Transcript P6 and NetBurst Microarchitecture - ECE Users Pages

ECE4100/6100
H-H. S. Lee
ECE4100/6100 Guest Lecture:
P6 & NetBurst Microarchitecture
Prof. Hsien-Hsin Sean Lee
School of ECE
Georgia Institute of Technology
February 11, 2003
1
ECE4100/6100
H-H. S. Lee
Why studies P6 from last millennium?
A paradigm shift from Pentium
A RISC core disguised as a CISC
Huge market success:
Microarchitecture
And stock price
Architected by former VLIW and RISC folks
Multiflow (pioneer in VLIW architecture for superminicomputer)
Intel i960 (Intel’s RISC for graphics and embedded
controller)
Netburst (P4’s microarchitecture)
is based on P6
2
ECE4100/6100
H-H. S. Lee
P6 Basics
 One implementation of IA32 architecture
 Super-pipelined processor
 3-way superscalar
 In-order front-end and back-end
 Dynamic execution engine (restricted dataflow)
 Speculative execution
 P6 microarchitecture family processors include
Pentium Pro
Pentium II (PPro + MMX + 2x caches—16KB I/16KB D)
Pentium III (P-II + SSE + enhanced MMX, e.g. PSAD)
Celeron (without MP support)
Later P-II/P-III/Celeron all have on-die L2 cache
3
ECE4100/6100
H-H. S. Lee
x86 Platform Architecture
Host Processor
P6 Core
L1
Cache
(SRAM) Back-Side
L2 Cache
(SRAM)
On-die
or on-package
Bus
GPU
Graphics
Processor
Front-Side
Bus
AGP
System
Memory
(DRAM)
MCH
ICH
Local
Frame
Buffer
chipset
PCI
USB
4
I/O
ECE4100/6100
H-H. S. Lee
Pentium III Die Map
5



















EBL/BBL – External/Backside Bus logic
MOB - Memory Order Buffer
Packed FPU - Floating Point Unit for SSE
IEU - Integer Execution Unit
FAU - Floating Point Arithmetic Unit
MIU - Memory Interface Unit
DCU - Data Cache Unit (L1)
PMH - Page Miss Handler
DTLB - Data TLB
BAC - Branch Address Calculator
RAT - Register Alias Table
SIMD - Packed Floating Point unit
RS - Reservation Station
BTB - Branch Target Buffer
TAP – Test Access Port
IFU - Instruction Fetch Unit and L1 I-Cache
ID - Instruction Decode
ROB - Reorder Buffer
MS - Micro-instruction Sequencer
ECE4100/6100
H-H. S. Lee
ISA Enahncement (on top of Pentium)
CMOVcc / FCMOVcc r, r/m
Conditional moves (predicated move) instructions
Based on conditional code (cc)
FCOMI/P : compare FP stack and set integer flags
RDPMC/RDTSC instructions
Uncacheable Speculative Write-Combining (USWC) —
weakly ordered memory type for graphics memory
MMX in Pentium II
SIMD integer operations
SSE in Pentium III
Prefetches (non-temporal nta + temporal t0, t1, t2), sfence
SIMD single-precision FP operations
6
ECE4100/6100
H-H. S. Lee
RS Disp
Exec / WB
ROB
DIS
RET1 RET2
EX
31 32 33 42 43
Dcache2
DCache1
MOB disp
31 32 33 42 43
…….. 40 41 42 43
81 82 83
7
MOB Scheduling
Delay
…
Ret ROB rd
RRF wr
Retirement in-order boundary
DCache1
DCache2
81 82 83
Ret ptr wr
..
81 82 83
…
RAT
Exec n
Exec2
31 32 33 ..
