Optimization of java.io.DataInputStream

[Alex Yursha]

Well, I agree that the code is more readable when we use shift
consistently, if you think it is worth some redundant bytes in a class file.


On Tue, Nov 12, 2013 at 6:00 PM, Vitaly Davidovich <vitalyd at gmail.com>wrote:

> Roger,
>
> I think a simple comment is better than leaving head scratching code
> behind, especially since it'll impact interpreter and byte code size.
>
> Sent from my phone
> On Nov 12, 2013 9:58 AM, "roger riggs" <roger.riggs at oracle.com> wrote:
>
> > Hi,
> >
> > At the source level the style makes it clear where the data is to be
> > aligned.
> > There many other places that use that style "<< 0" to indicate alignment
> > of the data
> > when assembling a whole from parts.
> > It is assumed the compiler/runtime will omit unnecessary operations; the
> > compiler does not.
> >
> > $.02, Roger
> >
> > On 11/12/2013 9:45 AM, Vitaly Davidovich wrote:
> >
> >> Interesting - was this file codegen'd or something? Why are these nop
> >> shifts there?
> >>
> >> Anyway, I'd expect JIT does the right thing here and doesn't actually
> >> issue
> >> this instruction.  Are you optimizing for interpreter or meta data size
> >> then?
> >>
> >> Sent from my phone
> >> On Nov 12, 2013 3:45 AM, "Alex Yursha" <alexyursha at gmail.com> wrote:
> >>
> >>  The following methods in java.io.DataInputStream perform right shift to
> >>> zero positions in their implementations:
> >>> - short readShort()
> >>> - int readUnsignedShort()
> >>> - char readChar()
> >>> - int readInt()
> >>> - long readLong()
> >>> - String readUTF(DataInput in)
> >>>
> >>> For example:
> >>>
> >>>      public final short readShort() throws IOException {
> >>>          int ch1 = in.read();
> >>>          int ch2 = in.read();
> >>>          if ((ch1 | ch2) < 0)
> >>>              throw new EOFException();
> >>>         return (short)((ch1 << 8) + *(ch2 << 0)*);
> >>>      }
> >>>
> >>> It can be optimizied as follows:
> >>>
> >>>      public final short readShort() throws IOException {
> >>>          int ch1 = in.read();
> >>>          int ch2 = in.read();
> >>>          if ((ch1 | ch2) < 0)
> >>>              throw new EOFException();
> >>>          return (short)((ch1 << 8) + *ch2*);
> >>>      }
> >>>
> >>>
> >>> This optimization saves 2 bytecode instructions in the class file code
> >>> arrays of each method mentioned above.
> >>>
> >>>
> >
>