Chapter …
Morgan Kaufmann Publishers 13 November, 2018
Chapter 4 — The Processor 1
Chapter 4 — The Processor — 140
Instruction-Level Parallelism (ILP)
Pipelining: executing multiple instructions in
parallel
To increase ILP
Deeper pipeline
Less work per stage shorter clock cycle
Multiple issue
Replicate pipeline stages multiple pipelines
Start multiple instructions per clock cycle
CPI < 1, so use Instructions Per Cycle (IPC)
E.g., 4GHz 4-way multiple-issue
16 BIPS, peak CPI = 0.25, peak IPC = 4
But dependencies reduce this in practice
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Multiple Issue
Static multiple issue
Compiler groups instructions to be issued together
Packages them into “issue slots”
Compiler detects and avoids hazards
Dynamic multiple issue
CPU examines instruction stream and chooses
instructions to issue each cycle
Compiler can help by reordering instructions
CPU resolves hazards using advanced techniques at
runtime
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Chapter 4 — The Processor — 142
Speculation
“Guess” what to do with an instruction
Start operation as soon as possible
Check whether guess was right
If so, complete the operation
If not, roll-back and do the right thing
Common to static and dynamic multiple issue
Examples
Speculate on branch outcome
Roll back if path taken is different
Speculate on load
Roll back if location is updated
Chapter 4 — The Processor — 143
Compiler/Hardware Speculation
Compiler can reorder instructions
e.g., move load before branch
Can include “fix-up” instructions to recover
from incorrect guess
Hardware can look ahead for
instructions to execute
Buffer results until it determines they are
actually needed
Flush buffers on incorrect speculation
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Chapter 4 — The Processor — 144
Speculation and Exceptions
What if exception occurs on a
speculatively executed instruction?
e.g., speculative load before null-pointer
check
Static speculation
Can add ISA support for deferring
exceptions
Dynamic speculation
Can buffer exceptions until instruction
completion (which may not occur)
Chapter 4 — The Processor — 145
Static Multiple Issue
Compiler groups instructions into “issue
packets”
Group of instructions that can be issued on
a single cycle
Determined by pipeline resources required
Think of an issue packet as a very long
instruction
Specifies multiple concurrent operations
Very Long Instruction Word (VLIW)
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Chapter 4 — The Processor — 146
Scheduling Static Multiple Issue
Compiler must remove some/all hazards
Reorder instructions into issue packets
No dependencies with a packet
Possibly some dependencies between
packets
Varies between ISAs; compiler must know!
Pad with nop if necessary
Chapter 4 — The Processor — 147
MIPS with Static Dual Issue
Two-issue packets
One ALU/branch instruction
One load/store instruction
64-bit aligned
ALU/branch, then load/store
Pad an unused instruction with nop
Address Instruction type Pipeline Stages
n ALU/branch IF ID EX MEM WB
n + 4 Load/store IF ID EX MEM WB
n + 8 ALU/branch IF ID EX MEM WB
n + 12 Load/store IF ID EX MEM WB
n + 16 ALU/branch IF ID EX MEM WB
n + 20 Load/store IF ID EX MEM WB
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Chapter 4 — The Processor — 148
MIPS with Static Dual Issue
Chapter 4 — The Processor — 149
Hazards in the Dual-Issue MIPS
More instructions executing in parallel
EX data hazard
Forwarding avoided stalls with single-issue
Now can’t use ALU result in load/store in same
packet
add $t0, $s0, $s1
load $s2, 0($t0)
Split into two packets, effectively a stall
Load-use hazard
Still one cycle use latency, but now two instructions
More aggressive scheduling required
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Chapter 4 — The Processor — 150
Scheduling Example
Schedule this for dual-issue MIPS
