Integrated: JDK-8311035: CDS should not use dump time JVM narrow Klass encoding to pre-compute Klass ids

[Thomas Stuefe]

On Wed, 28 Jun 2023 07:48:33 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

> I recently spent too much time trying to understand the interleaving of narrow Klass decoding with CDS. Thanks to @iklam for clarifying some details. This patch aims to make the code easier to understand and more correct.
> 
> (*2023-07-05 Updated to fit patch description to the agreed final form*)
> 
> ----
> 
> CDS narrow Klass handling plays a role for archived heap objects. 
> 
> When dumping heap objects, we must recompute their narrow Klass ids since the relative positions of archived Klass instances change compared to their live counterparts in the dump time JVM.
> We recompute those narrow Klass ids using the following encoding scheme:
> - base = future assumed mapping start address
> - shift = dump time (!) JVMs encoding shift  (A)
> see `ArchiveHeapWriter::update_header_for_requested_obj` https://github.com/openjdk/jdk/blob/c3f10e847999ec254893de5a1a5de32fd07f715a/src/hotspot/share/cds/archiveHeapWriter.cpp#L419-L425
> 
> At CDS runtime, we load the CDS archive, then place the class space behind it. We then initialize narrow Klass encoding for the resulting combined Klass range such that:
> - encoding base is the range start address (mapping base)
> - encoding shift is always zero
> see `CompressedKlassPointers::initialize` : https://github.com/openjdk/jdk/blob/c3f10e847999ec254893de5a1a5de32fd07f715a/src/hotspot/share/oops/compressedOops.cpp#L195-L231
> 
> The lengthy comment there is misleading and partly wrong (regrettable since it was mine :)
> 
> At dump time, when initializing the JVM, we also set the encoding base to klass range start and shift to zero (also `CompressedKlassPointers::initialize`). That is the shift we later use for (A); hence, that shift is zero.
> 
> -------------------
> 
> There are minor things wrong with the current code. Mainly, the *dump time* VM's narrow Klass encoding scheme is irrelevant for the encoding employed on the future runtime archive since we recalculate the narrow Klass ids for archived heap objects. That means:
> 
> - In `CompressedKlassPointers::initialize`, there is no need to fix the encoding base and shift for the *dump time* JVM. Dump time JVM can use whatever base+shift it pleases; it can use the same code path as when CDS is off (e.g. use zero-based encoding).
> 
> - In `ArchiveHeapWriter::update_header_for_requested_obj`, we should not use the dump time JVM shift for pre-computing the narrow Klass ids. Instead, we should use the *minimal shift needed for the maximal possible future Klass range size*. The comment in ArchiveHeapWriter explains this in g...

This pull request has now been integrated.

Changeset: 0616648c
Author:    Thomas Stuefe <stuefe at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/0616648c59215d001211423402c6444ce228f01e
Stats:     159 lines in 8 files changed: 70 ins; 57 del; 32 mod

8311035: CDS should not use dump time JVM narrow Klass encoding to pre-compute Klass ids

Reviewed-by: iklam

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PR: https://git.openjdk.org/jdk/pull/14688