Proposal for Direct-X-Buffer change to improve allocation of zero capacity buffers

[Rob McKenna]

Hi Jon,

I think nio-dev at openjdk.java.net would be a more appropriate venue for this discussion.

	-Rob

On 01/08/16 12:24, Jon V. wrote:
> The Direct Buffer does not optimize the allocation of zero capacity
> buffers.  I am proposing a simple if( cap != 0 )  condition that will
> bypass most of the setup process.
> 
> Here is the current constructor:
> 
>     Direct$Type$Buffer$RW$(int cap) {                   // package-private
> #if[rw]
>         super(-1, 0, cap, cap);
>         boolean pa = VM.isDirectMemoryPageAligned();
>         int ps = Bits.pageSize();
>         long size = Math.max(1L, (long)cap + (pa ? ps : 0));
>         Bits.reserveMemory(size, cap);
> 
>         long base = 0;
>         try {
>             base = unsafe.allocateMemory(size);
>         } catch (OutOfMemoryError x) {
>             Bits.unreserveMemory(size, cap);
>             throw x;
>         }
>         unsafe.setMemory(base, size, (byte) 0);
>         if (pa && (base % ps != 0)) {
>             // Round up to page boundary
>             address = base + ps - (base & (ps - 1));
>         } else {
>             address = base;
>         }
>         cleaner = Cleaner.create(this, new Deallocator(base, size, cap));
>         att = null;
> #else[rw]
>         super(cap);
> #end[rw]
>     }