Optimizing byte reverse code for int value

[Hiroshi H Horii]

> I suggest replacing the use of loadI by endianness specific code 
> (which could possibly use lwbrx on big endian).

I believe that lwbrx is necessary for BE.

> Surely the source code needs fixing.  It could be:
> 
> public float readFloat(InputStream in) throws IOException {
>   readFully(in, aByteBuffer, 0, 4);
>   int val = aByteBuffer.getInt(0);
> 
>   return Float.intBitsToFloat(val);
> }

In my understanding, ByteBuffer.getInt() does the similar thing. I guess 
that application does not use ByteBuffer only for calling getInt().

Heap-X-Buffer.java.template
    public int getInt() {
        return Bits.getInt(this, ix(nextGetIndex(4)), bigEndian);
    }

Bits.java
    static int getInt(ByteBuffer bb, int bi, boolean bigEndian) {
        return bigEndian ? getIntB(bb, bi) : getIntL(bb, bi) ;
    }
    static int getIntL(ByteBuffer bb, int bi) {
        return makeInt(bb._get(bi + 3),
                       bb._get(bi + 2),
                       bb._get(bi + 1),
                       bb._get(bi    ));
    }
    static private int makeInt(byte b3, byte b2, byte b1, byte b0) {
        return (((b3       ) << 24) |
                ((b2 & 0xff) << 16) |
                ((b1 & 0xff) <<  8) |
                ((b0 & 0xff)      ));
    }

Heap-X-Buffer.java.template
    byte _get(int i) {                          // package-private
        return hb[i];
    }

Direct-X-Buffer.java.template
    byte _get(int i) {                          // package-private
        return unsafe.getByte(address + i);
    }

Regards,
Hiroshi

Andrew Haley <aph at redhat.com> wrote on 2017/04/07 17:36:13:

> From: Andrew Haley <aph at redhat.com>
> To: Michihiro Horie/Japan/IBM at IBMJP, ppc-aix-port-
> dev at openjdk.java.net, hotspot-dev at openjdk.java.net
> Cc: Hiroshi H Horii/Japan/IBM at IBMJP, volker.simonis at sap.com
> Date: 2017/04/07 17:37
> Subject: Re: Optimizing byte reverse code for int value
> 
> On 07/04/17 06:49, Michihiro Horie wrote:
> > 
> > Would you please review our change for JDK10 on ppc64?
> > Issue: https://bugs.openjdk.java.net/browse/JDK-8178294
> > Webrev: http://cr.openjdk.java.net/~horii/8178294/webrev.00/
> > 
> > This change adds two conversion rules of reversing contiguous 4 bytes 
for
> > int value.
> > The first conversion rule finds a pattern below and emits a lwz 
instruction
> > instead.
> 
> Surely the source code needs fixing.  It could be:
> 
> public float readFloat(InputStream in) throws IOException {
>   readFully(in, aByteBuffer, 0, 4);
>   int val = aByteBuffer.getInt(0);
> 
>   return Float.intBitsToFloat(val);
> }
> 
> Then there would be no need for a special ppc64 pattern.
> 
> Andrew.
> 


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