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Zero based compressed oops
(To do: format the following nicely.}
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Compressed oops use an arbitrary address for the narrow oop base which is calculated as java heap base minus one (protected) page size for implicit NULL checks to work. This means a generic field reference is next:
| No Format |
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<narrow-oop-base> + (<narrow-oop> << 3) + <field-offset>. |
If
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the
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narrow
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oop
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base
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can
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be
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made
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to
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be
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zero
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(the
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java
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heap
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doesn't
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actually
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have
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to
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start
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at
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offset
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zero),
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then
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a
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generic
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field
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reference
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can
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be
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just next:
| No Format |
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(<narrow-oop << 3) + <field-offset>, allowing to save the heap base add (current Register Allocator does not allow to save register). Also with zero base the null check of compressed oop is not needed. Current code for decoding compressed oops looks like this: |
Theoretically it allows to save the heap base add (current Register Allocator does not allow to save register). Also with zero base the null check of compressed oop is not needed.
Current code for decoding compressed oops looks like this:
| No Format |
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if (<narrow-oop> == NULL) <wide_oop> = NULL else <wide_oop> = <narrow-oop-base> + (<narrow-oop> << 3) |
With
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zero
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narrow oop base
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the
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code
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is
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much
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simpler. It needs only shift to decode/encode a compressed oop:
| No Format |
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<wide_oop> = <narrow-oop> << 3 |
Also
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if
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java
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heap
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size
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<
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4Gb
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and
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it
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can
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be
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moved
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into
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low
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virtual
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address
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space
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(below 4Gb) then
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compressed
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oops
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can
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be
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used
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without
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encoding/decoding.
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Zero
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based
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implementation
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tries
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to
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allocated
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java
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heap
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using
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different
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strategies
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based
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on
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the
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heap
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size
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and
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a
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platform
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it
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runs
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on.
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First,
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it
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tries
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to
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allocate
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java
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heap
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below
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4Gb
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to
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use
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compressed
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oops
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without
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decoding
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if
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heap
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size
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<
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4Gb.
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If
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it
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fails
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or
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heap
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size
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>
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4Gb
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it
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will
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try
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to
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allocate
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the
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heap
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below
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32Gb
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to
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use
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zero
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based
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compressed
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oops.
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If
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this
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also
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fails
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it
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will
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switch
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to
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regular
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compressed
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oops
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with
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narrow
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oop
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base.
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