SLIDE 1
Outline
1
Motivation
2
Type analysis
3
Coloring
4
Evaluation
2 Motivation Type analysis Coloring Evaluation Efficient Compilation of .NET Programs for Embedded Systems
Efficient Compilation of .NET Programs for Embedded Systems Olivier - - PowerPoint PPT Presentation
Efficient Compilation of .NET Programs for Embedded Systems Olivier - - PowerPoint PPT Presentation
Efficient Compilation of .NET Programs for Embedded Systems Olivier Sallenave Roland Ducournau Cortus SA LIRMM (France) MASPEGHI/ICOOOLPS 2010 Maribor, Slovenia Outline Motivation 1 Type analysis 2 Coloring 3 Evaluation 4
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SLIDE 3
Outline
1
Motivation
2
Type analysis
3
Coloring
4
Evaluation
3 Motivation Type analysis Coloring Evaluation Efficient Compilation of .NET Programs for Embedded Systems
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Embedded systems Long development cycles
Specific constraints Increasing design complexity
Low-end systems
Still programmed in C or assembly
The problem
High-level languages poorly adapted for constrained devices
4 Motivation Type analysis Coloring Evaluation Efficient Compilation of .NET Programs for Embedded Systems
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Approach
Global compilation
Closed-world assumption
Knowledge of the whole program ⇒ more efficient implementation
Implications
Disable dynamic loading and reflection
5 Motivation Type analysis Coloring Evaluation Efficient Compilation of .NET Programs for Embedded Systems
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Objective
Enable .NET for devices with <100KB memory
Target architecture
Cortus APS3 (32-bit RISC processor) Small size Low power consumption
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Compilation scheme
Ahead-of-time compilation
C# code bytecode CIL Microsoft C# compiler C# code CIL bytecode code C
code machine APS3
aps3−gcc Global compilation
No runtime penalty (no JIT) Compile under the closed-world assumption Focus on optimizing object mechanisms
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SLIDE 8
Outline
1
Motivation
2
Type analysis
3
Coloring
4
Evaluation
8 Motivation Type analysis Coloring Evaluation Efficient Compilation of .NET Programs for Embedded Systems
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Type analysis Type analysis
Constructs a call graph of the program Approximates dynamic types Many algorithms exist (+/- precision)
Optimizations
Monomorphic method calls → implement as static calls Safe subtype checks → remove Unreachable members/code → remove
9 Motivation Type analysis Coloring Evaluation Efficient Compilation of .NET Programs for Embedded Systems
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Implementation details Our algorithm
Based on RTA (maintain a set of instantiated classes) Meta-model to represent classes/members Takes into account .NET specificities
Meta-model
Distinguishes identity/implementations of methods Dead identity → remove entry in method tables Dead implementation → remove code
Stack state simulation
.NET VM is stack-based ⇒ static type of operands ? Compute stack state for each instruction
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Array covariance (Java/.NET) Principle
B <: A ⇒ B[] <: A[] <: subtyping relationship
class B : A { } ... void meth(A[] tab, A x, int i) { tab[i] = x ; }
Drawback
Array assignments are unsafe → need subtype check
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Eliminating subtype checks
P(x) = {possible types of x} approximate x’s type
x isa t ?
True if ∀p ∈ P(x), p <: t False if ∀p ∈ P(x), ¬(p <: t) <: subtyping relationship Otherwise, need subtype check
The problem for array assignments
t is unknown at compile-time (element type of the assigned array) ⇒ approximate it as well
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Optimizing array covariance
P(x) = {possible types of x} approximate x’s E(tab) = {possible element types for tab} and tab’s types
tab[i] = x
Safe if ∀p ∈ P(x), ∀e ∈ E(tab), p <: e Error if ∀p ∈ P(x), ∀e ∈ E(tab), ¬(p <: e) Otherwise, need subtype check
Effectiveness
Efficient if a few array types are instantiated
13 Motivation Type analysis Coloring Evaluation Efficient Compilation of .NET Programs for Embedded Systems
SLIDE 14
Outline
1
Motivation
2
Type analysis
3
Coloring
4
Evaluation
14 Motivation Type analysis Coloring Evaluation Efficient Compilation of .NET Programs for Embedded Systems
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Single subtyping Virtual method tables
Straightforward extensions of the superclass table
No binary tree dispatch
No branch prediction Tends to increase code size
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Multiple subtyping Insert empty entries in method tables
Maintain single subtyping invariants Drawback is additional space cost Minimize the number of holes is NP-hard
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Coloring interfaces Abstract types
Their method tables do not exist at runtime Consider it in the coloring algorithm → allocate holes in abstract tables first
17 Motivation Type analysis Coloring Evaluation Efficient Compilation of .NET Programs for Embedded Systems
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Implementation details What remains in method tables
Only necessary data Methods called polymorphically at least once Type IDs for subtype check targets (Cohen test) Some holes
Fill empty entries
Insertion of holes is the drawback of coloring ⇒ use them to store static fields
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Outline
1
Motivation
2
Type analysis
3
Coloring
4
Evaluation
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Program reduction Dead types
reduction classes 552 57% interfaces 24 54%
Dead members
non-virtual reduction fields 795 33% methods 886 41% virtual methods (id) 384 66% methods (impl) 651 69% bytecode size 82KB 26%
20 Motivation Type analysis Coloring Evaluation Efficient Compilation of .NET Programs for Embedded Systems
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Object mechanisms Compile-time resolution
method calls resolved static 2999 – virtual 599 85% subtype checks downcasts 84 11% array assignments 199 81%
Runtime structures
entries in reduction method tables 2974 82% instance layouts* 707 15%
* one layout per class
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Summary Summary
Noticeable reduction of the programs Array covariance is almost safe (almost no overhead)
Bench programs (.NET Micro Framework)
Only a few holes should result from coloring : Most classes do not implement interfaces Average method table size is small Holes can be filled with static fields
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Future work Future work
Null check elimination Genericity (mixed implementation) Evaluate on bigger programs Profiling Garbage collection
Additional features
Generate debug informations Multithreading
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Thank you !
http://www.cortus.com
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