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Storage Allocation for Embedded Processors

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... of pointer expressions grow linearly in the size of the program ... Measure the execution time and code size of 6 benchmarks ... Improve program size and speed ... – PowerPoint PPT presentation

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Title: Storage Allocation for Embedded Processors


1
Storage Allocation for
Embedded Processors

By Jan Sjodin Carl von Platen Present by Xie
Lei ( PLS Lab)
2
Memory In Embedded Sys
  • Several memory areas (on-chip, off-chip)
  • SRAM
  • DRAM
  • E2PROM …
  • Different properties
  • access time
  • size
  • restricted to native pointer types

3
Pointer Types
  • Different in length
  • 8-bit
  • 16-bit . . .
  • Different in cost
  • 8-bit is cheaper than 16-bit

4
Problem to solve
  • Allocate data and select pointer types in the
    most efficient way
  • Frequently accessed data in fast memory
  • Frequently used pointers expression have cheap
    pointer type
  • Manual ? Automatic

5
Road Map
  • Brief Background
  • Memory Organization Model
  • Cost Model
  • Formulation Discussion
  • Implementation
  • Experiments Results
  • Conclusion

6
Why Allocation ?
  • Different pointer types to access different types
    of memories improves performance
  • execution speed
  • program size
  • energy consumption
  • production cost . . .

7
Allocation Problem
  • Allocating each variable of a program in a memory
    segment
  • Assigning a native pointer type to each pointer
    expression of the same program
  • Dependency latter depends on former
  • ---take both into consider
    simultaneously

8
Current solution Drawback
  • Manually locate variables and select native
    pointer types pragmas or keywords
  • Drawbacks
  • time-consuming
  • source code non-portable

9
New Solution
  • A model which can describe architectures with
    irregular memory organization and several pointer
    types.
  • Derive integer linear program from the model
  • Solve the program Solve the allocation problem
  • Optimal solution ( under assumption )

10
An Important Condition
  • Condition All memory accesses in the program
    should be known
  • -- whole program optimization ( WPO )
  • points-to set Set of the objects a restricted
    pointer may point to
  • Reason
  • The points-to set of a pointer must be
    allocated where that pointer can reach

11
Road Map
  • Brief Background
  • Memory Organization Model
  • Cost Model
  • Formulation Discussion
  • Implementation
  • Experiment Results
  • Conclusion

12
Features in Embedded Processors
  • On-chip memory efficiently
  • Zero page use smaller pointer types , fast ,
    compact code
  • Harvard architectures different native pointer
    representations and addressing mode are used for
    program and data address space

13
AVR RAM Configuration
14
Separate VS Single address space (1)
  • Separate address space
  • disjoint sets of pointer types
  • know points-to sets to allocate data
  • use separate address buses
  • different memory access instructions use
    different pointer types as operands

15
Separate VS Single address space (2)
  • Single address space
  • A general native pointer type
  • No restriction on allocation
  • Try to use cheaper pointer type ( need to
    know the point-to set to decide if that pointer
    type can work)

16
Code Size Contrast
17
An Abstract Memory Model
  • Memory Model Memory Segments, Pointer Types ,
    Relation
  • Memory Segments subsets of the total memory
    space, uniform with properties( speed, addressing
    modes). M1 , M2 , M3 . . . MS
  • Pointer Types P1 , P2 , P3 . . . PT
  • Relation Pointer Type to Memory Segments

18
Memory Model
  • Size( Mj ) number of addressable units of
    storage in Memory Segment Mj
  • Relation function
  • Mem( Pk ) j Pk can point to Mj
  • In which Pk Pointer
    Type
  • Memory Model can be expressed by a table

19
Example AVR
20
Road Map
  • Brief Background
  • Memory Organization Model
  • Cost Model
  • Formulation Discussion
  • Implementation
  • Experiment Results
  • Conclusion

21
Two Kinds of Cost
  • Cost Static cost Dynamic cost
  • Static cost the size of a program, expressed by
    the sum of all statements in instruction set
    STMTR
  • Dynamic cost execution time, for each statement
    need to be scaled by the estimate execution
    frequency of the statement

22
Expressions
  • V v1 , v2 . . . vN set of variables
    referred to in the source program
  • Size(v) number of allocation units required by
    v
  • STMTR set of individual occurrences of
    instructions
  • E e1 , e2 . . . eM set of pointer
    expressions

