Scala Compiler2.12.21 < Back
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# Packages

-  package root

  The Scala compiler and reflection APIs.

  The Scala compiler and reflection APIs.

  Definition Classes
    root
-  package scala
  Definition Classes
    root
-  package tools
  Definition Classes
    scala
-  package nsc
  Definition Classes
    tools
-  package backend
  Definition Classes
    nsc
-  package jvm
  Definition Classes
    backend
-  package opt
  Definition Classes
    jvm
- BoxUnbox
- ByteCodeRepository
- BytecodeUtils
- CallGraph
- ClosureOptimizer
- CopyProp
- InlineInfoAttribute
- InlineInfoAttributePrototype
- Inliner
- InlinerHeuristics
- LabelNotLive
- LocalOpt
- LocalOptImpls
- LruMap
- RemovePair
- RemovePairDependency

c

scala.tools.nsc.backend.jvm.opt

# LocalOpt 

###

#### abstract class LocalOpt extends AnyRef

Optimizations within a single method. Certain optimizations enable others, for example removing unreachable code can render a `try` block empty and enable removeEmptyExceptionHandlers. The latter in turn enables more unreachable code to be eliminated (the `catch` block), so there is a cyclic dependency. Optimizations that depend on each other are therefore executed in a loop until reaching a fixpoint.

The optimizations marked UPSTREAM enable optimizations that were already executed, so they cause another iteration in the fixpoint loop.

nullness optimizations: rewrite null-checking branches to GOTO if nullness is known + enables downstream

- unreachable code (null / non-null branch becomes unreachable)
- box-unbox elimination (may render an escaping consumer of a box unreachable)
- stale stores (aload x is replaced by aconst_null if it's known null)
- simplify jumps (replaces conditional jumps by goto, so may enable goto chains)

unreachable code / DCE (removes instructions of basic blocks to which there is no branch) + enables downstream:

- stale stores (loads may be eliminated, removing consumers of a store)
- empty handlers (try blocks may become empty)
- simplify jumps (goto l; [dead code]; l: ..) => remove goto
- stale local variable descriptors
- (not box-unbox, which is implemented using prod-cons, so it doesn't consider dead code)

note that eliminating empty handlers and stale local variable descriptors is required for correctness, see the comment in the body of `methodOptimizations`.

box-unbox elimination (eliminates box-unbox pairs within the same method) + enables UPSTREAM:

- nullness optimizations (a box extraction operation (unknown nullness) may be rewritten to a read of a non-null local. example in doc comment of box-unbox implementation)
- further box-unbox elimination (e.g. an Integer stored in a Tuple; eliminating the tuple may enable eliminating the Integer) + enables downstream:
- copy propagation (new locals are introduced, may be aliases of existing)
- stale stores (multi-value boxes where not all values are used)
- redundant casts (`("a", "b")._1`: the generic `_1` method returns `Object`, a cast to String is added. The cast is redundant after eliminating the tuple.)
- empty local variable descriptors (local variables that were holding the box may become unused)

- push-pop (due to artifacts of eliminating runtime type tests on primitives)

copy propagation (replaces LOAD n to the LOAD m for the smallest m that is an alias of n) + enables downstream:

- stale stores (a stored value may not be loaded anymore)
- store-load pairs (a load n may now be right after a store n) + NOTE: copy propagation is only executed once, in the first fixpoint loop iteration. none of the other optimizations enables further copy prop. we still run it as part of the loop because it requires unreachable code to be eliminated.

stale stores (replace STORE by POP) + enables downstream:

- push-pop (the new pop may be the single consumer for an instruction)

redundant casts: eliminates casts that are statically known to succeed (uses type propagation) + enables UPSTREAM:

- box-unbox elimination (a removed checkcast may be a box consumer) + enables downstream:
- push-pop for closure allocation elimination (every indyLambda is followed by a checkcast, see scala/bug#9540)

push-pop (when a POP is the only consumer of a value, remove the POP and its producer) + enables UPSTREAM:

- stale stores (if a LOAD is removed, a corresponding STORE may become stale)
- box-unbox elimination (push-pop may eliminate a closure allocation, rendering a captured box non-escaping) + enables downstream:
- store-load pairs (a variable may become non-live)
- stale handlers (push-pop removes code)
- simplify jumps (push-pop removes code)

store-load pairs (remove `STORE x; LOAD x` if x is otherwise not used in the method) + enables downstream:

- empty handlers (code is removes, a try block may become empty
- simplify jumps (code is removed, a goto may become redundant for example)
- stale local variable descriptors

empty handlers (removes exception handlers whose try block is empty) + enables UPSTREAM:

- unreachable code (catch block becomes unreachable)
- box-unbox (a box may be escape in an operation in a dead handler) + enables downstream:
- simplify jumps

simplify jumps (various, like `GOTO l; l: ...`, see doc comments of individual optimizations) + enables UPSTREAM

- unreachable code (`GOTO a; a: GOTO b; b: ...`, the first jump is changed to `GOTO b`, the second becomes unreachable)
- store-load pairs (a `GOTO l; l: ...` is removed between store and load)
- push-pop (`IFNULL l; l: ...` is replaced by `POP`)

The following cleanup optimizations don't enable any upstream optimizations, so they can be executed once at the end, when the above optimizations reach a fixpoint.

empty local variable descriptors (removes unused variables from the local variable table)

empty line numbers (eliminates line number nodes that describe no executable instructions)

At this point, we used to filter out redundant label nodes (sequences of labels without any executable instructions in between). However, this operation is relatively expensive, and unnecessary: labels don't exist in the classfile, they are lowered to bytecode offsets, so redundant labels disappear by design.