AGU
…
DEC2
Mob wakeup
Blocking
memory
pipeline
81: Mem/FP WB
82: Int WB
83: Data WB
DEC1
ROB Scheduling
Delay
MOB blk
MOB wr
Non-blocking
memory pipeline
RS Scheduling
Delay
IFU3
31 32 33
AGU
Multi-cycle inst
pipeline
…
IFU2
82 83
FE in-order boundary
Single-cycle inst
pipeline
IDQ
RAT
20 21 22
RS schd
Dec2
Br Dec
11 12 13 14 15 16 17
RS Write
In-order FE
I-Cache
ILD
Rotate
Dec1
Next IP
P6 Pipelining
IFU1
91 92 93
ECE4100/6100
H-H. S. Lee
P6 Microarchitecture
External bus
Data Cache
Unit (L1)
Chip boundary
Bus Cluster
Memory
Order Buffer
Bus interface unit
Memory
Cluster
AGU
Instruction
Instruction Fetch
Fetch Unit
Unit
MMX
IEU/JEU
IEU/JEU
Control
Flow
BTB/BAC
FEU
Instruction Fetch Cluster
(Restricted)
Data
Flow
MIU
Instruction
Instruction
Decoder
Decoder
Microcode
Sequencer
Register
Alias Table
Reservation
Station
Allocator
Issue Cluster
8
ROB &
Retire RF
Out-of-order
Cluster
ECE4100/6100
H-H. S. Lee
Instruction Fetching Unit
data
Other fetch
requests
addr
Streaming
Buffer
Select
mux
Instruction
buffer
Next PC
Mux
Linear Address
Instruction
Cache
ILD
Length
marks
Instruction
rotator
Victim
Cache
P.Addr
Instruction
TLB
Prediction
marks
#bytes
consumed
by ID
Branch Target Buffer
 IFU1: Initiate fetch, requesting 16 bytes at a time
 IFU2: Instruction length decoder, mark instruction boundaries, BTB makes prediction
 IFU3: Align instructions to 3 decoders in 4-1-1 format
9
ECE4100/6100
H-H. S. Lee
Dynamic Branch Prediction
W0
W1
W2
Pattern History Tables
(PHT)
W3
New (spec) history
512-entry BTB
1 1 1 0 1
Branch History Register 0
(BHR)
0000
0001
0010
Spec. update
1101
1110 1
1111
Prediction
0
2-bit sat. counter
Rc: Branch Result




Similar to a 2-level PAs design
Associated with each BTB entry
W/ 16-entry Return Stack Buffer
4 branch predictions per cycle
(due to 16-byte fetch per cycle)
 Static prediction provided by Branch Address
Calculator when BTB misses (see prior slide)
10
ECE4100/6100
H-H. S. Lee
Static Branch Prediction
No
No
Unconditional
PC-relative?
BTB miss?
Yes
Yes
PC-relative?
No
Return?
Yes
No
BTB’s
decision
Yes
No
Indirect
jump
Conditional?
Yes
Taken
Backwards?
