Automatic Instruction Scheduler Automatic Instruction Scheduler Retargeting by Reverse-EngineeringRetargeting by Reverse-Engineering

Matthew J. Bridges, Neil Vachharajani, Guilherme Ottoni, David I. August

Liberty Research GroupDepartment of Computer Science

Princeton University

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Structural HazardsStructural Hazards

Add

Shl

Sub

Xor

Code

Dynamic Schedule

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Xor

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Static Schedule

Xor

Resource Usage

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Xor

ALU0

ALU0

ALU1

ALU1

ALU1ALU0

ALU1ALU0

ALU1ALU0

Resources

Tim

e

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Structural HazardsStructural Hazards

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Sub

Xor

Code

Resource Usage

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Xor

ALU0

ALU0

ALU1

ALU1

ALU1ALU0

ALU1ALU0

ALU1ALU0

Resources

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Static Schedule

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Dynamic Schedule

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Encoding Structural HazardsEncoding Structural Hazards

Processor Manuals

Machine Description

SchedulerHazard

Detector

Processor Manuals

Pros:

Easily Available

Standard Methodology

Cons:

Processor manual does not describe the actual machine

Takes several man-weeks to translate processor manual into machine description

Machine description written incorrectly

Compiler

Offline

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Processor ManualsProcessor ManualsIA64 Instructions Set Reference Vol. 3 ~370 pages of ISA description

Itanium 2 Processor Reference Manual~200 pages of microarchitecural details

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Humans aren’t PerfectHumans aren’t Perfect

OP_SHIFT (alt(ALT_SHIFT_REG));

OP_SHIFT (alt||(ALT_SHIFT_IMM));

ALT_SHIFT_REG (format (OF_IREG_IREG resv(RL_Itype));ALT_SHIFT_IMM (format (OF_IREG_IMM6 resv(RL_Itype));

Itanium Instruction Set Reference Vol. 3: Page 3:212

Itanium 2 Processor Reference Manual: Page 27

IMPACT Itanium 2 Machine Description

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Encoding Structural HazardsEncoding Structural HazardsArchitecture Description

Language[LISA][nML]

Pros:

Machine description accurately describes processor

Cons:

ADLs are limited

Not used for general purpose processors

Often unavailable

ADL

Machine Description

SchedulerHazard

Detector

Compiler

Offline

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Encoding Structural HazardsEncoding Structural HazardsQuery machine while

scheduling

[Baker `91][Dupre `04]

Pros:

Avoids human errors

Easily available

Cons:

Increase in compile time

Unable to cross-compile

SchedulerHazard

Detector

Compiler

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Encoding Structural HazardsEncoding Structural Hazards

Machine Description

Query machine a priori

Pros:

Avoids human errors

Easily available

Avoid scheduling overhead

Cons:

?

SchedulerHazard

Detector

Compiler

OfflineStructural HazardDetermination Algo.

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Limits of QueryingLimits of Querying

Machine A

Infinite

Finite

Machine B

Infinite

Finite

Goal: Perfectly characterize the machine

Goal: Characterize the machine

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Identifying a Subspace to ExploreIdentifying a Subspace to Explore

I I I

I I I

I I I

Width (w)

Dep

th (

p)

Instructions (I)

# Possible Instruction Schedules

€

∞

€

∞

€

Iw∗p

€

3006*8

€

10107MillenniaMyopia

€

Iw

€

3006 230 MillenniaPipelined

Itanium 2 Time

€

∞

€

C + w −1

w

⎛

⎝ ⎜

⎞

⎠ ⎟ 1.3 HoursCategorization

€

24 + 6 −1

6

⎛

⎝ ⎜

⎞

⎠ ⎟

100 Schedules/Second

Assumption

€

I + w −1

w

⎛

⎝ ⎜

⎞

⎠ ⎟ 3No Order

€

300 + 6 −1

6

⎛

⎝ ⎜

⎞

⎠ ⎟ Millennia

I I I

Width (w)

Instructions (I)

Width (w)

Instructions (I)

€

L

€

K

€

J

Width (w)

Categories (C)

€

CL

€

CJ

€

CK

Width (w)

Instructions (I)

€

L

€

K

€

JGeneral

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Cost of AssumptionsCost of Assumptions

1.00

1.02

1.04

1.06

1.08

1.10

1.12

1.14

1.16

1.18

1.20

TI TMS320C3x Sparc V8 Itanium Itanium 2

Speedup Over No Hazard Detection

Ignore Order Pipelined Manual Resource Maps

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Reverse-Engineering AlgorithmReverse-Engineering Algorithm1. All Instructions in a Single Category

2. Split Categories• Perform a Random Walk

• Longer walk = more accurate categories

3. Extract Canonical Instructions

4. Exhaustive Exploration of Canonical Instructions …

5. Update machine description

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Cost of Finding CategoriesCost of Finding Categories

0

10

20

30

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50

60

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80

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100

1 10 100 1000

Time (Minutes)

% of No Order Speedup

Itanium 2 Itanium Sparc V8 TI TMS320C3x

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ConclusionConclusionObtaining structural hazards for use in instruction scheduling is error prone

Processors manuals wrong or confusingManual translation is error proneADLs aren’t available

We can automatically extract structural hazards useful for scheduling

It is impossible to determine all structural hazards a prioriIt is possible to find sufficient hazard information to produce good schedules Algorithm achieves 81-100% of manual resource map performance

Greatly reduces the time needed to retarget the instruction scheduler of a compiler

Facilitates Design Space Exploration

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Thank YouThank You