On Fri, 26 Jul 2024 15:13:06 GMT, Andrew Haley <a...@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: > > Fix test failure Oh, it comes as a surprise to me... I was under impression that the first thing hand-coded assembly variants do is check for `bitmap != SECONDARY_SUPERS_BITMAP_FULL`. At least, it was my recollection from working on [JDK-8180450](https://bugs.openjdk.org/browse/JDK-8180450). (And the initial version of the patch with the check in `Klass::lookup_secondary_supers_table()` and `_bitmap(SECONDARY_SUPERS_BITMAP_FULL)` only reassured me that's still the case.) > The root cause of this bug is that we're not maintaining the invariant > popcount(bitmap) == secondary_supers()->length(). The invariant holds only when `bitmap != SECONDARY_SUPERS_BITMAP_FULL`. It does help that even in case of non-hashed `secondary_supers` list `secondary_supers()->length() >= popcount(bitmap)`, but initial probing becomes much less efficient (a random probe followed by a full linear pass over secondary supers list). Alternatively, all table lookups can be adjusted to start with `bitmap != SECONDARY_SUPERS_BITMAP_FULL` checks before probing the table. It does add a branch on the fast path (and slightly increases inlined snippet code size), but the branch is highly predictable and works on a value we need anyway. So, I would be surprised to see any performance effects from it. IMO it's easier to reason and more flexible: `SECONDARY_SUPERS_BITMAP_FULL == bitmap` simply disables all table lookups and unconditionally falls back to linear search. ------------- PR Comment: https://git.openjdk.org/jdk/pull/19989#issuecomment-2253281836