Taken
Yes
Taken
11
No
Taken
Not Taken
Taken
ECE4100/6100
H-H. S. Lee
X86 Instruction Decode
IFU3
complex
(1-4)
Microinstruction
sequencer
(MS)





simple
(1)
simple
(1)
Instruction decoder queue
(6 ops)
4-1-1 decoder
Decode rate depends on instruction alignment
DEC1: translate x86 into micro-operation’s (ops)
DEC2: move decoded ops to ID queue
MS performs translations either
Next 3 inst
#Inst to dec
S,S,S
3
S,S,C
First 2
S,C,S
First 1
S,C,C
First 1
C,S,S
3
C,S,C
First 2
C,C,S
First 1
C,C,C
First 1
S: Simple
C: Complex
 Generate entire op sequence from microcode ROM
 Receive 4 ops from complex decoder, and the rest from microcode ROM
12
ECE4100/6100
H-H. S. Lee
Allocator
The interface between in-order and out-of-order
pipelines
Allocates
“3-or-none” ops per cycle into RS, ROB
“all-or-none” in MOB (LB and SB)
Generate physical destination Pdst from the ROB
and pass it to the Register Alias Table (RAT)
Stalls upon shortage of resources
13
ECE4100/6100
H-H. S. Lee
Register Alias Table (RAT)
FP
TOS
Adjust
Integer
RAT
Array
Array
Physical
Src (Psrc)
FP
RAT
Array
Int and FP Overrides
In-order queue
Logical Src
Renaming Example
RRF PSrc
EAX 0 25
RAT
PSrc’s
EBX
ECX
0
EDX
0
1
2
ECX
15
Allocator
Physical ROB Pointers
RRF
ROB
 Register renaming for 8 integer registers, 8 floating point (stack) registers and flags: 3 op per cycle
 40 80-bit physical registers embedded in the ROB (thereby, 6 bit to specify PSrc)
 RAT looks up physical ROB locations for renamed sources based on RRF bit
14
ECE4100/6100
H-H. S. Lee
Partial Register Width Renaming
FP
TOS
Adjust
Size(2) RRF(1)
Array
Physical
Src
FP
RAT
Array
Int and FP Overries
In-order queue
Logical Src
Integer
RAT
Array
INT Low Bank
(32b/16b/L): 8 entries
INT High Bank (H):
4 entries
RAT
Physical Src
Allocator
op0:
op1:
op2:
op3:
MOV
MOV
ADD
ADD
Physical ROB Pointers from Allocator
 32/16-bit accesses:
 Read from low bank
 Write to both banks
 8-bit RAT accesses: depending on which Bank is being written
15
PSrc(6)
AL
AH
AL
AH
=
=
=
=
(a)
(b)
(c)
(d)
ECE4100/6100
H-H. S. Lee
Partial Stalls due to RAT
read
AX
EAX
write
CMP
INC
JBE
EAX, EBX
ECX
XX
; stall
Partial flag stalls (1)
MOVB AL, m8 ;
ADD EAX, m32 ; stall
Partial register stalls
TEST EBX, EBX
LAHF
; stall
XOR EAX, EAX
MOVB AL, m8 ;
ADD EAX, m32 ; no stall
Partial flag stalls (2)
Idiom Fix (1)
 JBE reads both ZF and CF while
INC affects (ZF,OF,SF,AF,PF)
 LAHF loads low byte of EFLAGS
SUB EAX, EAX
MOVB AL, m8 ;
ADD EAX, m32 ; no stall
Idiom Fix (2)
 Partial register stalls: Occurs when writing a smaller (e.g. 8/16-bit)
register followed by a larger (e.g. 32-bit) read
 Partial flags stalls: Occurs when a subsequent instruction read more
flags than a prior unretired instruction touches
16
ECE4100/6100
H-H. S. Lee
Reservation Stations
WB bus 0
Port 0
IEU0
Fadd
Fmul
Imul
Div
WB bus 1
Port 1
IEU1
JEU
Pfadd
Pfshuf
Loaded data
RS
Port 2
AGU0 Ld addr
LDA
MOB
Port 3
AGU1 St addr
STA
DCU
STD
St data
Port 4
ROB




Retired
RRF
data
Gateway to execution: binding max 5 op to each port per cycle
20 op entry buffer bridging the In-order and Out-of-order engine
RS fields include op opcode, data valid bits, Pdst, Psrc, source data, BrPred, etc.
Oldest first FIFO scheduling when multiple ops are ready at the same cycle
17
Pfmul
ECE4100/6100
H-H. S. Lee
ReOrder Buffer
 A 40-entry circular buffer
Similar to that described in [SmithPleszkun85]
157-bit wide
Provide 40 alias physical registers
 Out-of-order completion
 Deposit exception in each entry
 Retirement (or de-allocation)
RS
ALLOC
ROB
RAT
After resolving prior speculation
Handle exceptions thru MS
Clear OOO state when a mis-predicted branch or
exception is detected
3 op’s per cycle in program order
For multi-op x86 instructions: none or all (atomic)
18
RRF
..
