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Escape Analysis
(To do: Format this nicely.)
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From:
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Vladimir
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Kozlov
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Date:
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May
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15,
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2009
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1:47:21
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PM
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PDT
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_C2
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implements the
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flow-insensitive
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escape
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analysis
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algorithm
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described
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in:
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[Choi99] Jong-Deok Shoi, Manish Gupta, Mauricio Seffano, Vugranam C. Sreedhar, Sam Midkiff, "Escape Analysis for Java", Procedings of ACM SIGPLAN OOPSLA Conference, November 1, 1999 |
The
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analysis
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requires
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construction
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of
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a
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"connection
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graph"
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(CG)
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for
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the
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method
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being
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analyzed.
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The
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nodes
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of
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the
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connection
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graph
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are:
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- Java objects (JO) - Local variables (LV) - Fields of an object (OF), these also include array elements |
C2
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does
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not
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have
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local
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variables.
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However
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for
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the
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purposes
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of
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constructing
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the
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connection
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graph,
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the
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following
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IR
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nodes
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are
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treated
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as
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local
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variables:
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|---|
Phi (pointer values) LoadP, LoadN Proj#5 (value returned from call nodes including allocations) CheckCastPP, CastPP, EncodeP, DecodeN Return (GlobalEscape) |
The
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LoadP,
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Proj
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and
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CheckCastPP
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behave
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like
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variables
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assigned
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to
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only
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once.
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Only
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a
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Phi
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can
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have
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multiple
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assignments.
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Each
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input
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to
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a
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Phi
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is
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treated
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as
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an
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assignment
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to
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it.
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The
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following
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node
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types
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are
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JavaObject:
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top() Allocate AllocateArray Parm (for incoming object arguments, GlobalEscape) CastX2P ("unsafe" operations, GlobalEscape) CreateEx (GlobalEscape) ConP, ConN (GlobalEscape except for null) LoadKlass, LoadNKlass (GlobalEscape) ThreadLocal (ArgEscape) |
AddP
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nodes
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are
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fields.
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After
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building
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the
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graph,
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a
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pass
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is
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made
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over
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the
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nodes,
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deleting
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deferred
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nodes
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and
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copying
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the
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edges
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from
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the
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target
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of
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the
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deferred
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edge
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to
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the
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source.
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This
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results
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in
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a
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graph
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with
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no
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deferred
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edges,
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only:
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|---|
LV -P> JO OF -P> JO (the object whose oop is stored in the field) JO -F> OF It makes a pass over the nodes and determines nodes escape state: GlobalEscape - An object escapes the method and thread (stored into a static field or stored into a field of an escaped object or returned as the result of the current method). ArgEscape - An object passed as argument or referenced by argument but not globally escape during a call (by analyzing the bytecode of called method). NoEscape - A scalar replaceable object. After escape analysis C2 eliminates scalar replaceable object allocations and associated locks. C2 also eliminates locks for all non globally escaping objects. C2 does NOT replace a heap allocation with a stack allocation for non globally escaping objects. |
After that escape analysis makes a pass over the nodes and determines nodes escape state:
- GlobalEscape - An object escapes the method and thread (stored into a static field or stored into a field of an escaped object or returned as the result of the current method).
- ArgEscape - An object passed as argument or referenced by argument but not globally escape during a call (by analyzing the bytecode of called method).
- NoEscape - A scalar replaceable object.
After escape analysis C2 eliminates scalar replaceable object allocations and associated locks. C2 also eliminates locks for all non globally escaping objects. C2 does NOT replace a heap allocation with a stack allocation for non globally escaping objects.
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