Note on a method's maxLocals / maxStack: the backend only uses those values for running Analyzers. The values can be conservative approximations: if an optimization removes code and the maximal stack size is now smaller, the larger maxStack value will still work fine for running an Analyzer (just that frames allocate more space than required). The correct max values written to the bytecode are re-computed during classfile serialization. To keep things simpler, we don't update the max values in every optimization:

- we do it in `removeUnreachableCodeImpl`, because it's quite straightforward
- maxLocals is updated in `compactLocalVariables`, which runs at the end of method optimizations

Note on updating the call graph: whenever an optimization eliminates a callsite or a closure instantiation, we eliminate the corresponding entry from the call graph.

Source
  LocalOpt.scala

Linear Supertypes
AnyRef, Any
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Ordering

1. Alphabetic
2. By Inheritance

Inherited

1. LocalOpt
2. AnyRef
3. Any

Implicitly

1. by any2stringadd
2. by StringFormat
3. by Ensuring
4. by ArrowAssoc

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Visibility

1. Public
2. All

### Instance Constructors

1.  new LocalOpt()

### Abstract Value Members

1.  abstract val postProcessor: PostProcessor

### Concrete Value Members

1.  final def !=(arg0: Any): Boolean
   Definition Classes
     AnyRef → Any
2.  final def ##(): Int
   Definition Classes
     AnyRef → Any
3.  def +(other: String): String
   Implicit
     This member is added by an implicit conversion from LocalOpt to any2stringadd[LocalOpt] performed by method any2stringadd in scala.Predef.
   Definition Classes
     any2stringadd
4.  def ->[B](y: B): (LocalOpt, B)
   Implicit
     This member is added by an implicit conversion from LocalOpt to ArrowAssoc[LocalOpt] performed by method ArrowAssoc in scala.Predef.
   Definition Classes
     ArrowAssoc
   Annotations
     @inline()
5.  final def ==(arg0: Any): Boolean
   Definition Classes
     AnyRef → Any
6.  final def asInstanceOf[T0]: T0
   Definition Classes
     Any
7.  val boxUnbox: BoxUnbox { val postProcessor: LocalOpt.this.postProcessor.type }
8.  def clone(): AnyRef
   Attributes
     protected[lang]
   Definition Classes
     AnyRef
   Annotations
     @throws( ... ) @native()
9.  val copyProp: CopyProp { val postProcessor: LocalOpt.this.postProcessor.type }
10.  def eliminateRedundantCasts(method: MethodNode, owner: InternalName): Boolean

    Eliminate `CHECKCAST` instructions that are statically known to succeed.

    Eliminate `CHECKCAST` instructions that are statically known to succeed. This is safe if the tested object is null: `null.asInstanceOf` always succeeds.

    The type of the tested object is determined using a NonLubbingTypeFlowAnalyzer. Note that this analysis collapses LUBs of non-equal references types to Object for simplicity. Example: given `B <: A <: Object`, the cast in `(if (..) new B else new A).asInstanceOf[A]` would not be eliminated.

    Note: we cannot replace `INSTANCEOF` tests by only looking at the types, `null.isInstanceOf` always returns false, so we'd also need nullness information.

11.  def ensuring(cond: (LocalOpt) ⇒ Boolean, msg: ⇒ Any): LocalOpt
    Implicit
      This member is added by an implicit conversion from LocalOpt to Ensuring[LocalOpt] performed by method Ensuring in scala.Predef.
    Definition Classes
      Ensuring
12.  def ensuring(cond: (LocalOpt) ⇒ Boolean): LocalOpt
    Implicit
      This member is added by an implicit conversion from LocalOpt to Ensuring[LocalOpt] performed by method Ensuring in scala.Predef.
    Definition Classes
      Ensuring
13.  def ensuring(cond: Boolean, msg: ⇒ Any): LocalOpt
    Implicit
      This member is added by an implicit conversion from LocalOpt to Ensuring[LocalOpt] performed by method Ensuring in scala.Predef.
    Definition Classes
      Ensuring
14.  def ensuring(cond: Boolean): LocalOpt
    Implicit
      This member is added by an implicit conversion from LocalOpt to Ensuring[LocalOpt] performed by method Ensuring in scala.Predef.
    Definition Classes
      Ensuring
15.  final def eq(arg0: AnyRef): Boolean
    Definition Classes
      AnyRef
16.  def equals(arg0: Any): Boolean
    Definition Classes
      AnyRef → Any
17.  def finalize(): Unit
    Attributes
      protected[lang]
    Definition Classes
      AnyRef
    Annotations
      @throws( classOf[java.lang.Throwable] )
18.  final def getClass(): Class[_]
    Definition Classes
      AnyRef → Any
    Annotations
      @native()
19.  def hashCode(): Int
    Definition Classes
      AnyRef → Any
    Annotations
      @native()
20.  final def isInstanceOf[T0]: Boolean
    Definition Classes
      Any
21.  def methodOptimizations(method: MethodNode, ownerClassName: InternalName): Boolean