.
(exp) code assist
MS
ECE4100/6100
H-H. S. Lee
Memory Execution Cluster
RS / ROB
LD
STA
STD
Load Buffer
DTLB
FB
DCU
LD
STA
Store Buffer
EBL
Memory Cluster Blocks
 Manage data memory accesses
 Address Translation
 Detect violation of access ordering
 Fill buffers in DCU (similar to MSHR
[Kroft’81]) for handling cache misses (nonblocking)
19
ECE4100/6100
H-H. S. Lee
Memory Order Buffer (MOB)




Allocated by ALLOC
A second order RS for memory operations
1 op for load; 2 op’s for store: Store Address (STA) and Store Data (STD)
MOB
 16-entry load buffer (LB)
 12-entry store address buffer (SAB)
 SAB works in unison with
 Store data buffer (SDB) in MIU
 Physical Address Buffer (PAB) in DCU
 Store Buffer (SB): SAB + SDB + PAB
 Senior Stores
 Upon STD/STA retired from ROB
 SB marks the store “senior”
 Senior stores are committed back in program order to memory when bus idle or SB full
 Prefetch instructions in P-III
 Senior load behavior
 Due to no explicit architectural destination
20
ECE4100/6100
H-H. S. Lee
Store Coloring
x86 Instructions
op’s
mov (0x1220), ebx
std
sta
std
sta
ld
ld
std
sta
ld
mov (0x1110), eax
mov ecx, (0x1220)
mov edx, (0x1280)
mov (0x1400), edx
mov edx, (0x1380)
(ebx)
0x1220
(eax)
0x1100
(edx)
0x1400
store color
2
2
3
3
3
3
4
4
4
 ALLOC assigns Store Buffer ID (SBID) in program order
 ALLOC tags loads with the most recent SBID
 Check loads against stores with equal or younger SBIDs for potential
address conflicts
 SDB forwards data if conflict detected
21
ECE4100/6100
H-H. S. Lee
Memory Type Range Registers (MTRR)
Control registers written by the system (OS)
Supporting Memory Types
UnCacheable (UC)
Uncacheable Speculative Write-combining (USWC or WC)
Use a fill buffer entry as WC buffer
WriteBack (WB)
Write-Through (WT)
Write-Protected (WP)
E.g. Support copy-on-write in UNIX, save memory space by allowing child
processes to share with their parents. Only create new memory pages when child
processes attempt to write.
Page Miss Handler (PMH)
Look up MTRR while supplying physical addresses
Return memory types and physical
address to DTLB
22
ECE4100/6100
H-H. S. Lee
Intel NetBurst Microarchitecture
Pentium 4’s microarchitecture, a post-P6 new generation
Original target market: Graphics workstations, but … the
major competitor screwed up themselves…
Design Goals:
Performance, performance, performance, …
Unprecedented multimedia/floating-point performance
Streaming SIMD Extensions 2 (SSE2)
Reduced CPI
Low latency instructions
High bandwidth instruction fetching
Rapid Execution of Arithmetic & Logic operations
Reduced clock period
New pipeline designed for scalability
23
ECE4100/6100
H-H. S. Lee
Innovations Beyond P6
Hyperpipelined technology
Streaming SIMD Extension 2
Enhanced branch predictor
Execution trace cache
Rapid execution engine
Advanced Transfer Cache
Hyper-threading Technology (in Xeon and Xeon MP)
24
ECE4100/6100
H-H. S. Lee
Pentium 4 Fact Sheet
 IA-32 fully backward compatible
 Available at speeds ranging from 1.3 to ~3 GHz
 Hyperpipelined (20+ stages)
 42+ million transistors
 0.18 μ for 1.7 to 1.9GHz; 0.13μ for 1.8 to 2.8GHz;
 Die Size of 217mm2
 Consumes 55 watts of power at 1.5Ghz
 400MHz (850) and 533MHz (850E) system bus
 512KB or 256KB 8-way full-speed on-die L2 Advanced Transfer Cache
(up to 89.6 GB/s @2.8GHz to L1)
 1MB or 512KB L3 cache (in Xeon MP)
 144 new 128 bit SIMD instructions (SSE2)
 HyperThreading Technology (only enabled in Xeon and Xeon MP)
25
ECE4100/6100
H-H. S. Lee
Recent Intel IA-32 Processors
26
ECE4100/6100
H-H. S. Lee
Building Blocks of Netburst
System bus
Bus Unit
L1 Data Cache
Level 2 Cache
Execution Units
Memory subsystem
INT and FP Exec. Unit
Fetch/
Dec
ETC
μROM
OOO
logic
Branch history update
BTB / Br Pred.
