Automatic Instruction Scheduler Retargeting by Reverse-Engineering

1
Automatic Instruction Scheduler Retargeting by
Reverse-Engineering

• Matthew J. Bridges, Neil Vachharajani, Guilherme
  Ottoni, David I. August
• Liberty Research Group
• Department of Computer Science
• Princeton University

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Structural Hazards
Code
Resources
Add
ALU1
ALU0
Time
Shl
ALU1
ALU0
Sub
ALU1
ALU0
Xor
3
Structural Hazards
Code
Resources
Add
ALU1
ALU0
Shl
Time
ALU1
ALU0
Sub
ALU1
ALU0
Xor
4
Encoding Structural Hazards

• 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

Processor Manuals
Offline
Compiler
Machine Description
Hazard Detector
Scheduler
5
Processor Manuals
IA64 Instructions Set Reference Vol. 3 370
pages of ISA description
Itanium 2 Processor Reference Manual200 pages
of microarchitecural details
6
Humans arent Perfect
IMPACT Itanium 2 Machine Description
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
3212
Itanium 2 Processor Reference Manual Page 27
7
Encoding Structural Hazards

• Architecture Description LanguageLISAnML
• Pros
• Machine description accurately describes
  processor
• Cons
• ADLs are limited
• Not used for general purpose processors
• Often unavailable

ADL
Offline
Compiler
Machine Description
Hazard Detector
Scheduler
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Encoding Structural Hazards

• Query machine while scheduling
• Baker 91Dupre 04
• Pros
• Avoids human errors
• Easily available
• Cons
• Increase in compile time
• Unable to cross-compile

Hazard Detector
Scheduler
Compiler
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Encoding Structural Hazards

• Query machine a priori
• Pros
• Avoids human errors
• Easily available
• Avoid scheduling overhead
• Cons
• ?

Hazard Detector
Scheduler
Machine Description
Compiler
Offline
Structural HazardDetermination Algo.
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Limits of Querying
Goal Perfectly characterize the machine
Goal Characterize the machine
Machine A
Machine B
Finite
Finite
Infinite
Infinite
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Identifying a Subspace to Explore
Possible Instruction Schedules
Itanium 2
Time
Assumption
General
100 Schedules/Second
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Cost of Assumptions
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Reverse-Engineering Algorithm
1. All Instructions in a Single Category
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Cost of Finding Categories
15
Conclusion

• Obtaining structural hazards for use in
  instruction scheduling is error prone
• Processors manuals wrong or confusing
• Manual translation is error prone
• ADLs arent available
• We can automatically extract structural hazards
  useful for scheduling
• It is impossible to determine all structural
  hazards a priori
• It 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 You