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scala.tools.nsc.backend.jvm.opt

LocalOptImpls

object LocalOptImpls

Source
LocalOpt.scala
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  1. def compactLocalVariables(method: MethodNode): Boolean

    Compact the local variable slots used in the method's implementation.

    Compact the local variable slots used in the method's implementation. This prevents having unused slots for example after eliminating unreachable code.

    This transformation reduces the size of the frame for invoking the method. For example, if the method has an ISTORE instruction to the local variable 3, the maxLocals of the method is at least 4, even if some local variable slots below 3 are not used by any instruction.

    This could be improved by doing proper register allocation.

  2. def removeEmptyExceptionHandlers(method: MethodNode): Set[TryCatchBlockNode]

    Remove exception handlers that cover empty code blocks.

    Remove exception handlers that cover empty code blocks. A block is considered empty if it consist only of labels, frames, line numbers, nops and gotos.

    There are no executable instructions that we can assume don't throw (eg ILOAD). The JVM spec basically says that a VirtualMachineError may be thrown at any time: http://docs.oracle.com/javase/specs/jvms/se8/html/jvms-6.html#jvms-6.3

    Note that no instructions are eliminated.

    returns

    the set of removed handlers

  3. def removeEmptyLabelNodes(method: MethodNode): Boolean

    Removes unreferenced label declarations, also squashes sequences of label definitions.

    Removes unreferenced label declarations, also squashes sequences of label definitions.

    [ops]; Label(a); Label(b); [ops];

    > subs([ops], b, a); Label(a); subs([ops], b, a);

  4. def removeEmptyLineNumbers(method: MethodNode): Boolean

    Removes LineNumberNodes that don't describe any executable instructions.

    Removes LineNumberNodes that don't describe any executable instructions.

    This method expects (and asserts) that the start label of each LineNumberNode is the lexically preceding label declaration.

  5. def removeUnusedLocalVariableNodes(method: MethodNode)(firstLocalIndex: Int = parametersSize(method), renumber: (Int) ⇒ Int = identity): Boolean

    Remove all non-parameter entries from the local variable table which denote variables that are not actually read or written.

    Remove all non-parameter entries from the local variable table which denote variables that are not actually read or written.

    Note that each entry in the local variable table has a start, end and index. Two entries with the same index, but distinct start / end ranges are different variables, they may have not the same type or name.

  6. def simplifyJumps(method: MethodNode): Boolean

    Apply various simplifications to branching instructions.