoc. search)
• Snapshots of rename table taken at every branch to recover mispredicts
• On exception, renaming undone in reverse order of issue (MIPS R10000)
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Pipeline Design with Physical Regfile
Branch
Resolution
kill
Branch
Prediction
PC
Fetch
kill
kill
Decode &
Rename
Update predictors
kill
Out-of-Order
Reorder Buffer
In-Order
Commit
In-Order
Physical Reg. File
Branch
ALU MEM
Unit
Store
Buffer
D$
Execute
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CS152 Administrivia
• Quiz 4, Tuesday April 7, Complex Pipelining
• Quiz 5 and 6 moved back one class:
– Quiz 5, Thursday April 23
– Quiz 6, Thursday May 7
• Also, PS/Lab 5/6 moved back one class
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Memory Dependencies
st r1, (r2)
ld r3, (r4)
When can we execute the load?
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In-Order Memory Queue
• Execute all loads and stores in program order
=> Load and store cannot leave ROB for execution until
all previous loads and stores have completed
execution
• Can still execute loads and stores speculatively, and
out-of-order with respect to other instructions
• Need a structure to handle memory ordering…
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Conservative O-o-O Load Execution
st r1, (r2)
ld r3, (r4)
• Split execution of store instruction into two phases: address
calculation and data write
• Can execute load before store, if addresses known and r4 != r2
• Each load address compared with addresses of all previous
uncommitted stores (can use partial conservative check i.e.,
bottom 12 bits of address)
• Don’t execute load if any previous store address not known
(MIPS R10K, 16 entry address queue)
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Address Speculation
st r1, (r2)
ld r3, (r4)
• Guess that r4 != r2
• Execute load before store address known
• Need to hold all completed but uncommitted
load/store addresses in program order
• If subsequently find r4==r2, squash load and all
following instructions
=> Large penalty for inaccurate address speculation
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Memory Dependence Prediction
(Alpha 21264)
st r1, (r2)
ld r3, (r4)
• Guess that r4 != r2 and execute load before store
• If later find r4==r2, squash load and all following
instructions, but mark load instruction as store-wait
• Subsequent executions of the same load instruction
will wait for all previous stores to complete
• Periodically clear store-wait bits
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Speculative Loads / Stores
Just like register updates, stores should not modify
the memory until after the instruction is committed
- A speculative store buffer is a structure introduced to hold
speculative store data.
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Speculative Store Buffer
Speculative
Store
Buffer
V
V
V
V
V
V
S
S
S
S
S
S
Load Address
Tag
Tag
Tag
Tag
Tag
Tag
Data
Data
Data
Data
Data
Data
L1 Data
Cache
Tags
Data
Store Commit Path
Load Data
• On store execute:
– mark entry valid and speculative, and save data and tag of instruction.
• On store commit:
– clear speculative bit and eventually move data to cache
• On store abort:
– clear valid bit
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Speculative Store Buffer
Speculative
Store
Buffer
V
V
V
V
V
V
S
S
S
S
S
S
Tag
Tag
Tag
Tag
Tag
Tag
Load Address
Data
Data
Data
Data
Data
Data
L1 Data
Cache
Tags
Store Commit Path
Data
Load Data
• If data in both store buffer and cache, which should we use?
Speculative store buffer
• If same address in store buffer twice, which should we use?
Youngest store older than load
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Datapath: Branch Prediction
and Speculative Execution
Update predictors
Branch
Prediction
kill
kill
Branch
Resolution
kill
kill
PC
Fetch
Decode &
Rename
Reorder Buffer
Commit
Reg. File
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Branch
ALU MEM
Unit
Execute
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Store
Buffer
D$
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Instruction Flow in Unified Physical
Register File Pipeline
• Fetch
– Get instruction bits from current guess at PC, place in fetch buffer
– Update PC using sequential address or branch predictor (BTB)
• Decode
– Take instruction from fetch buffer
– Allocate resources to execute instruction:
» Destination physical register, if instruction writes a register
» Entry in reorder buffer to provide in-order commit
» Entry in issue window to wait for execution
» Entry in memory buffer, if load or store
– Decode will stall if resources not available
– Rename source and destination registers
– Check source registers for readiness
– Insert instruction into issue window+reorder buffer+memory buffer
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Memory Instructions
• Split store instruction into two pieces during decode:
– Address calculation, store-address
– Data movement, store-data
• Allocate space in program order in memory buffers during
decode
• Store instructions:
–
–
–
–
Store-address calculates address and places in store buffer
Store-data copies store value into store buffer
Store-address and store-data execute independently out of issue window
Stores only commit to data cache at commit point
• Load instructions:
– Load address calculation executes from window
– Load with completed effective address searches memory buffer
– Load instruction may have to wait in memory buffer for earlier store ops
to resolve
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Issue Stage
• Writebacks from completion phase “wakeup” some
instructions by causing their source operands to
become ready in issue window
– In more speculative machines, might wake up waiting loads in
memory buffer
• Need to “select” some instructions for issue
– Arbiter picks a subset of ready instructions for execution
– Example policies: random, lower-first, oldest-first, critical-first
• Instructions read out from issue window and sent to
execution
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Execute Stage
• Read operands from physical register file and/or
bypass network from other functional units
• Execute on functional unit
• Write result value to physical register file (or store
buffer if store)
• Produce exception status, write to reorder buffer
• Free slot in instruction window
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Commit Stage
• Read completed instructions in-order from
reorder buffer
– (may need to wait for next oldest instruction to complete)
• If exception raised
– flush pipeline, jump to exception handler
• Otherwise, release resources:
– Free physical register used by last writer to same architectural
register
– Free reorder buffer slot
– Free memory reorder buffer slot
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Acknowledgements
• These slides contain material developed and
copyright by:
–
–
–
–
–
–
Arvind (MIT)
Krste Asanovic (MIT/UCB)
Joel Emer (Intel/MIT)
James Hoe (CMU)
John Kubiatowicz (UCB)
David Patterson (UCB)
• MIT material derived from course 6.823
• UCB material derived from course CS252
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