RFR: 8331341: secondary_super_cache does not scale well: C1 and interpreter [v5]

[Vladimir Ivanov]

On Mon, 22 Jul 2024 17:19:46 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 incrementally with one additional commit since the last revision:
> 
>   Review comments

My take on the questions you raised.

> Should Klass::linear_search_secondary_supers() const call set_secondary_super_cache()? (Strong no from me. It's UB.)

Agree. I'm fine with addressing that separately (as I mentioned earlier).

> Should we use a straight linear search for secondary C++ supers in the runtime, i.e.not changing it for now?

Slightly in favor of keeping `Klass::is_subtype_of()` simple, but I'm fine with it either way.

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PR Comment: https://git.openjdk.org/jdk/pull/19989#issuecomment-2246108523