Front-end
Retire
27
Out-of-Order Engine
ECE4100/6100
H-H. S. Lee
Pentium 4 Microarchitectue
BTB (4k entries)
I-TLB/Prefetcher
Trace Cache BTB
(512 entries)
64 bits
IA32 Decoder
Code ROM
Execution Trace Cache
op Queue
Allocator / Register Renamer
Memory op Queue
Memory scheduler
INT / FP op Queue
64-bit
System
Bus
Quad
Pumped
400M/533MHz
3.2/4.3 GB/sec
BIU
Fast Slow/General FP scheduler Simple FP
INT Register File / Bypass Network
FP RF / Bypass Ntwk
U-L2 Cache
FP
FP
AGU
AGU
2x ALU 2x ALU Slow ALU
256KB 8-way
Move
Simple Simple Complex
MMX
Ld addr St addr
128B line, WB
Inst.
Inst.
Inst.
SSE/2
48 GB/s
256 bits
L1 Data Cache (8KB 4-way, 64-byte line,28WT, 1 rd + 1 wr port)
@1.5Gz
ECE4100/6100
H-H. S. Lee
Pipeline Depth Evolution
PREF DEC
DEC
EXEC
WB
P5 Microarchitecture
IFU1
IFU2
IFU3
DEC1
DEC2
RAT
ROB
DIS
EX
RET1 RET2
P6 Microarchitecture
TC NextIP
TC Fetch Drive Alloc
Rename Queue
Schedule
Dispatch
NetBurst Microarchitecture
29
Reg File
Exec Flags Br Ck Drive
ECE4100/6100
H-H. S. Lee
Execution Trace Cache
Primary first level I-cache to replace conventional L1
Decoding several x86 instructions at high frequency is difficult, take
several pipeline stages
Branch misprediction penalty is horrible
lost 20 pipeline stages vs. 10 stages in P6
Advantages
Cache post-decode ops
High bandwidth instruction fetching
Eliminate x86 decoding overheads
Reduce branch recovery time if TC hits
Hold up to 12,000 ops
6 ops per trace line
Many (?) trace lines in a single trace
30
ECE4100/6100
H-H. S. Lee
Execution Trace Cache
Deliver 3 op’s per cycle to OOO engine
X86 instructions read from L2 when TC misses (7+ cycle latency)
TC Hit rate ~ 8K to 16KB conventional I-cache
Simplified x86 decoder
Only one complex instruction per cycle
Instruction > 4 op will be executed by micro-code ROM (P6’s MS)
Perform branch prediction in TC
512-entry BTB + 16-entry RAS
With BP in x86 IFU, reduce 1/3 misprediction compared to P6
Intel did not disclose the details of BP algorithms used in TC and x86
IFU (Dynamic + Static)
31
ECE4100/6100
H-H. S. Lee
Out-Of-Order Engine
Similar design philosophy with P6 uses
Allocator
Register Alias Table
128 physical registers
126-entry ReOrder Buffer
48-entry load buffer
24-entry store buffer
32
ECE4100/6100
H-H. S. Lee
Register Renaming Schemes
Data
Status
RRF
Front-end RAT
EAX
EBX
ECX
EDX
ESI
EDI
ESP
EBP
Retirement RAT
EAX
EBX
ECX
EDX
ESI
EDI
ESP
EBP
P6 Register Renaming
RF (128-entry) ROB (126)
Allocated sequentially
ROB (40-entry)
Allocated sequentially
RAT
EAX
EBX
ECX
EDX
ESI
EDI
ESP
EBP
..