23
Functions
  • PtrExp STMTR ? E
  • allow the client of the storage
    allocator to modify the type of single pointer
    expressions independently
  • Seg(v) V? M , the segment where a variable is
    allocated
  • PtrT(e) E?P, the pointer type a pointer
    expression is assigned

24
Cost of A Statement ( 1 )
  • For each statement in STMTR , we can have pointer
    cost and variable cost
  • ptrcostS ( e, t ) pointer cost , the cost
    contribution of selecting t as the pointer
    type for pointer expression e
  • varcostS ( v, m ) variable cost , the cost
    contribution of variable v if it is located
    in memory segment m

25
Cost of A Statement ( 2 )
  • The cost of a particular solution is defined in
    terms of the cost contribution of each statement
    in STMTR
  • Cost(S) ? (e?E) ptrcostS ( e, PtrT (e) )
  • ? (v?V) varcostS ( v, Seg(v)
    )

26
Road Map
  • Brief Background
  • Memory Organization Model
  • Cost Model
  • Formulation Discussion
  • Implementation
  • Experiment Results
  • Conclusion

27
BIP Formulation
  • BIP Binary Integer Program
  • The storage allocation problem is formulated as
    a BIP
  • Based on the model of Memory Organization and the
    cost model

28
Feasible Solution
  • Two condition must be satisfied
  • Total size of the variables in a single memory
    segment size of the segment
  • Pointer expression that may point to a variable
    must be assigned a type that can access the
    segment where that variable is
  • Find a feasible solution with minimal cost !

29
BIP ( 1 )
  • Each variable can be placed in exactly one memory
    segment
  • xi1 xi2 . . . xiS1 , 1 i N
  • Total size of the variables in a segment can not
    be greater than the size of the segment
  • Size(v1) x1m Size(v2) x2m . . . Size(vN) xNm
  • Size(Mm) , 1 m S

30
BIP ( 2 )
  • Each pointer expression have a unique pointer
    type
  • yj1 yj2 . . . yjT 1 , 1 j M
  • Type of a pointer expression be general enough
    for all the variables in its points-to set
  • Seg(v) ? Mem ( PtrT( ej ) ) , v ?Pt ( ej
    )

31
BIP ( 3 )
  • Objective function is the cost of the solution

32
Discussion Scalability
  • Number of variables and number of pointer
    expressions grow linearly in the size of the
    program
  • Number of memory segments and the number of
    pointer types depend on target architecture , so
    can be regarded as constants

33
Discussion Accuracy
  • Fundamental to the model cost of a statement
    can be expressed as linear combination of cost
    from variables and pointer expressions ---- Not
    precise
  • Some architectures may represent pointers with
    multiple machine words, so loading a pointer need
    several instructions

34
Road Map
  • Brief Background
  • Memory Organization Model
  • Cost Model
  • Formulation Discussion
  • Implementation
  • Experiment Results
  • Conclusion

35
Compiler Framework
36
WPO Prototype
  • Points-to analysis
  • estimate the points-to set of each pointer
  • ILP solver
  • Information about all global variables and
    pointers is put into the solver to get an
    allocation

37
Modified ICCAVR
  • Compile the C source code and use the
    allocation information by WPO to modify the
    memory attributes of variables
  • Allocation information is used during the parsing
    stage to modify the memory attributes of variables

38
Road Map
  • Brief Background
  • Memory Organization Model
  • Cost Model
  • Formulation Discussion
  • Implementation
  • Experiments Results
  • Conclusion

39
Experiments
  • Measure the execution time and code size of 6
    benchmarks
  • Results are compared with code compiled on the
    standard ICCAR compiler
  • Different version of AVR microcontroller ,
    internal memory vary from 0 to 4KB or more ( 64KB
    and 16KB)

40
Results
  • Results vary a lot with benchmarks
  • CPU intensive or Memory intensive
  • Code size decreased slightly for most benchmarks
    ( Special statemate )
  • Execution time improved for most benchmarks( also
    Statemate )

41
Road Map
  • Brief Background
  • Memory Organization Model
  • Cost Model
  • Formulation Discussion
  • Implementation
  • Experiments Results
  • Conclusion

42
Conclusion
  • Model of memory hierarchies for storage
    allocation for embedded system processors
  • Implement a memory allocator using ILP to get an
    optimal allocation under the memory model
  • Improve program size and speed
  • Automatically done , portable , robust no need
    to include target-specific information in source
    code

43
  • The End
  • Thank You !
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