[PATCH] 7901019: jol estimates do not take AllocationStyle into the account

[Aleksey Shipilev]

Hi,

On 06/06/2016 12:47 AM, Тимур Гибадуллин wrote:
> Hey there! I fixed tests, there were several bugs during determining
> allocation types of java types, also I removed autoAlign option from
> HotspotLayouter, because it seems, that now it is unnecessary, since we
> emulate original Hotspot algorithm of layouting.

Actually, autoAlign is needed to allow layout simulation (same with
take{Hierarchy|Super}Gaps). HotspotLayouter in default mode should match
the current VM layout precisely.

I have corrected a few things in the patch, and added a new tests that
matches the actual VM layout against HotspotLayouter:
  http://cr.openjdk.java.net/~shade/7901019/AllocationStyle-v2.patch

This identified at least one bug: new code takes all fields from the
entire hierarchy, and lays them out. This is not how a real layouter
works: it works on fields of each class in isolation.

Actual layout:

Class9943 object internals:
 OFFSET  SIZE    TYPE DESCRIPTION                    VALUE
      0    12         (object header)                N/A
     12     1 boolean Class9942.field0               N/A
     13     3         (alignment/padding gap)        N/A
     16     8  double Class9943.field0               N/A
     24     2   short Class9943.field1               N/A
     26     6         (loss due to the next object alignment)
Instance size: 32 bytes
Space losses: 3 bytes internal + 6 bytes external = 9 bytes total

VM Layout Simulation (current, field allocation style: 2)
Class9943 object internals:
 OFFSET  SIZE    TYPE DESCRIPTION                    VALUE
      0    12         (object header)                N/A
     12     4         (alignment/padding gap)        N/A
     16     8  double Class9943.field0               N/A
     24     2   short Class9943.field1               N/A
     26     1 boolean Class9942.field0               N/A
     27     5         (loss due to the next object alignment)
Instance size: 32 bytes
Space losses: 4 bytes internal + 5 bytes external = 9 bytes total

Notice how Class9942 field is laid out after all field in patched
HotspotLayouter. This is incorrect. Once you fix that,
take{Hierarchy|Super}Gaps gain meaning: hierarchy gap is a gap due to
"next class field block" alignment, and super gap is the hole left in
superclass after layout, that may be used to put some subclass fields.

Thanks,
-Aleksey