.
..
.
Data
..
.
..
.
Status
NetBurst Register Renaming
33
ECE4100/6100
H-H. S. Lee
Micro-op Scheduling
 op FIFO queues
Memory queue for loads and stores
Non-memory queue
 op schedulers
Several schedulers fire instructions to execution (P6’s RS)
4 distinct dispatch ports
Maximum dispatch: 6 ops per cycle (2 fast ALU from Port 0,1 per cycle; 1 from
ld/st ports)
Exec Port 0
Fast ALU
(2x pumped)
FP
Move
Exec Port 1
Fast ALU
(2x pumped)
•Add/sub
•FP/SSE Move •Add/sub
•Logic
•FP/SSE Store
•Store Data •FXCH
•Branches
INT
Exec
•Shift
•Rotate
FP
Exec
•FP/SSE Add
•FP/SSE Mul
•FP/SSE Div
34 •MMX
Load Port
Store Port
Memory
Load
Memory
Store
•Loads
•LEA
•Prefetch
•Stores
ECE4100/6100
H-H. S. Lee
Data Memory Accesses
8KB 4-way L1 + 256KB 8-way L2 (with a HW prefetcher)
Load-to-use speculation
Dependent instruction dispatched before load finishes
Due to the high frequency and deep pipeline depth
Scheduler assumes loads always hit L1
If L1 miss, dependent instructions left the scheduler receive
incorrect data temporarily – mis-speculation
Replay logic – Re-execute the load when mis-speculated
Independent instructions are allowed to proceed
Up to 4 outstanding load misses (= 4 fill buffers in original P6)
Store-to-load forwarding buffer
 24 entries
 Have the same starting physical address
 Load data size <= store data size
35
ECE4100/6100
H-H. S. Lee
Streaming SIMD Extension 2
P-III SSE (Katmai New Instructions: KNI)
Eight 128-bit wide xmm registers (new architecture state)
Single-precision 128-bit SIMD FP
Four 32-bit FP operations in one instruction
Broken down into 2 ops for execution (only 80-bit data in ROB)
64-bit SIMD MMX (use 8 mm registers — map to FP stack)
Prefetch (nta, t0, t1, t2) and sfence
P4 SSE2 (Willamette New Instructions: WNI)
Support Double-precision 128-bit SIMD FP
Two 64-bit FP operations in one instruction
Throughput: 2 cycles for most of SSE2 operations (exceptional examples:
DIVPD and SQRTPD: 69 cycles, non-pipelined.)