    Run method-level optimizations, see comment on class LocalOpt.

    Run method-level optimizations, see comment on class LocalOpt.

    Returns `true` if the bytecode of `method` was changed.

22.  def methodOptimizations(clazz: ClassNode): Boolean

    Remove unreachable instructions from all (non-abstract) methods and apply various other cleanups to the bytecode.

    Remove unreachable instructions from all (non-abstract) methods and apply various other cleanups to the bytecode.

    clazz

      The class whose methods are optimized

    returns

      `true` if unreachable code was eliminated in some method, `false` otherwise.

23.  def minimalRemoveUnreachableCode(method: MethodNode): Boolean

    Remove unreachable code from a method.

    Remove unreachable code from a method.

    This implementation only removes instructions that are unreachable for an ASM analyzer / interpreter. This ensures that future analyses will not produce `null` frames. The inliner depends on this property.

    returns

      A set containing the eliminated instructions

24.  final def ne(arg0: AnyRef): Boolean
    Definition Classes
      AnyRef
25.  final def notify(): Unit
    Definition Classes
      AnyRef
    Annotations
      @native()
26.  final def notifyAll(): Unit
    Definition Classes
      AnyRef
    Annotations
      @native()
27.  def nullnessOptimizations(method: MethodNode, ownerClassName: InternalName): Boolean

    Apply various optimizations based on nullness analysis information.

    Apply various optimizations based on nullness analysis information.

    - IFNULL / IFNONNULL are rewritten to GOTO if nullness is known
    - IF_ACMPEQ / IF_ACMPNE are rewritten to GOTO if the both references are known null, or if one is known null and the other known not-null
    - ALOAD is replaced by ACONST_NULL if the local is known to hold null
    - ASTORE of null is removed if the local is known to hold null
    - INSTANCEOF of null is replaced by `ICONST_0`
    - scala.runtime.BoxesRunTime.unboxToX(null) is rewritten to a zero-value load
28.  def removeUnreachableCodeImpl(method: MethodNode): Boolean

    Removes unreachable basic blocks, returns `true` if instructions were removed.

    Removes unreachable basic blocks, returns `true` if instructions were removed.

    When this method returns, each `labelNode.getLabel` has a status set whether the label is live or not. This can be queried using `BackendUtils.isLabelReachable`.

29.  final def synchronized[T0](arg0: ⇒ T0): T0
    Definition Classes
      AnyRef
30.  def toString(): String
    Definition Classes
      AnyRef → Any
31.  final def wait(): Unit
    Definition Classes
      AnyRef
    Annotations
      @throws( ... )
32.  final def wait(arg0: Long, arg1: Int): Unit
    Definition Classes
      AnyRef
    Annotations
      @throws( ... )
33.  final def wait(arg0: Long): Unit
    Definition Classes
      AnyRef
    Annotations
      @throws( ... ) @native()
34.  def →[B](y: B): (LocalOpt, B)
    Implicit
      This member is added by an implicit conversion from LocalOpt to ArrowAssoc[LocalOpt] performed by method ArrowAssoc in scala.Predef.
    Definition Classes
      ArrowAssoc

### Deprecated Value Members

1.  def formatted(fmtstr: String): String
   Implicit
     This member is added by an implicit conversion from LocalOpt to StringFormat[LocalOpt] performed by method StringFormat in scala.Predef.
   Definition Classes
     StringFormat
   Annotations
     @deprecated @inline()
   Deprecated

     (Since version 2.12.16) Use `formatString.format(value)` instead of `value.formatted(formatString)`, or use the `f""` string interpolator. In Java 15 and later, `formatted` resolves to the new method in String which has reversed parameters.

### Inherited from AnyRef

### Inherited from Any

### Inherited by implicit conversion any2stringadd from LocalOpt to any2stringadd[LocalOpt]

### Inherited by implicit conversion StringFormat from LocalOpt to StringFormat[LocalOpt]

### Inherited by implicit conversion Ensuring from LocalOpt to Ensuring[LocalOpt]

### Inherited by implicit conversion ArrowAssoc from LocalOpt to ArrowAssoc[LocalOpt]

### Ungrouped