A Framework for Unrestricted Whole-Program Optimization

1
A Framework for UnrestrictedWhole-Program
Optimization

• Spyridon Triantafyllis, Matthew J. Bridges,
  Easwaran Raman, Guilherme Ottoni, David I. August
• The Liberty Research Group
• Department of Computer Science
• Princeton University

2
Procedure-Based Compilation

• Procedure-Based Compilation
• Pros
• Well Known
• Cons
• Can not exploit opportunities that cross
  procedures

g()
h()
f(a,b,c)
if (EOB)
zxy f(x,y,5)
f(1,2,3)
fill B
dab
z
ret
3
Interprocedural Analysis

• Interprocedural Analysis
• Sharir78 Morel78 Reps95
• Pros
• Increases available information
• Enables some optimization across procedure
  boundaries
• Cons
• Has to analyze the entire program
• Optimizations need to respect the procedure
  boundary

g()
h()
f(a,b,c)
if (EOB)
zxy f(x,y,5)
f(1,2,3)
fill B
dab
z
ret
4
Interprocedural Analysis Interprocedural Opti

• Interprocedural Analysis
• Sharir78 Morel78 Reps95
• Pros
• Increases available information
• Enables some optimization across procedure
  boundaries
• Cons
• Has to analyze the entire program
• Optimizations need to respect the procedure
  boundary
• Most optimizations will still be intraprocedural

g()
h()
f(a,b,c,z)
if (EOB)
zxy f(x,y,5,z)
f(1,2,3,2)
fill B
dz
z
ret
5
Inlining

• Inlining
• Scheifler 77 Hwu89 Chang92
• Pros
• Increases optimization scope
• Enables specialization
• Doesnt require opti to understand
  interprocedural concerns
• Cons
• Hard limit on procedure size
• Unnecessary code growth

g()
zxy
if (EOB)
fill B
f(a,b,c)
if (EOB)
dz
fill B
h()
dab
jump
f(1,2,3)
z
ret
6
Partial Inlining

• Partial Inlining
• Suganuma03Way00
• Pros
• Can alleviate some code growth
• Cons
• Gains are limited

g()
zxy
f()
fill B
if (EOB)
f()
return
f(a,b,c)
if (EOB)
dz

h()
fill B
dab
jump
f(1,2,3)
ret
z
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Why Procedures?

• Procedures
• Calling convention boundary
• Single-Entry, Single-Exit
• Pros
• Implicit correlated edges - context sensitivity
• Natural unit for divide conquer compilation
• Cons
• Optimized for software-engineering
• Restricts optimization

if (EOB)
zxy
fill B
dab
dz
z
ret
We dont have to use procedures!
8
The Whole-Program CFG

• Retain useful traits of procedures
• Correlated edges
• Compilation unit
• Goal Obtain an optimizable whole-program
  representation
• Increase optimization scope
• Allow all opti to operate on increased scope
  without change
• Targeted code growth

if (EOB)
zxy
fill B
dab
z
jump
9
The Whole-Program CFG

• Retain useful traits of procedures
• Correlated edges
• Compilation unit
• Goal Obtain an optimizable whole-program
  representation
• Increase optimization scope
• Allow all opti to operate on increased scope
  without change
• Targeted code growth

Represent calls and returns as special
control-flow transitions Sharir78
10
Whole-Program Optimizations

• Optimization destroys the programs procedural
  structure!
• Example Superblock Formation Hwu92
• Unconventional call structures!
• Many-to-many call lt-gt return relation
• Must rediscover structure for summary edges

A
(1
B
B
(2
G
D
C
C
H
)2
E
E
)1
)1
F
F
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Analyzing the Whole-Program CFG

• Context-Sensitive
• Interprocedural AnalysisSharir78
• meet over all realizable paths
• Identify Entry-Exit Pairs (EEP)
• Correlated call return arcs
• Allows use of summary edges
• Blocks may belong to more than one EEP

A
(1
B
B
(2
G
D
C
C
H
)2
E
E
)1
)1
F
F
12
Determining a Compilation Unit Region Formation

• Region Formation Hank95
• arbitrarily shaped, compiler-selected compilation
  unit

A
(1
B

• Region Selection
• Select seed add neighbors
• (profile, structure, dataflow )

B
(2
G
D

• Success Criteria
• Optimizability vs. compile time
• Few too small or too big regions
• Intra-region transitions inter-region
  transitions
• Encapsulation
• Make regions independently optimizable

C
C
H
)2
E
E
)1
)1
F
F
Compiler is free to select its own optimization
units!
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Evaluation Framework The Velocity Compiler
Frontend
Frontend
Frontend

• Evaluation
• Inliner Opti. ported from IMPACT
• Targeting Itanium 2

Procedures
Procedures
Procedures
Inlining
WCFG
DetermineCompilation Unit
Region Form.
Regions
Procedures
Superblock
Superblock
Superblock
Classical ILP Optimizer
Classical ILP Optimizer
Classical ILP Optimizer
Optimize Compilation Unit
Scheduling
Scheduling
Scheduling
Executable
Executable
Executable
Baseline
Inlining
PBE
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Code Growth
1.45
1.23
Code Size
15
Speedup - Train Input
1.07
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Conclusion

• Procedure boundaries restrict optimization!
• Ways to deal with procedures exist, but limited
• Interprocedural analysis opti Scales badly,
  not always possible
• Inlining Unnecessary Code growth
• Procedures are not the right compilation unit
• PBE offers unrestricted and practical
  whole-program optimization
• An expanded form of interprocedural analysis
• New region formation framework and heuristics
• An interprocedural region encapsulation method