Package mockit.asm.controlFlow

Class Frame

java.lang.Object
mockit.asm.controlFlow.Frame
public final class Frame extends Object
Information about the input and output stack map frames of a basic block.

Frames are computed in a two steps process: during the visit of each instruction, the state of the frame at the end of current basic block is updated by simulating the action of the instruction on the previous state of this so called "output frame". In visitMaxStack, a fix point algorithm is used to compute the "input frame" of each basic block, i.e. the stack map frame at the beginning of the basic block, starting from the input frame of the first basic block (which is computed from the method descriptor), and by using the previously computed output frames to compute the input state of the other blocks.

All output and input frames are stored as arrays of integers. Reference and array types are represented by an index into a type table (which is not the same as the constant pool of the class, in order to avoid adding unnecessary constants in the pool - not all computed frames will end up being stored in the stack map table). This allows very fast type comparisons.

Output stack map frames are computed relatively to the input frame of the basic block, which is not yet known when output frames are computed. It is therefore necessary to be able to represent abstract types such as "the type at position x in the input frame locals" or "the type at position x from the top of the input frame stack" or even "the type at position x in the input frame, with y more (or less) array dimensions". This explains the rather complicated type format used in output frames.

This format is the following: DIM KIND VALUE (4, 4 and 24 bits). DIM is a signed number of array dimensions (from -8 to 7). KIND is either BASE, LOCAL or STACK. BASE is used for types that are not relative to the input frame. LOCAL is used for types that are relative to the input local variable types. STACK is used for types that are relative to the input stack types. VALUE depends on KIND. For LOCAL types, it is an index in the input local variable types. For STACK types, it is a position relatively to the top of input frame stack. For BASE types, it is either one of the constants defined below, or for OBJECT and UNINITIALIZED types, a tag and an index in the type table.

Output frames can contain types of any kind and with a positive or negative dimension (and even unassigned types, represented by 0 - which does not correspond to any valid type value). Input frames can only contain BASE types of positive or null dimension. In all cases the type table contains only internal type names (array type descriptors are forbidden - dimensions must be represented through the DIM field).

The LONG and DOUBLE types are always represented by using two slots (LONG + TOP or DOUBLE + TOP), for local variable types as well as in the operand stack. This is necessary to be able to simulate DUPx_y instructions, whose effect would be dependent on the actual type values if types were always represented by a single slot in the stack (and this is not possible, since actual type values are not always known - cf LOCAL and STACK type kinds).

  • Method Summary

    Modifier and Type
    Method
    Description
    void
    execute(@org.checkerframework.checker.index.qual.NonNegative int opcode)
    Simulates the action of the given zero-operand instruction on the output stack frame.
    void
    execute(@org.checkerframework.checker.index.qual.NonNegative int opcode, TypeOrMemberItem item)
    Simulates the action of the given instruction on the output stack frame.

    Methods inherited from class java.lang.Object

    clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

  • Method Details

    • execute

      public void execute(@org.checkerframework.checker.index.qual.NonNegative int opcode)
      Simulates the action of the given zero-operand instruction on the output stack frame.

    • execute

      public void execute(@org.checkerframework.checker.index.qual.NonNegative int opcode, @NonNull TypeOrMemberItem item)
      Simulates the action of the given instruction on the output stack frame.
      Parameters:
      opcode - the opcode of the instruction
      item - the operand of the instruction