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abstract class CopyProp extends AnyRef

Source
CopyProp.scala
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Instance Constructors

  1. new CopyProp()

Type Members

  1. case class ProducedValue(producer: AbstractInsnNode, size: Int) extends Product with Serializable

Abstract Value Members

  1. abstract val postProcessor: PostProcessor

Concrete Value Members

  1. def copyPropagation(method: MethodNode, owner: InternalName): Boolean

    For every xLOAD n, find all local variable slots that are aliases of n using an AliasingAnalyzer and change the instruction to xLOAD m where m is the smallest alias.

    For every xLOAD n, find all local variable slots that are aliases of n using an AliasingAnalyzer and change the instruction to xLOAD m where m is the smallest alias. This leaves behind potentially stale xSTORE n instructions, which are then eliminated by eliminateStaleStores.

  2. def eliminatePushPop(method: MethodNode, owner: InternalName): Boolean

    When a POP instruction has a single producer, remove the POP and eliminate the producer by bubbling up the POPs.

    When a POP instruction has a single producer, remove the POP and eliminate the producer by bubbling up the POPs. For example, given ILOAD 1; ILOAD 2; IADD; POP we first eliminate the POP, then the IADD, then its inputs, so the entire sequence goes away. If a producer cannot be eliminated (need to keep side-effects), a POP is inserted.

    A special case eliminates the creation of unused objects with side-effect-free constructors: NEW scala/Tuple1; DUP; ALOAD 0; INVOKESPECIAL scala/Tuple1.<init>; POP The POP has a single producer (the DUP), it's easy to eliminate these two. A special case is needed to eliminate the INVOKESPECIAL and NEW.

  3. def eliminateStaleStores(method: MethodNode, owner: InternalName): Boolean

    Eliminate xSTORE instructions that have no consumer.

    Eliminate xSTORE instructions that have no consumer. If the instruction can be completely eliminated, it is replaced by a POP. The eliminatePushPop cleans up unnecessary POPs.

    Note that an ASOTRE can not always be eliminated: it removes a reference to the object that is currently stored in that local, which potentially frees it for GC (scala/bug#5313). Therefore we replace such stores by POP; ACONST_NULL; ASTORE x.

  4. def eliminateStoreLoad(method: MethodNode): Boolean

    Remove xSTORE n; xLOAD n pairs if

    Remove xSTORE n; xLOAD n pairs if

    • the local variable n is not used anywhere else in the method (1), and
    • there are no executable instructions and no live labels (jump targets) between the two (2)

    Note: store-load pairs that cannot be eliminated could be replaced by DUP; xSTORE n, but that's just cosmetic and doesn't help for anything.

    (1) This could be made more precise by running a prodCons analysis and checking that the load is the only user of the store. Then we could eliminate the pair even if the variable is live (except for ASTORE, scala/bug#5313). Not needing an analyzer is more efficient, and catches most cases.

    (2) The implementation uses a conservative estimation for liveness (if some instruction uses local n, then n is considered live in the entire method). In return, it doesn't need to run an Analyzer on the method, making it more efficient.

    This method also removes ACONST_NULL; ASTORE n if the local n is not live. This pattern is introduced by eliminateStaleStores.

    The implementation is a little tricky to support the following case: ISTORE 1; ISTORE 2; ILOAD 2; ACONST_NULL; ASTORE 3; ILOAD 1 The outer store-load pair can be removed if two the inner pairs can be.