RFR: 8331341: secondary_super_cache does not scale well: C1 and interpreter [v8]

[Vladimir Ivanov]

On Thu, 25 Jul 2024 16:05:49 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> This patch expands the use of a hash table for secondary superclasses
>> to the interpreter, C1, and runtime. It also adds a C2 implementation
>> of hashed lookup in cases where the superclass isn't known at compile
>> time.
>> 
>> HotSpot shared runtime
>> ----------------------
>> 
>> Building hashed secondary tables is now unconditional. It takes very
>> little time, and now that the shared runtime always has the tables, it
>> might as well take advantage of them. The shared code is easier to
>> follow now, I think.
>> 
>> There might be a performance issue with x86-64 in that we build
>> HotSpot for a default x86-64 target that does not support popcount.
>> This means that HotSpot C++ runtime on x86 always uses a software
>> emulation for popcount, even though the vast majority of machines made
>> for the past 20 years can do popcount in a single instruction. It
>> wouldn't be terribly hard to do something about that.
>> 
>> Having said that, the software popcount is really not bad.
>> 
>> x86
>> ---
>> 
>> x86 is rather tricky, because we still support
>> `-XX:-UseSecondarySupersTable` and `-XX:+UseSecondarySupersCache`, as
>> well as 32- and 64-bit ports. There's some further complication in
>> that only `RCX` can be used as a shift count, so there's some register
>> shuffling to do. All of this makes the logic in macroAssembler_x86.cpp
>> rather gnarly, with multiple levels of conditionals at compile time
>> and runtime.
>> 
>> AArch64
>> -------
>> 
>> AArch64 is considerably more straightforward. We always have a
>> popcount instruction and (thankfully) no 32-bit code to worry about.
>> 
>> Generally
>> ---------
>> 
>> I would dearly love simply to rip out the "old" secondary supers cache
>> support, but I've left it in just in case someone has a performance
>> regression.
>> 
>> The versions of `MacroAssembler::lookup_secondary_supers_table` that
>> work with variable superclasses don't take a fixed set of temp
>> registers, and neither do they call out to to a slow path subroutine.
>> Instead, the slow patch is expanded inline.
>> 
>> I don't think this is necessarily bad. Apart from the very rare cases
>> where C2 can't determine the superclass to search for at compile time,
>> this code is only used for generating stubs, and it seemed to me
>> ridiculous to have stubs calling other stubs.
>> 
>> I've followed the guidance from @iwanowww not to obsess too much about
>> the performance of C1-compiled secondary supers lookups, and to prefer
>> simplicity over absolute performance. Nonetheless, this i...
>
> Andrew Haley has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 40 commits:
> 
>  - Merge branch 'clean' into JDK-8331658-work
>  - Cleanup
>  - temp
>  - Merge branch 'JDK-8331658-work' of https://github.com/theRealAph/jdk into JDK-8331658-work
>  - Review comments
>  - Review comments
>  - Review comments
>  - Review comments
>  - Review feedback
>  - Review feedback
>  - ... and 30 more: https://git.openjdk.org/jdk/compare/34ee06f5...248f44dc

src/hotspot/share/oops/klass.inline.hpp line 82:

> 80: // subtype check: true if is_subclass_of, or if k is interface and receiver implements it
> 81: inline bool Klass::is_subtype_of(Klass* k) const {
> 82:   guarantee(secondary_supers() != nullptr, "must be");

Minor point: considering libjvm contains hundreds of copies, does it make sense to turn it into an assert instead? For example, on AArch64 the check costs 36 bytes [1] in product build.  

[1]

libjvm.dylib[0x1306d4] <+28>:  ldr    x9, [x8, #0x28]
libjvm.dylib[0x1306d8] <+32>:  cbz    x9, 0x1307dc              ; <+292> [inlined] Klass::is_subtype_of(Klass*) const at klass.inline.hpp:82:3
...
libjvm.dylib[0x1307dc] <+292>: adrp   x0, 3200
libjvm.dylib[0x1307e0] <+296>: add    x0, x0, #0x663            ; "open/src/hotspot/share/oops/klass.inline.hpp"
libjvm.dylib[0x1307e4] <+300>: adrp   x2, 3200
libjvm.dylib[0x1307e8] <+304>: add    x2, x2, #0x690            ; "guarantee(secondary_supers() != nullptr) failed"
libjvm.dylib[0x1307ec] <+308>: adrp   x3, 3200
libjvm.dylib[0x1307f0] <+312>: add    x3, x3, #0x6c0            ; "must be"
libjvm.dylib[0x1307f4] <+316>: mov    w1, #0x52
libjvm.dylib[0x1307f8] <+320>: bl     0x54f870                  ; report_vm_error at debug.cpp:181

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

PR Review Comment: https://git.openjdk.org/jdk/pull/19989#discussion_r1692288943