Remove op_index and end_sequence from Crystal::DWARF::LineNumbers::Row

[HertzDevil]

Although these two fields are part of the DWARF line numbers state machine, they have no use when obtaining the line number for a given PC address. Removing them reduces sizeof(Crystal::DWARF::LineNumbers::Row) from 40 to 24 bytes on 64-bit targets, and from 28 to 20 bytes on 32-bit targets. As it turns out, Crystal::DWARF::LineNumbers#@matrix is one of the largest contributors to heap consumption in the Crystal runtime. We could check it by calculating the difference in GC.stats.total_bytes immediately before and after Exception::CallStack.load_debug_info_impl is called:

struct Exception::CallStack
  def self.load_debug_info : Nil
    return if ENV["CRYSTAL_LOAD_DEBUG_INFO"]? == "0"

    unless @@dwarf_loaded
      @@dwarf_loaded = true
      begin
        old = GC.stats.total_bytes
        load_debug_info_impl
        STDERR.puts(GC.stats.total_bytes - old)
      rescue ex
        @@dwarf_line_numbers = nil
        @@dwarf_function_names = nil
        Crystal::System.print_exception "Unable to load dwarf information", ex
      end
    end
  end
end

Using CRYSTAL_LOAD_DEBUG_INFO=1, the values are:

• x86_64-windows-gnu, empty file: 7,677,008 -> 5,793,856 (-24.5%)
• x86_64-linux-gnu, empty file: 5,021,184 -> 3,607,840 (-28.1%)
• aarch64-apple-dawrin, empty file: 4,844,144 -> 3,453,024 (-28.7%)
• x86_64-windows-gnu, compiler: 190,008,704 -> 127,562,288 (-32.9%)
• x86_64-linux-gnu, compiler: 171,804,448 -> 110,186,544 (-35.9%)
• aarch64-apple-dawrin, compiler: 172,521,040 -> 110,276,752 (-36.1%)

We could also profile GC.stats.heap_size in the same way: (these values are less reliable because the GC might simply decide to skip a GC cycle on some runs)

• x86_64-windows-gnu, empty file: 3,858,432 -> 2,863,104 (-25.8%)
• x86_64-linux-gnu, empty file: 3,235,840 -> 2,285,568 (-29.4%)
• aarch64-apple-dawrin, empty file: 3,325,952 -> 2,392,064 (-28.1%)
• x86_64-windows-gnu, compiler: 76,259,328 -> 58,658,816 (-23.1%)
• x86_64-linux-gnu, compiler: 68,206,592 -> 44,556,288 (-34.7%)
• aarch64-apple-dawrin, compiler: 59,293,696 -> 38,764,544 (-34.6%)