RFR: 8305896: Alternative full GC forwarding [v3]

[Roman Kennke]

> Currently, the full-GC modes of Serial, Shenandoah and G1 GCs are forwarding objects by over-writing the object header with the new object location. Unfortunately, for compact object headers ([JDK-8294992](https://bugs.openjdk.org/browse/JDK-8294992)) this would not work, because the crucial class information is also stored in the header, and we could no longer iterate over objects until the headers would be restored. Also, the preserved-headers tables would grow quite large.
> 
> I propose to use an alternative algorithm for full-GC (sliding-GC) forwarding that uses a special encoding so that the forwarding information fits into the lowest 32 bits of the header.
> 
> It exploits the insight that, with sliding GCs, objects from one region will only ever be forwarded to one of two possible target regions. For this to work, we need to divide the heap into equal-sized regions. This is already the case for Shenandoah and G1, and can easily be overlaid for Serial GC, by assuming either the whole heap as a single region (if it fits) or by using SpaceAlignment-sized virtual regions.
> 
> We also build and maintain a table that has N elements, where N is the number of regions. Each entry is two addresses, which are the start-address of the possible target regions for each source region.
> 
> With this, forwarding information would be encoded like this:
>  - Bits 0 and 1: same as before, we put in '11' to indicate that the object is forwarded.
>  - Bit 2: Used for 'fallback'-forwarding (see below)
>  - Bit 3: Selects the target region 0 or 1. Look up the base address in the table (see above)
>  - Bits 4..31 The number of heap words from the target base address
> 
> This works well for all sliding GCs in Serial, G1 and Shenandoah. The exception is in G1, there is a special mode called 'serial compaction' which acts as a last-last-ditch effort to squeeze more space out of the heap by re-forwarding the tails of the compaction chains. Unfortunately, this breaks the assumption of the sliding-forwarding-table. When that happens, we initialize a fallback table, which is a simple open hash-table, and set the Bit 2 in the forwarding to indicate that we shall look up the forwardee in the fallback-table.
> 
> All the table accesses can be done unsynchronized because:
> - Serial GC is single-threaded anyway
> - In G1 and Shenandoah, GC worker threads divide up the work such that each worker does disjoint sets of regions.
> - G1 serial compaction is single-threaded
> 
> The change introduces a new (experimental) flag -XX:[+|-]UseAltGCForwarding. This flag is not really intended to be used by end-users. Instead, I intend to programatically enable it with compact object headers once they arrive (i.e. -XX:+UseCompactObjectHeaders would turn on -XX:+UseAltGCForwarding), and the flag is also useful for testing purposes. Once compact object headers become the default and only implementation, the flag and old implementation could be removed. Also, [JDK-8305898](https://bugs.openjdk.org/browse/JDK-8305898) would also use the same flag to enable an alternative self-forwarding approach (also in support of compact object headers).
> 
> The change also adds a utility class GCForwarding which calls the old or new implementation based on the flag. I think it would also be used for the self-forwarding change to be proposed soon (and separately).
> 
> I also experimented with a different forwarding approach that would use per-region hashtables, but shelved it for now, because performance was significantly worse than the sliding forwarding encoding. It will become useful later when we want to do 32bit compact object headers, because then, the sliding encoding will not be sufficient to hold forwarding pointers in the header.
> 
> Testing:
>  - [x] hotspot_gc -UseAltGCForwarding
>  - [x] hotspot_gc +UseAltGCForwarding
>  - [x] tier1 -UseAltGCForwarding
>  - [x] tier1 +UseAltGCForwarding
>  - [x] tier2 -UseAltGCForwarding
>  - [x] tier2 +UseAltGCForwarding

Roman Kennke has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:

 - Merge branch 'master' into JDK-8305896
 - Clarify comments for SlidingForwarding
 - Add gtest for SlidingForwarding
 - Some cleanups
 - Fix 32 bit builds
 - Improve handling of Serial GC spaces; some refactorings
 - Implement fallback-hashtable for G1 serial compaction
 - 8305896: Alternative full GC forwarding

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/13582/files
  - new: https://git.openjdk.org/jdk/pull/13582/files/ca81f54c..30f19716

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13582&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13582&range=01-02

  Stats: 41686 lines in 715 files changed: 26160 ins; 11223 del; 4303 mod
  Patch: https://git.openjdk.org/jdk/pull/13582.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13582/head:pull/13582

PR: https://git.openjdk.org/jdk/pull/13582