optimizing acmp in L-World

[John Rose]

On Feb 8, 2018, at 2:58 PM, David Simms <david.simms at oracle.com> wrote:
> 
> UseCompressedClassPointers

Ah, right, that's the detail I missed.  So the tag bit
(for value klasses) is 0x01 for compressed klasses,
and a larger power of two otherwise.

FTR, the code already does what I was expecting,
and uses a higher bit to tag value-type instanceKlass
pointers, using alignment to make detagging unnecessary.

The assembly code for acmp will (probably) need to
adjust (down-shift?) the extracted tag bit, so that the
perturbed oop pointer doesn't accidentally refer to a
nearby heap node.

A variety of perturbations might be workable:

intptr_t x = …;
intptr_t tag = x->klass() & color_mask;
x = x + (tag >> (exact_log2(color_mask)-1))
//or
x = x ^ (-tag);
//or
x = x | (-tag);
//or
x = x >> tag;  //maybe
//etc.

The basic requirement is that, once you perturb the
oop by the color bit, that oop pointer is not bitwise-equal
to any valid oop in the universe (including the original
unperturbed oop).  The perturbed pointer should not be
null (all zero bits) either.

— John