Enhanced 128-bit SIMD MMX using xmm registers
36
ECE4100/6100
H-H. S. Lee
Examples of Using SSE
X3 X2 X1 X0 xmm1
X3 X2 X1 X0
xmm1
X3 X2 X1 X0 xmm1
Y3 Y2 Y1 Y0 xmm2
Y3 Y2 Y1 Y0
xmm2
Y3 Y2 Y1 Y0 xmm2
xmm1
Y3 .. Y0 Y3 .. Y0 X3 .. X0 X3 .. X0
op
op
op
op
X3 op Y3 X2 op Y2 X1 op Y1X0 op Y0
op
xmm1
Packed SP FP operation
(e.g. ADDPS xmm1, xmm2)
X3 X2 X1
X0 op Y0
Scalar SP FP operation
(e.g. ADDSS xmm1, xmm2)
37
Y3 Y3 X0 X1 xmm1
Shuffle FP operation (8-bit imm)
imm8)
(e.g. SHUFPS xmm1, xmm2, 0xf1)
ECE4100/6100
H-H. S. Lee
Examples of Using SSE and SSE2
SSE
X3 X2 X1 X0 xmm1
X3 X2 X1 X0
xmm1
X3 X2 X1 X0 xmm1
Y3 Y2 Y1 Y0 xmm2
Y3 Y2 Y1 Y0
xmm2
Y3 Y2 Y1 Y0 xmm2
xmm1
Y3 .. Y0 Y3 .. Y0 X3 .. X0 X3 .. X0
op
op
op
op
X3 op Y3 X2 op Y2 X1 op Y1X0 op Y0
op
xmm1
Packed SP FP operation
(e.g. ADDPS xmm1, xmm2)
X3 X2 X1
X0 op Y0
Scalar SP FP operation
(e.g. ADDSS xmm1, xmm2)
Y3 Y3 X0 X1 xmm1
Shuffle FP operation (8-bit imm)
(e.g. SHUFPS xmm1, xmm2, imm8)
0xf1)
SSE2
X1
X0
xmm1
X1
X0
xmm1
X1
X0
Y1
Y0
xmm2
Y1
Y0
xmm2
Y1
Y0
op
op
X1 op Y1 X0 op Y0 xmm1
Packed DP FP operation
(e.g. ADDPD xmm1, xmm2)
op
X1
X0 op Y0 xmm1
Scalar DP FP operation
(e.g. ADDSD xmm1, xmm2)
38
Y1 or Y0 X1 or X0
Shuffle FP
DP operation (2-bit imm)
(e.g. SHUFPD
imm2)
SHUFPS xmm1, xmm2, imm8)
ECE4100/6100
H-H. S. Lee
HyperThreading
In Intel Xeon Processor and Intel Xeon MP
Processor
Enable Simultaneous Multi-Threading (SMT)
Exploit ILP through TLP (—Thread-Level Parallelism)
Issuing and executing multiple threads at the same
snapshot
Single P4 Xeon appears to be 2 logical processors
Share the same execution resources
Architectural states are duplicated in hardware
39
ECE4100/6100
H-H. S. Lee
Multithreading (MT) Paradigms
Unused
Thread 1
Thread 2
Thread 3
Thread 4
Thread 5
Execution Time
FU1 FU2 FU3 FU4
Conventional
Superscalar
Single
Threaded
Chip
Fine-grained
Coarse-grained
Multithreading Multithreading Multiprocessor
(CMP)
(cycle-by-cycle (Block Interleaving)
Interleaving)
40
Simultaneous
Multithreading
ECE4100/6100
H-H. S. Lee
More SMT commercial processors
Intel Xeon Hyperthreading
Supports 2 replicated hardware contexts: PC (or IP) and
architecture registers
New directions of usage
Helper (or assisted) threads (e.g. speculative precomputation)
Speculative multithreading
Clearwater (once called Xtream logic) 8 context SMT
“network processor” designed by DISC architect
(company no longer exists)
SUN 4-SMT-processor CMP?
41
ECE4100/6100
H-H. S. Lee
Speculative Multithreading
 SMT can justify wider-than-ILP datapath
 But, datapath is only fully utilized by multiple threads
 How to speed up single-thread program by utilizing multiple threads?
 What to do with spare resources?
Execute both sides of hard-to-predictable branches
Eager execution or Polypath execution
Dynamic predication
Send another thread to scout ahead to warm up caches & BTB
Speculative precomputation
Early branch resolution
Speculatively execute future work
Multiscalar or dynamic multithreading
e.g. start several loop iterations concurrently as different threads, if data
dependence is detected, redo the work
Run a dynamic compiler/optimizer on the side
Dynamic verification
DIVA or Slipstream Processor
42