SPARC <was> Re: RFR 8151163 All Buffer implementations should leverage Unsafe unaligned accessors

[Peter Levart]

On 03/31/2016 08:33 AM, John Rose wrote:
> On Mar 30, 2016, at 2:36 AM, Paul Sandoz <paul.sandoz at oracle.com> wrote:
>> When access is performed in loops this can cost, as the alignment checks are not hoisted out. Theoretically could for regular 2, 4, 8 strides through the buffer contents. For such cases alignment of the base address can be checked. Not sure how complicated that would be to support.
>>
>> I lack knowledge of the SPARC instruction set to know if we could do something clever as an intrinsic.
> A couple of partial thoughts:
>
> If we had bitfield type inference, we would be able to deduce that the low bits of p and p+8 are the same.  Graal has this (because I gave them the formulae[1]).  C2 may be too brittle to add it into TypeInt.  Bitfield inference on expressions of the form p&7 and (p+8)&7 would allow commoning tests in an unrolled loop, hoisting the alignment logic to the top of the loop body, and (perhaps) through the phi to the loop head.
>
> [1]: http://hg.openjdk.java.net/graal/graal-core/file/ea5cc66ec5f2/graal/com.oracle.graal.compiler.common/src/com/oracle/graal/compiler/common/type/IntegerStamp.java#l460 <http://hg.openjdk.java.net/graal/graal-core/file/ea5cc66ec5f2/graal/com.oracle.graal.compiler.common/src/com/oracle/graal/compiler/common/type/IntegerStamp.java#l460>
>
> An intrinsic that would guide the JIT more explicitly would (I think) need an extra argument.  Something like:  getLongUnaligned(p, uo, ao), where uo and ao are both longs, but only ao is required to be naturally aligned ((ao&7)==0).  Yuck.  Maybe this could be pattern-matched in C2; it would be a kludge.
>
> — John

Yes, an API that allows "indexed" unaligned access might be helpful. For 
example (in Unsafe):

     public final long getLongUnaligned(Object o, long offset, int i) {
         long iOffset = (long) i << 3;
         if ((offset & 7) == 0) {
             return getLong(o, offset + iOffset);
         } else if ((offset & 3) == 0) {
             return makeLong(getInt(o, offset + iOffset),
                             getInt(o, offset + iOffset + 4));
         } else if ((offset & 1) == 0) {
             return makeLong(getShort(o, offset + iOffset),
                             getShort(o, offset + iOffset + 2),
                             getShort(o, offset + iOffset + 4),
                             getShort(o, offset + iOffset + 6));
         } else {
             return makeLong(getByte(o, offset),
                             getByte(o, offset + iOffset + 1),
                             getByte(o, offset + iOffset + 2),
                             getByte(o, offset + iOffset + 3),
                             getByte(o, offset + iOffset + 4),
                             getByte(o, offset + iOffset + 5),
                             getByte(o, offset + iOffset + 6),
                             getByte(o, offset + iOffset + 7));
         }
     }


..with usage in ByteBufferAs-X-Buffer instead of:

       public $type$ get(int i) {
           $memtype$ x = unsafe.get$Memtype$Unaligned(bb.hb, 
byteOffset(checkIndex(i)),
               {#if[boB]?true:false});
           return $fromBits$(x);
       }


the following could then be used:

       public $type$ get(int i) {
           $memtype$ x = unsafe.get$Memtype$Unaligned(bb.hb, bb.address, 
checkIndex(i),
               {#if[boB]?true:false});
           return $fromBits$(x);
       }


Regards, Peter