Loop: lw $t0, 0($s1) # $t0=array element
addu $t0, $t0, $s2 # add scalar in $s2
sw $t0, 0($s1) # store result
addi $s1, $s1,–4 # decrement pointer
bne $s1, $zero, Loop # branch $s1!=0
ALU/branch Load/store cycle
Loop: nop lw $t0, 0($s1) 1
addi $s1, $s1,–4 nop 2
addu $t0, $t0, $s2 nop 3
bne $s1, $zero, Loop sw $t0, 4($s1) 4
IPC = 5/4 = 1.25 (c.f. peak IPC = 2)
Chapter 4 — The Processor — 151
Loop Unrolling
Replicate loop body to expose more
parallelism
Reduces loop-control overhead
Use different registers per replication
Called “register renaming”
Avoid loop-carried “anti-dependencies”
Store followed by a load of the same register
Aka “name dependence”
Reuse of a register name
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Chapter 4 — The Processor — 152
Loop Unrolling Example
IPC = 14/8 = 1.75
Closer to 2, but at cost of registers and code size
ALU/branch Load/store cycle
Loop: addi $s1, $s1,–16 lw $t0, 0($s1) 1
nop lw $t1, 12($s1) 2
addu $t0, $t0, $s2 lw $t2, 8($s1) 3
addu $t1, $t1, $s2 lw $t3, 4($s1) 4
addu $t2, $t2, $s2 sw $t0, 16($s1) 5
addu $t3, $t4, $s2 sw $t1, 12($s1) 6
nop sw $t2, 8($s1) 7
bne $s1, $zero, Loop sw $t3, 4($s1) 8
Pop Quiz
Loop unrolling is always beneficial:
A: True
B: False
Chapter 4 — The Processor — 153
Morgan Kaufmann Publishers 13 November, 2018
Chapter 4 — The Processor 8
Chapter 4 — The Processor — 154
Dynamic Multiple Issue
“Superscalar” processors
CPU decides whether to issue 0, 1, 2, …
each cycle
Avoiding structural and data hazards
Avoids the need for compiler scheduling
Though it may still help
Code semantics ensured by the CPU
Chapter 4 — The Processor — 155
Dynamic Pipeline Scheduling
Allow the CPU to execute instructions
out of order to avoid stalls
But commit result to registers in order
Example
lw $t0, 20($s2)
addu $t1, $t0, $t2
sub $s4, $s4, $t3
slti $t5, $s4, 20
Can start sub while addu is waiting for lw
Morgan Kaufmann Publishers 13 November, 2018
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Chapter 4 — The Processor — 156
Dynamically Scheduled CPU
Results also sent
to any waiting
reservation stations
Reorders buffer for
register writes
Can supply
operands for
issued instructions
Preserves
dependencies
Hold pending
operands
Chapter 4 — The Processor — 157
Register Renaming
Reservation stations and reorder buffer
effectively provide register renaming
On instruction issue to reservation
station
If operand is available in register file or
reorder buffer
Copied to reservation station
No longer required in the register; can be
overwritten
If operand is not yet available
It will be provided to the reservation station by
a function unit
Register update may not be required
Morgan Kaufmann Publishers 13 November, 2018
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Chapter 4 — The Processor — 158
Speculation
Predict branch and continue issuing
Don’t commit until branch outcome
determined
Load speculation
Avoid load and cache miss delay
Predict the effective address
Predict loaded value
Load before completing outstanding stores
Bypass stored values to load unit
Don’t commit load until speculation cleared
Chapter 4 — The Processor — 159
Why Do Dynamic Scheduling?
Why not just let the compiler schedule
code?
Not all stalls are predicable
e.g., cache misses
Can’t always schedule around branches
Branch outcome is dynamically determined
Different implementations of an ISA
have different latencies and hazards
Morgan Kaufmann Publishers 13 November, 2018
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Chapter 4 — The Processor — 160
Does Multiple Issue Work?
Yes, but not as much as we’d like
Programs have real dependencies that limit
ILP
Some dependencies are hard to eliminate
e.g., pointer aliasing
Some parallelism is hard to expose
Limited window size during instruction issue
Memory delays and limited bandwidth
Hard to keep pipelines full
Speculation can help if done well
The BIG Picture