RFR: JDK-8311035: CDS should not use dump time JVM narrow Klass encoding to pre-compute Klass ids [v9]

[Ashutosh Mehra]

On Wed, 5 Jul 2023 17:15:18 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 rang...
>
> Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Feedback Ashu

Marked as reviewed by ashu-mehra at github.com (no known OpenJDK username).

Thanks for the changes. lgtm!

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PR Review: https://git.openjdk.org/jdk/pull/14688#pullrequestreview-1515041679
PR Comment: https://git.openjdk.org/jdk/pull/14688#issuecomment-1622211868