Class: Omnizip::Filters::BcjPpc

Inherits:
FilterBase
  • Object
show all
Defined in:
lib/omnizip/filters/bcj_ppc.rb

Overview

BCJ filter for PowerPC executables.

This filter preprocesses PowerPC machine code by converting relative addresses in B/BL (Branch/Branch and Link) instructions to absolute addresses. PowerPC uses 4-byte aligned instructions with big-endian encoding.

The filter improves compression by making branch targets position-independent.

Constant Summary collapse

OPCODE_BASE =

PPC B/BL instruction base (0x48000000)

0x48000000
OPCODE_MASK =

Mask for checking B/BL instructions

0xFC000003
OPCODE_PATTERN =

Expected pattern for B/BL with link bit (0x48000001)

0x48000001
INSTRUCTION_SIZE =

Size of PPC instruction (4 bytes, big-endian)

4
OFFSET_MASK =

Offset mask (26-bit offset in instruction)

0x03FFFFFC

Class Method Summary collapse

Instance Method Summary collapse

Class Method Details

.metadata ⇒ Hash

Get metadata about this filter.

Returns:

  • (Hash) —

    Filter metadata



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# File 'lib/omnizip/filters/bcj_ppc.rb', line 129

def metadata
  {
    name: "BCJ-PPC",
    description: "Branch converter for PowerPC executables",
    architecture: "PowerPC",
    alignment: 4,
    endian: "big",
  }
end

Instance Method Details

#decode(data, position = 0) ⇒ String

Decode (postprocess) PowerPC executable data after decompression.

Reverses the encoding by converting absolute addresses back to relative addresses.

Parameters:

  • data (String) —

    Binary executable data

  • position (Integer) (defaults to: 0) —

    Current stream position

Returns:

  • (String) —

    Decoded binary data



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# File 'lib/omnizip/filters/bcj_ppc.rb', line 94

def decode(data, position = 0)
  return data.dup if data.bytesize < INSTRUCTION_SIZE

  result = data.b
  size = data.bytesize & ~(INSTRUCTION_SIZE - 1)
  i = 0
  pc = position - INSTRUCTION_SIZE

  while i < size
    instruction = extract_uint32_be(result, i)
    pc += INSTRUCTION_SIZE

    # Check for B/BL instruction
    if (instruction & OPCODE_MASK) == OPCODE_PATTERN
      # Extract absolute address
      absolute = instruction & OFFSET_MASK

      # Convert to relative offset
      offset = absolute - pc

      # Encode back
      new_instruction = OPCODE_BASE | (offset & OFFSET_MASK) | 1
      write_uint32_be(result, i, new_instruction)
    end

    i += INSTRUCTION_SIZE
  end

  result
end

#encode(data, position = 0) ⇒ String

Encode (preprocess) PowerPC executable data for compression.

Scans for B/BL instructions (0x48xxxxxx) and converts relative addresses to absolute addresses.

Parameters:

  • data (String) —

    Binary executable data

  • position (Integer) (defaults to: 0) —

    Current stream position

Returns:

  • (String) —

    Encoded binary data



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# File 'lib/omnizip/filters/bcj_ppc.rb', line 54

def encode(data, position = 0)
  return data.dup if data.bytesize < INSTRUCTION_SIZE

  result = data.b
  size = data.bytesize & ~(INSTRUCTION_SIZE - 1)
  i = 0
  pc = position - INSTRUCTION_SIZE

  while i < size
    instruction = extract_uint32_be(result, i)
    pc += INSTRUCTION_SIZE

    # Check for B/BL instruction (0x48xxxxxx with proper flags)
    if (instruction & OPCODE_MASK) == OPCODE_PATTERN
      # Extract 24-bit offset (bits 6-29), sign-extend
      offset = instruction & OFFSET_MASK
      offset = sign_extend_26(offset)

      # Convert to absolute address
      absolute = offset + pc

      # Encode back
      new_instruction = OPCODE_BASE | (absolute & OFFSET_MASK) | 1
      write_uint32_be(result, i, new_instruction)
    end

    i += INSTRUCTION_SIZE
  end

  result
end